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ORGANIC CHEMISTRY LABORATORY I
CH 337
To accompany CH 334 series
The PDF of the lab manual can be downloaded on the CH 337 D2L webpage
Fall 2012
PORTLAND STATE UNIVERSITY
Department of Chemistry
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TABLE OF CONTENTS
Ch 337 LAB SCHEDULE 1
ORGANIC CHEMISTRY LABORATORY 2
Laboratory Safety Rules and Procedures
3
Checking Into Organic Laboratory
6
Format for Organic Chemistry Lab Reports 7
GUIDELINES FOR USE OF THE LAB REPORT FORM
11
Experiment 1: The Purity and Purification of Solids -- Part I - Melting Points
Experiment 2: The Purity & Purification of Solids -- Part II Recrystallization
Experiment 3: The Purity and Purification of Liquids 24
Experiment 4: Isolation of Eugenol from Cloves Using Steam Distillation 29
Experiment 5: ISOLATION OF CAFFEINE FROM TEA LEAVES
33
Experiment 6: Cyclohexene from Cyclohexanol
36
Guidelines for Preparing ‘Schematic Diagram’ of Work-up and Purification.
Experiment 7: Cholesterol from Human Gallstones 41
APPENDIX A Composition and densities of Common Acids and Bases 45
APPENDIX B GENERAL PROCEDURE FOR RECRYSTALLIZATION
APPENDIX C Pasteur pipet 47
An Annoying Thing about Using a Pasteur Pipet
47
Using a Pasteur Pipet as a Filtering Device 47
APPENDIX D DIRECTIONS FOR OBTAINING INFRARED SPECTRA
I. Sample Preparation
Liquid Samples - NaCl Windows
Solid Samples - KBr Pellet Method
Solid Samples - Cast Film Method
II. Operation of IR Instrument
III. Characteristic IR Absorptions
APPENDIX E Recrystallization Using A Craig Tube
52
Introduction
Procedure
APPENDIX F REFRACTIVE INDEX
54
APPENDIX G The Use of Gas Chromatography
56
APPENDIX H DRYING 57
Drying Solids
Drying Liquids (Solvents or Solutions)
TABLE I. Drying Agents Usable with Organic Liquids
13
20
40
46
49
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Ch 337 LAB SCHEDULE
Fall 2011
Texts
"Organic Laboratory Techniques," 3rd edition, Fessenden & Fessenden & Feist (purchase at PSU Bookstore)
"Organic Chemistry Laboratory I, CH 337," PSU (purchase at Smart Copy, 1915 SW 6th Ave. or as a PDF on D2L)
Notes:
Experiments begin on the first day of lab. Safety goggles and notebook are needed for the first lab.
You must read the lab manual and the corresponding FFF references before coming to the lab.
Lab reports are due one lab period after the experiment is completed.
WEEK
1
2
EXPERIMENT
REFERENCES
Check in.
Begin Exp. 1: Melting Points or Fractional distillation
Melting Points or fractional distillation
(Report of the experiment you started first: due week 3)
Melting Points or fractional distillation
(Report of the experiment you started second: due week 4)
3
Begin Exp. 2: Recrystallization (report due week 5).
4
Begin Exp. 4: Isolation of Eugenol from Cloves. (report due week 6)
5
6
Begin Exp. 5: Caffeine from Tea. (report due week 7)
7
Begin Exp. 6: Cyclohexene from Cyclohexanol.
(Report due by week 8)
8
Begin Exp. 7: Cholesterol from Human Gallstones
***(report due by Dec 5) ***
Thanksgiving break no classes
9
10
Final Exam
Complete all work.
Clean up and check out.
Wed, December 7th, 8:00 - 9:50 am, Hoffmann Hall
FFF, p 23-38
Lab Manual, 20-23
Lab Manual, Appendix B
Lab Manual, 29-32
FFE 89-90 Hickman Still
Lab Manual, Appendix F
FFF, p 49-54, 58-61, 111-113
Lab Manual, p 33-35
Lab Manual, p 49-52, 55-56
Lab Manual, p 36-40
Lab Manual, p 41-44
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ORGANIC CHEMISTRY LABORATORY
Texts
"Organic Laboratory Techniques," 3rd edition, Fessenden & Fessenden & Feist (purchase at bookstore)
"Organic Chemistry Laboratory I, CH 337M," PSU (purchase at Smart Copy)
Course Grading
Lab Reports
Lab Technique
50%
15%
Notebook
Final Exam
10%
25%
Lab reports should be typed. Reports must be written according to the guideline described in "Format for Organic Chemistry Lab Reports" (Lab
Manual). Reports are due one lab period after the experiment is completed. If you cannot turn in a report on time due to medical or family
emergencies, consult with the instructor in advance to arrange a mutually agreeable alternative due date. Otherwise, 5 points/day will be deducted
for late reports. Good scientific writing is to the point. Try and write your lab reports in a clear and concise fashion.
Lab Technique will be evaluated by your lab instructor. The evaluation will include general understanding of the lab, ability to work
independently, wearing safety goggles, careful use of chemicals, apparatus and instruments, cleaning up your bench and spilled chemicals, etc.
Notebook will also be evaluated by your lab instructor at periodic intervals throughout the term. See the discussion on Laboratory Notebook on
how to record a good lab notebook.
This course has a minimal requirement of 50% on the final exam in order to receive a grade above C (no exceptions will be made). There will be
NO makeup final exam
.
A
85 -100
C+
68 - 69
A
83 - 84
C
58 -67
B+
81 - 82
C-
55 - 57
B
72 - 80
D
45 -55
B
70 - 71
F
< 45
You must do ALL assigned experiments unless the laboratory instructor changes schedules for the whole section. There is a limited opportunity
to make up work during other scheduled laboratory periods.
An incomplete grade is rarely given in laboratory courses where registration is required to reserve a space to work. If you have extreme medical
or family emergencies and cannot complete this course, immediately contact the professor in charge of this course (not your TA) to discuss
alternatives.
Lab Organization
Materials. You will need the following items for this lab:
Safety goggles. This department requires a type of safety goggles that shield the eyes from all sides and has no vents. They are available at the
PSU bookstore, or from the chemistry stockroom in an economical package that includes.
Notebook. A bound notebook with numbered pages is required (for example: Vernon Royal No. 11208 composition book, 120 pages, 9 1/2" x 7
1/2".) The same notebook may be used for both terms of organic lab.
Laboratory Clean-Up. It is everyone's responsibility to maintain a safe and organized working space in the lab. This
includes the proper disposal of waste chemicals, keeping the balance areas and the instruments free of chemicals
and spill, and making sure that your bench is clean before you leave. It is part of your Lab Technique to be
evaluated by your instructor.
Working in Other Lab Sections. If you need to work in a lab period other than your own, first check with the
teaching assistant in the lab you want to work in to be sure that there is room. If there is room, fill out a checkout slip for your key, have it initialed by the teaching assistant and obtain your drawer key from the stockroom.
Return the key to the stockroom when you leave the lab. You should have the TA sign your notebook after you
finish the lab. Your report will not be graded unless it is signed by the lab instructor.
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Glassware Breakage. It is your responsibility to pay for any broken glassware. Please be careful.
Fines for Late Turn-Ins. You will occasionally need to check out items of specialized equipment not available in your drawer. These must be
returned at the end of the period because the other lab sections will need them. To encourage prompt return of these items, the stockroom has
established a system of fines if they are turned in late. Fines are posted in the lab. If you do not check out of your locker before the end of the
term deadline, a $50 fee will be charged to your student account.
Preparing for the Final Exam
All students take the final at the same time. The final is designed to assess your understanding of lab operations and the chemical reactions
encountered in the lab.
***Hints for the final exam***
For each experiment review what you did, why you did it, the equipment you used, the concept of how and why it works. Go back over the
questions that were part of the lab report. For experiments that involved chemical synthesis, be sure that you understand such things as the
chemical reaction, the basic three-part organization of most syntheses as explained in the lab manual, and the function of the various chemicals,
solvents, etc. Again, review the questions that were part of the report.
Laboratory Safety Rules and Procedures
Safety Rules
The guidelines below are established for your and your classmates’ personal safety. Failure to adhere to the guidelines below will result in a loss
of Lab Technique points.
• Personal Protective Equipment (PPE) is used to protect you from serious injuries or illnesses resulting from
contact with chemical hazards in the laboratory. Spills and other accidents can occur when least expected. For
this reason it is necessary to wear proper PPE. The PPE for student labs consist of goggles, gloves and clothing.
Proper PPE is required for all students or they will be asked to leave the lab
•Goggles – Goggles must be worn at all times: unless the TA informs you it is safe not to wear them. You should not wear contact lenses in a
chemical laboratory. Chemical vapors may become trapped behind the lenses and cause eye damage. Some chemicals will dissolve “soft” contact
lenses. The most important aspect of having the goggles fit comfortably is the proper adjustment of the strap length. Adjust the strap length so
that the goggles fit comfortably securely and are not too tight. If you find that your goggles tend to fog, you can pick-up anti-fog tissue from the
stockroom. If you are caught not wearing goggles you will loose 10 technique points. This is your warning.
• Gloves – Gloves should be worn to protect the hands from chemicals. Gloves are provided through your student fees and are located in the
student labs. For health and safety reasons it is important to always remove at least one glove when leaving the student laboratory, this prevents
things such as door handles from getting contaminated. If you are wearing gloves outside the lab you will loose technique points.
• Clothing – Dress appropriately for laboratory work. You must wear shoes that cover your entire foot, including the heel. They should fit up
near your ankle; leather is preferred but any non-porous material is okay. Your clothing must cover your torso and legs down to your knees. Short
shorts, short skirts, tank tops and halter tops are not allowed.
• Eating, drinking and smoking are prohibited in the laboratory at ALL times. Wash your hands after finishing lab work and refrain from quick
trips to the hall to drink or eat during lab. If you take a break, be certain to remove gloves and wash hands before ingesting food or drink.
• Never work alone in the laboratory or in the absence of the instructor.
• Headphones may not be worn in lab.
• Do not text or talk on your cell phone in the lab. If you have to take a call please step out into the hall.
• If you are horse playing or screwing around you WILL be excused from the lab and loose every points for that
lab.
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Safety Procedures
• Know location of safety equipment; fire extinguisher, fire blanket, first aid kit, safety shower, eyewash
fountain and all exits.
• In case of fire or accident, call the instructor at once.
• Small fires may be extinguished by wet towels.
• If a person’s clothing catches fire, roll the person in the fire blanket to extinguish the flames.
• In case of a chemical spill on the body or clothing, stand under the safety shower and flood the affected area
with water. Remove clothing to minimize contamination with the chemical.
• If evacuation of the lab is necessary, leave through any door that is safe, or not obstructed; doors that lead to
other labs may be the best choice. Leave the building by the nearest exit and meet your TA on the field next to
Hoffmann Hall. This would also be the meeting place in the event of an earthquake or other emergency. It is
good to know the nearest exits of your lab on the first day of class.
• Spilled chemicals must be cleaned up immediately. If the material is corrosive or flammable, ask the instructor
for assistance. If acids or bases are spilled on the floor or bench, neutralize with sodium bicarbonate, then
dilute with water. Most other chemicals can be sponged off with water.
• Avoid contact with blood or bodily fluids. Notify the instructor or stockroom personnel if ANY blood is spilled
in the lab so that proper clean up and disposal procedures may be followed.
Laboratory Procedures and Protocol
General Etiquette:
• Leave all equipment and work areas as you would wish to find them.
• Keep your lab bench area neat and free of spilled chemicals. Your book bag, coat, etc., should be kept in the
designated area at the entrance to the lab, not at your bench.
• All chemical waste must be disposed of in proper containers. Proper disposal of chemicals is important for
student safety and proper disposal. Putting chemicals into the wrong containers can lead to injury from
unexpected chemical reactions. Mixing waste makes it more difficult and expensive for PSU to dispose of.
Waste jars for each experiment will be provided in the lab. They will be labeled specifying which contents
should be placed inside. It is important that you replace the lids to the waste containers. Do not put anything
down the sink unless you are explicitly told to dispose of it this way. Your instructor will provide specific
disposal guidelines when needed. Following these guidelines assists us in lowering the environmental impact of
the labs.
There are several locations for very specific waste.
Chemical waste – these containers are ONLY for chemical waste generated in the lab. They are each
specifically labeled for each lab and waste type. READ THE LABELS.
Contaminated paper waste –is ONLY for paper towels used for clean-up of chemical spills.
Broken glass –is ONLY for broken glassware.
Gloves – is ONLY for used gloves.
• Clean your bench and equipment Clean all your glassware- dirty glassware is harder to clean later. Wash with
water and detergent scrubbing with a brush as necessary. Rinse well with water. Do not dry glassware with
compressed air, as it is contaminated with compressor oily. The water and gas should be turned off and your
equipment drawer locked.
• Clean the common areas before you leave the lab. Point deductions for the entire class will be imposed if
the instructor or stockroom is not satisfied.
• Return any special equipment to its proper location or the stockroom.
Handling Chemicals:
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Obtaining reagents:
• Read the label CAREFULLY. Chemicals are organized by experiment in secondary containment bins. Make
sure the chemical name and concentration match what is required by the experiment!
• Do not take reagents to your bench.
• Never pick up a bottle by its lid as the lid my not be secure. Pick up bottles by the label. Remember to wear
gloves while working with reagents.
• Do not put stoppers/lids from reagents down on the lab bench. They may become contaminated. Be sure that
the lids/stoppers are replaced when not is use.
• To minimize cross contamination, do not place your own pipet, dropper, or spatulas into the reagent jar. Pour
a small amount into a beaker and measure from that. Please pour on the conservative side to minimize waste
and cost of labs. You can always go back for more.
• Do not put any excess reagent back in the reagent jar. Treat it as waste and dispose of it properly.
• When weighing chemicals on the balances, never weigh directly onto the weighing pan. Weigh into a
weighing boat or beaker. Any spills on the balances MUST be cleaned up immediately. If you are unclear
how to clean a spill, notify your instructor. The balances you are using are precision pieces of equipment and
costs up to $4000.
• All chemicals should be treated as potentially hazardous and toxic. Never taste a chemical or solution. When
smelling a chemical, gently fan the vapors toward your nose.
• Any chemicals that come in contact with your skin should be immediately washed with soap and copious
amounts of water. Please inform the TA.
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Laboratory Procedures
• Never pipet any liquid directly by mouth! Use a rubber bulb to draw liquid into the pipet.
• Never weigh hot chemicals or hot glass wear.
• When heating a test tube, always use a test tube holder and never point the open end of the test tube toward
yourself or another person.
• Handling glass tubing or thermometers: to insert glass tubing into a rubber stopper, lubricate the glass tubing
with a drop of glycerin, hold the tubing in your hand close to the hole, and keep all glass pieces wrapped in a
towel while applying gentle pressure with a twisting motion.
• To prepare a dilute acid solution from concentrated acid, acid should be added slowly to water with continuous
stirring. This process is strongly exothermic, and adding water to acid may result in a dangerous, explosive
spattering.
• Use the fume hood for all procedures that involve poisonous or objectionable gases or vapors.
• Never use an open flame and flammable liquids at the same time.
Checking Into Organic Laboratory
Notice To All Students in Chemistry Laboratories
Please read the notice that was handed out with the equipment list. It describes your responsibility for the equipment once you check in, and
discusses other procedures regarding the lab and stockroom.
Safety Equipment. Note the location of fire extinguishers, safety showers, eye wash fountain, first aid supplies, and sodium bicarbonate (for acid
spills).
Keys. After having locked your drawer at the end of the laboratory period, please return your key to the key board.
Checking Your Equipment
Please listen carefully to the TA!!! Read the section "Organic Lab Techniques" by FFF, and the pictures on the back of the check-out sheet will
help you identify the equipment.
Check to see that there are no chips, cracks, or other defects on the condenser and on the distilling column (large condenser). This is done by
gently tapping the condenser and column on the table top; a grating sound means the condenser or column is broken at the ring seals (the point at
which the outer tube joins the inner tube). Visually inspect the ring seals for cracks. Look down the column and check that the tips of the three
indentations are not broken and look at indentations through the side of the column.
Check the bottom of the round bottom flasks for "star" cracks and other cracks. Scratches are acceptable. Cracks can be detected as they "glisten"
when the equipment is rotated. The tube, (adapter thermo), has a glass and a rubber part. Check the bottom - especially where the side joins the
bottom - of beakers, Erlenmeyer flasks, and test tubes for "star" cracks and other cracks. Check pouring lips and rims of beakers, flasks, and test
tubes for chips. Each sample bottle is to have its own cap. The filter flask should have a piece of heavy-wall rubber tubing 15-18" long.
The stopcock of the separatory funnel should turn and the stopper for the top should come out. For some drawers, the same stopper fits the
ground glass joints and the separatory funnel; for most drawers there are two stoppers, one larger and one smaller. Separatory funnels with Teflon
stopcocks have a white washer next to the glass, a black "O" ring, and a Teflon "nut" on the end of the stopcock. All three parts must be there.
Stirring rods should be about 6" or longer. Thermometer should read room temperature; if it doesn't, it is probably broken (check the bulb for a
crack). You should have 2 pieces of condenser tubing (thin-wall rubber tubing), each at least 18" long.
If you need any equipment replaced, list the items on a pink slip and have the instructor sign the slip. Take the slip to the stockroom. On the
equipment list, note any student-induced imperfections in equipment not being replaced. Return the equipment sheet to the instructor.
Laboratory Notebook
READ "The Laboratory Notebook" section, p 8-12 in "Organic Laboratory Techniques" (FFF), the textbook for the laboratory. Pay particular
attention to their statements relating to amount to record, use of ink, lining out material, table of contents, and what to record.
The following points are of special interest:
A laboratory notebook is not a report; it is not intended to be a polished product. It should be a complete record of the successes and failures. The
polished report is written later, using information from the notebook.
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Think of the lab notebook as the "primary record". All data and observations are recorded directly in the
notebook rather than on slips of paper, etc. Take the notebook with you when you make weighing, keep it open
at your desk and make entries as appropriate.
Do not mix up lab lecture notes with the actual experiment. Put lab lecture notes somewhere else altogether, or
put them in a clearly marked section at the back of the notebook.
The goal in keeping a lab notebook is to have it complete, so that you or somebody else could repeat what you
did many years later from the information in the notebook.
5. Keep a running table of contents at the front of the book.
For each experiment, enter:
Date:
Title:
Purpose: a one or two-sentence overview of what you intend to accomplish.
References: indicate the texts or manuals from which you are getting directions and background information.
Balanced equation(s) when appropriate.
Procedure: summarize each step using your own words. DO NOT copy the entire experimental procedure from the lab manual
All of the above items MUST be completed BEFORE coming to the lab. Make a copy for your instructor. Your instructor will grade this portion
which accounts for 10% of your report. Errors or incomplete parts need to be corrected and finished before you can start the experiment.
Observations/results/data/etc: during the experiment, carefully record the observations you made for each step (quantities used, color changes,
time for different reactions, temperatures, etc.), any changes from the printed procedures, and the actual results (appearance and weight of the
product, melting point, degree and criteria of purity, etc.).
Conclusions: brief statements relating to the success of the experiment, confirmation of product identities, changes that might be made if you
were to run the experiment again, etc.
All original infrared spectra, gas chromatograms, and thin layer plates, taped into the notebook and appropriately labeled. Copies are to be turned
in with the reports.
If an experiment is not completed in one period, on the day when it is resumed, enter the new data and "Experiment so-and-so (continued)".
Have your instructor initial your notebook at the end of every lab period.
Format for Organic Chemistry Lab Reports
A good lab report that is brief, clear, and complete.
Introduction
One of the goals of this course is to learn how to report scientific information, which requires accuracy and attention to details. To some of you, it
may seem like focusing on minutiae, but a sloppy technical report gives the impression (probably correct) that the work was done with equal lack
of care. There are accepted ways of writing and presenting data that most scientists follow.
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All written reports for the organic laboratory are to follow so-called journal style. This format is based on a typical format of an article in a
chemistry research journal such as the Journal of the American Chemical Society (JACS). In any copy of a journal you will see some variations
on this format, but use exactly this style for your reports. You will probably experience some initial uncertainty regarding the degree of detail that
is appropriate. Consult with your lab instructor if necessary.
General Considerations
All lab reports must be typed.
Chemical structures and equations can be drawn on chemical drawing programs such as ChemDraw and ChemIntosh. These programs are
available on computers in the Chemistry Commons (Room 221, SB1). Although it is highly recommended that you learn to use the program, you
are not required to do so at this point.
Scientific articles and reports are typically written in the passive voice, and personal pronouns such as "I" or "we" are generally avoided. (It
creates a greater sense of objectivity.) For example, write: "The synthesis was carried out . . ." instead of "I carried out the synthesis . . ."; and "To
a 25-mL round-bottle flask was added . . . and . . . " instead of "I added . . . and . . . to 25-mL round-bottle flask".
Journal Format
Your lab reports should include the following sections, in this order. Also include the heading for each section except title and author
Title
Author
Abstract
Introduction
Experimental Section
Results and Discussion
Conclusions
References
Title. Keep it short, appropriately succinct, and descriptive. Often, the title as given in the lab manual is adequate.
Author. Your name and affiliation. For example:
Joleen Smythe
CH 337M Laboratory, Section 026
Department of Chemistry
Portland State University, Portland OR
Abstract. This is a very concise summary of the contents of the article. Although it appears at the beginning of the report, write this section last.
Tell the story of the experiment in the other sections, then summarize it here. Nothing new should be in the abstract, but all of the important
points of the paper should be briefly mentioned. The lengths of abstracts vary in the literature; make yours brief for your report. In general,
abstracts should state what was done, how it was done and the important results.
An abstract collects the important findings of a paper so that investigators skimming the abstract can decide if they want to invest time into
reading the rest of the paper. Many abstracts are now entered into computer databases, and an important new function of abstracts is to provide
keywords for computer searches.
Introduction. This section provides the setting for the information to be presented, discussing why the experiment was performed, what its
intention is. Often the historical background of the experiment is mentioned with a reference to the literature the work is based on. For your report
this should be a short section, a few sentences. Avoid extensive theoretical discussions.
Experimental Section. The Experimental Section is one of the most important segments in a laboratory or research report. It should provide a
description of experimental steps complete enough that someone with the some chemistry could run the experiment. The trick here is to be
concise; only give enough detail to duplicate the experiment, not a point-by-point description of every motion you made in the lab.
This section should begin with "Materials and Instruments". Identify the materials used, give chemical names of all compounds and the chemical
formula of compounds that are new or uncommon. List the instrument used to collect the specific data. For example: "The infrared spectra were
recorded on a FTIR spectrometer (Perkin Elmer Model xxx)".
DO NOT copy the entire experimental procedure in the lab manual; instead, summarize each step using your own words. Be sure to indicate any
changes you made from the procedure. Describe or draw the apparatus if it is not standard, or if you have made any changes. For certain
procedures, you may reference a written procedure in the literature, FFF, or the lab manual. For example, "Gas chromatograms were recorded
following the standard procedure.3" (Be sure to list the literature in the References Section of your report.)
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Results and Discussion. In this section you present and interpret your data. Present the numbers that your work generates, such as theoretical
yield and percent yield. Use chemical structures, equations, graphs, tables or figures when they will clarify your results. Present the results of any
purity analysis, such as a melting point, GC or IR spectra. Be sure to discuss what your numbers, figures, spectra, and graphs mean. Discuss
problems you encountered during the experiment and if a procedure did not work as expected, describe what went wrong and discuss what might
have caused the unexpected result. You may suggest modifications that might improve purity or yield.
Keep this as concise as you can but include adequate text to lead the reader through the ideas comfortably. Use the accepted format for tables and
graphs as described in the later section.
Conclusion. Frequently the conclusions that are arrived at in the course of interpreting the data in the previous section are restated here more
concisely. Do not repeat discussion points or include irrelevant materials. Your conclusions should be based on the evidence presented.
References. Include a numbered list of references, in the order in which they appeared in the sections above, where they are cited as superscripts.
For example, "It was determined some time ago that some amines,7 thiol anions,8 and biphosphate ions8 stimulate the decomposition of
MNNG."
Follow JACS style to cite your references. The following examples illustrate the correct punctuation and format.
For books:
author(s)
title (italics)
edition, if appropriate
publisher
place of publication
year of publication
volume, chapter, and/or page, if appropriate
Refer to the following for punctuation:
(1) Cross, A. S.; Jones, R. A. Introduction to Practical Infra-Red Spectroscopy, 3rd ed.; Plenum: New York, 1969; Chapter 2.
If a book has an editor instead of an author, this is shown after the title. See for example:
(2) Mapping Strategies in Chemical Oceanography; Zirino, A., Ed.; American Chemical Society: Washington, DC, 1985; Vol. 209.
For journals:
author(s)
journal title (italics)
year (boldface)
volume (italics)
pages
See example citation 3 for a journal citation:
(3) Burgi, H.; Dunitz, J. D.; Shepter, E. J. Am. Chem. Soc. 1993, 95, 5065-5067.
Special publications such as in-house lab manuals are problematic. You may reference the PSU lab manual as shown in example citation 4:
(4) Organic Chemistry Laboratory I, CH 337M; Department of Chemistry, Portland State University: Portland OR; p 1.
Tables and Graphs. The table and graph reproduced below have typical, acceptable features.
Tables should 1) be numbered (using Roman numerals), 2) have a brief title and 3) have appropriate column headings. Lower case letters are used
as superscripts for any explanatory material shown at the bottom. Numerical superscripts may be given, if appropriate, to refer to items in the list
of references.
Table I. Calculated Magnetic and Electronic Parameters for Cr2(O2CR)4L2
compound
E, cm-1
A, MHz
ddia, ppm
R = Me, L = MeOHa
R = Me, L = H2Ob
1004
980
0.3363
0.3677
1.81
1.68
10
R = Me, L = pyb
R = Me, L = MeCNc
R = CF3, L = Et2Od
R = NEt2, L = NHEt2e CH3
R = NEt2, L = NHEt2e CH2
958
926
462
611
578
0.3236
0.3478
0.1728
0.0106
0.0388
1.72
1.69
54.60
2.74
0.78
aRecorded in CD3OD. bRecorded in acetone-d6. cRecorded in CD3CN. dRecorded in Et2O. eRecorded in toluene-d8.
Graphs should be labeled as a numbered figure (using Arabic numerals) and have a descriptive title. For your graphs, do the following:
1. Use Excel or other drawing program. Hand-drawn graphs are not acceptable.
2. Label the axes, including units.
3. Assign the independent variable (the one you control) to the x axis (horizontal) and the dependent variable (the system’s response) to the y
axis (vertical).
4. Select a range of numbers for an axis so that most of the graph paper is used. You don’t necessarily have to start at zero.
5. On each axis, place tic marks at regular intervals.
6. Plot each data point as a point enclosed in a circle.
7. When drawing a curve through the data points, decide on the best fitting smooth curve. Don’t simply connect the points with straight lines. In
selecting the best fit curve, you can use your understanding of how the system normally behaves.
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GUIDELINES FOR USE OF THE LAB REPORT FORM
Use of Printed Report Forms
The Lab Report Form, when filled out, will serve as the report for most experiments involving synthesis of compounds. No other report is
needed. It is to be turned in along with any assigned questions. Products should also be turned in for inspection.
Items 1-9 will serve as your pre-lab (your notebook will also need to be prepared as usual and they MUST be completed BEFORE coming to the
lab. Again, this will account for 10% of your report grade. Make a photocopy and submit it to your lab instructor before the lab starts. Your
instructor will check the form at the beginning of the period and initial it. Any errors and incomplete parts need to be corrected and finished
before you could start the experiment.
The report form is designed to foster an understanding of what is occurring at every stage in an experimental procedure, what each operation is
designed to accomplish, and what each chemical does. The guidelines below should give you a cleaner idea of what is appropriate in each section.
Again, the Lab Report Form should be used for most experiments involving the synthesis of compounds. Refer to the Schedule if you are not
certain whether you need a Report Form for a particular lab. The Lab Report Form can be down-loaded from blackboard.
Guideline for Lab Report Form
Name ___________________________________ Instructor’s initials _________________________
1. Name of Experiment:
2. Purpose of Experiment:
Generally the purpose is to prepare a specific compound and/or to illustrate a particular reaction type or technique.
3. Balanced Equation(s) for Main Reaction(s) (Show Catalysts):
The equations give in the lab text and in "handout" procedures will often not be balanced. You must balance the equation. Sometimes more
than one equation is needed. A balanced equation is needed to determine the limiting reagent(s) and theoretical yield.
4. Summary of Experimental Procedure:
a. Reaction Stage:
Most organic preparations can be subdivided into the three stages indicated here. "Reaction" describes the
chemicals, operations, and conditions (time and temperature) necessary to convert starting materials to
products. Omit quantities; we are trying to get you to think in generalities about what is being done.
b. Work-up (isolation of crude product):
At the end of the reaction stage, the product is present but mixed with solvent, by-products, unreacted starting
materials, etc. "Work-up" is the term used to denote the process of separating the product from the other
materials and isolating the product in a still-impure form (called the crude product) suitable for the final
purification stage.
c. Final Purification:
The final purification of solids is generally by recrystallization. Liquids are generally purified by distillation.
Sometimes a product of sufficient purity is obtained directly from work-up and no final purification is carried
out.
5. Sketch of Apparatus:
Not necessary for simple operations or simple equipment, such as separatory funnels, Erlenmeyer flasks, etc.
6. Reasonable Stopping Places and Estimated Time Needed to Reach Stopping Places:
To help you plan your work, indicate at which point(s) it would be safe to interrupt the preparation and resume at a later date. The product
should not be left in contact with strong acids or bases for a long time. Generally, it is safe to stop after work-up, while over a drying agent, and
sometimes at other points. Estimate the time you will need to reach the safe stopping points.
7. (8 pts) List all chemicals used and their purpose or function:
reactants
12
catalysis
reaction solvents
drying agents
extraction solvents
other
(specify)
The reagents which are chemically transformed as shown on the left in the balanced equation(s) are called "reactants". The
catalyst is involved in the reaction mechanism, but it is not chemically transformed and is still present at the end of the reaction
stage.
Write the name or structure of each reagent listed on the left column. In addition, specify the purpose or function of each
reagent.
8. Data: (List only for the reactants given in the balanced equation(s) of Item 3; omit catalysts)
reactants
molecular weight
density (for liquids)
grams used
moles used
This section is for organizing information to determine limiting reagent(s) and to calculate the theoretical yield. Pay attention to significant
figures. They sometimes can make a difference in the theoretical yield calculations. Density is found in the handbook and is needed only for
liquids, which are often measured by volume. Densities of common inorganic acids are listed in Appendix of the laboratory manual.
product
molecular weight
theoretical yield (moles)
theoretical yield (grams)
Calculate the theoretical yield, in both moles and grams from the data computed above.
9. Limiting Reactant:
The reactant which, based on the balanced equation(s) and the above data, governs the maximum amount of product which theoretically could be
formed.
All of the above items must be completed and the completed preliminary laboratory report must be submitted to the laboratory instructor for
initialing before starting the experiment. Your lab notebook must still be prepared for the lab. The following items are to be completed after the
experiment is finished and then the complete report is given to the instructor.
10. Actual Yield (in grams):
Turn in product(s) which were obtained. Sample container should be labeled.
11. Percent Yield:
Report yields to two significant figures (e.g., 64% rather than 64.3%). Reaction yields are not that reproducible,
and the third figure is meaningless.
12. a. Observed Physical Properties of Product (e.g., color, mp, bp, etc):
Report the values you found for your product (usually mp range of solids or bp range of liquids is determined).
b. Literature Values for Physical Properties:
Compare the values you found with the values from the handbook for the same physical properties. Do not
report other physical properties. Give the reference to your source of literature values.
13. Tests (report results of chemical or physical (VPC, IR) tests run on product):
If qualitative chemical or physical (infrared, vapor phase chromatography) tests were run, report results. For
VPC, report the conditions used for the analysis, apparent percent purity of product, etc. Attach a copy of
spectra and chromatograms to this report. (The originals stay in the notebook.)
14. Comments and Conclusions:
Compare results of your experiments with those expected. Account for the difference-unusually good technique,
spilled product mixtures, variation in procedure followed, etc. Indicate how procedure might be improved.
15. Answers to Questions (attach to report):
13
Experiment 1: The Purity and Purification of Solids -- Part I - Melting Points
Introduction
In most applications, a chemist needs to be sure that compounds being worked with are reasonably pure. If they aren't, they have to be purified by
one of many processes, some you will be learning in future labs. Different methods to establish purity are used for solids and for liquids. In these
experiments, the methods applicable to solids will be introduced.
Reference
The discussion and comments given below are not intended to be complete, but supplement those given in the lab text, FFF, p. 39-46.
Melting Point as a Criterion of Purity
Observing the melting temperature of a substance is a very old technique that can be performed with rudimentary equipment, but it has not been
supplanted as a rapid, generally effective way to determine whether or not a given substance is pure. The melting point is routinely determined
when an organic solid is prepared. The utility of the technique depends on the fact that a pure substance melts sharply, (generally less than a
1-2˚C melting range) whereas an impure substance has a broad melting range (that is, the temperature will rise several or many degrees from the
onset of melting until the last crystal of solid melts). A 1-2˚C melting range is considered "satisfactory" for most purposes, but occasionally a
higher degree of purity is required; for example, if a sample is to be submitted for elemental analysis (%C, %H, etc.).
One additionally useful feature of the observed melting point that the method can be used to identify an unknown substance by comparing
melting points with a substance whose melting point is already reported in the chemical literature. If a sample is believed to be the known
compound, the two melting points must of course agree. Another useful aspect of melting point is that one can get an idea of the purity of a
sample, an impure substance will give a melting point that is depressed in comparison with the pure substance; its melting range will not only be
broader, but the top of the melting range will be lower than the pure substance.
Mixture Melting Points
A procedure known as mixture melting point can be carried out to verify the identity of a substance whose melting point has been determined,
and which is suspected of being a known compound whose melting point is already reported. If a sample of the pure comparison substance is
available, it is intimately mixed with roughly an equal amount of the unknown, and the melting point of the mixture is determined. If the two
substances are the same, the mixture melting point will be sharp and un-depressed; if they are different, the mixture is of course highly impure,
and its melting point will be broad and depressed.
Melting Point Techniques - The Capillary Method
The most common method for determining melting points is the capillary method, in which a small amount of sample is introduced into a glass
capillary that is gradually heated by means of an oil bath or electrically heated metal block. In this experiment you will use an electrically heated
Mel-Temp device (see FFF p. 42, Fig. 2.2 left). In subsequent experiments in which melting points are determined as part of the procedure, the
Mel-Temp will generally be used (there are enough of them if everyone isn't doing melting points at the same time). Become familiar with the
Mel-Temp device by carrying out your melting point runs with this equipment.
14
Observing the Melting Behavior
The diagram below illustrates the changes that can be observed in the capillary tube as the sample is heated.
"A" represents the sample appearance before melting.
"B" represents an initial softening and shrinking of the sample (called "sintering") which is sometimes observed before the onset of melting. It is
apparently due to a melting/resolidification process that takes place on the surface of the crystals, at a temperature below that of the bulk melting
point. The temperature at which the sample first begins to shrink is sometimes reported as the sintering point, but typically it is not recorded.
"C" represents the actual onset of melting. The lowest temperature at which the first liquid is visible in the capillary is defined as the lower bound
of the melting range.
"D" represents the point at which the last crystal in the tube melts to give a completely transparent liquid.
heat
heat
heat
o
o
o
A
B
C
D
The temperature at C and D are both reported; for example, mp 86.0-87.5˚C. This is the melting point range. Sometimes a sample melts so
sharply that it is impossible to specify the range; a single value is then reported. The first decimal place in the thermometer readings is usually
recorded, but it should be kept in mind that variations in heating rate, sample preparation, capillary thickness, acuteness of vision, etc., could
change this by a few tenths of a degree. For accurate work, the thermometer is calibrated using a series of compounds of known melting point,
and subsequent values are reported as "corrected"; e.g., mp 86.4˚C (corr.)
There are some compounds that are heat sensitive and decompose either before or at the melting point. This, of course, creates impurities that
then affect further melting behavior. If a sample becomes severely discolored or evolves a gas, decomposition is taking place. The closest
approximation to the melting point is then reported with the letter d or the abbreviation dec. following the temperature; e.g., mp 127-131˚C (dec.).
Many substances can exist in more than one crystalline form, and when such a substance is heated one crystalline form may be converted to
another with or without melting of the sample. Such changes are called phase transitions and should be reported when observed. For example, mp
40-41˚, re-solidify 43-44˚, re-melt 76-77˚C.
Note that the abbreviation mp is used even if there is a range in the melting temperature. This is grammatically incorrect, but it would be
awkward to have separate notation.
15
Experimental Procedure
This experiment consists of two unrelated parts. Details of sample preparation, filling capillaries, rate of heating, etc. are given in FFF, p. 43-45.
Use the 200 setting on the digital thermometers with the Mel-Temps.
Filling the melting-point capillary
1. Push the open end of a capillary
onto a small mound of sample in
a spotplate depression. (A).
2. Invert the capillary (B), grasp it at
the bottom, and tap it vigourously on
the desk top to bring the sample to
the bottom (C).
3. Repeat until the hieght of the
sample is about 2 mm (about
what you see in D).
4. Drop the loaded capillary two
or three times into a long glass tube
held vertically against the desk top.
A
B
C
D
Part A - Melting Point Diagram for Naphthalene and Biphenyl
In Part A, the melting points of pure naphthalene, pure biphenyl and mixtures of the two will be determined, and the resultant data will later be
plotted in the form of a melting point diagram. The diagram will then be used to determine (approximately, at best) the composition and
temperature at the eutectic point for these mixtures. The following samples are to be used (weight percent is given):
100 % Naphthalene (N)
100 % Biphenyl (B)
10 % N, 90 % B
30 % N, 70 % B
50 % N, 50 % B
70 % N, 30 % B
90 % N, 10 % B
NOTE: Very little amount of sample is needed for melting point experiments. Take only enough for one or two determinations (use your spot
plate). Otherwise, you aggravate the waste disposal problem.
Work with either pure naphthalene or pure biphenyl, as assigned by instructor, until you feel comfortable with the technique. Your values should
be reasonably close to those indicated on the sample bottles. Note, however, that your thermometers are not calibrated and could easily differ by a
degree or so from indicated values. Once you are familiar with the technique, only one mp determination is carried out for each sample.
Determine the melting point of one of the mixtures as assigned by your instructor. Report the value for this mixture as well as your pure
compound to the instructor before you leave the lab. The values from the class should be posted on the board which you will need to copy. The
combined values will be averaged for constructing the melting-point diagram which will be part of the report for this experiment. Guidelines for
writing the report are given at the end of the experiment.
Part B - Identification of an Unknown Using Mixture Melting Points
For this section of the experiment, the instructor will issue an unknown that is one of several known samples available in the lab. Determine the
melting point of the unknown then, carry out mixture melting point determinations using two known samples that have melting points closest to
16
that of the unknown. Identify the unknown on the basis of the presence or absence of a melting-point depression in the mixed samples. For
mixture melting points, fine pulverization with intimate mixing of the samples is essential. Use about equal amounts of both components, and
grind them using the spot plate. (Note that you will be carrying out a total of three mp determinations for this part of the experiment; the
unknown, then the unknown mixed individually with each of two known samples.)
Waste Disposal. Place your used capillaries and any unused solids in the waste jar provided.
Further Aspects of the Melting Behavior of Mixtures of Organic Solids
Figure 1 illustrates the usual way of graphically representing the equilibrium melting behavior of a typical system of two organic compounds. It is
referred to as a melting diagram for a binary (two-component) mixture. The diagram reproduced is actually that for mixtures of 1-naphthol and
naphthalene. The ordinate gives the temperature and the abscissa the relative molar amounts of each component. The far left of the abscissa
corresponds to pure 1-naphthol (mp 96 ˚C) and the far right to pure naphthalene (mp 80 ˚C).
The amount is usually given in mole fraction or mole percent. For a two component (A and B) mixture, the mole fraction of A is given by the
following formula:
moles A
XA = moles A + moles B
At any temperature and composition represented by a point below the dashed horizontal line, the sample is entirely solid. Along the solid curved
line, both solid and liquid are present. Above the curved line, only liquid is present.
H
OH
H
H
H
H 100
H
Point A
H
H
1-Naphthol
Point D
90
ºC
H
H
H
H
Point B
H
H
H
Naphthalene
80
70
Point C
(eutectic point)
60
0
10
100%
1-Naphthol
20
30
40
50
60
70
80
Composition
(mole percent naphthalene
)
90 100
100%
Naphthalene
Figure 1
The diagram is probably best understood by considering the behavior of an actual mixture of specific composition, for example 30 mole percent
of naphthalene. At 95 ˚C the sample is entirely liquid (Point A). If the sample is cooled (represented by the dotted line), no other change takes
place until Point B is reached. At that temperature, 83 ˚C, pure 1-naphthol begins to crystallize. If the temperature were then held constant, the
sample would remain unchanged, a small amount of crystalline 1-naphthol in the main body of liquid. The composition of the liquid has changed
slightly because some 1-naphthol has crystallized. If the temperature is lowered, more solid 2-naphthol separates, leaving the remaining liquid
richer and richer in naphthalene until the liquid has the composition represented by Point C. At that point, both 1-naphthol and naphthalene
crystallize together until no more liquid remains.
Point C is called the eutectic point (Greek: easy melting). Material having the composition represented by the eutectic point is called a eutectic
mixture, and the temperature the eutectic temperature.
17
If the original mixture had a composition somewhere to the right of the eutectic point (say 80 mole percent naphthalene) a liquid sample at 85 ˚C
(Point D) would first deposit crystals of naphthalene as it cooled. At the eutectic point, both 1-naphthol and naphthalene would again crystallize
together.
Consider now what happens in the reverse situation; that is, when a solid containing 30 mole percent naphthalene and 70 mole percent 1-naphthol
is heated. When the temperature reaches 61˚ (the eutectic temperature), both 1-naphthol and naphthalene melt together. The temperature of the
sample remains constant until no more solid naphthalene is present. The resulting liquid has the composition of the eutectic mixture. As heating is
continued, the temperature rises, and the remaining solid (which is entirely 1-naphthol) melts. As it does so, the liquid is enriched in 1-naphthol.
The composition of the liquid follows the curved line until Point B is reached, where the last crystal of 1-naphthol melts to give a homogeneous
liquid mixture. Continued heating simply raises the temperature of the liquid without causing any further phase changes (until the sample begins
to boil, that is.)
The above discussion elaborates on some of the generalizations typically made regarding melting behavior and the purity of solids; namely, that
an impure compound melts over a temperature range rather than sharply at a single temperature, and that the more nearly pure the sample the
higher its melting point. For example, if the original mixture of solids contained only 5 mole percent of naphthalene, the last crystal of 1-naphthol
would disappear at 94˚ rather than 83˚.
Another generalization usually made about melting points is that the more nearly pure a solid, the narrower will be the range in its melting point.
This is not true in principle; it only appears that way in practice. In either of the above two cases, melting would have begun at 61 oC, the eutectic
temperature; the sample containing only 5 mole percent of naphthalene actually has a broader range in its melting point. But in a typical
capillary-tube melting point determination of the more nearly pure of the above two samples, it would be impossible to see the small amount of
liquid which appeared at 61oC and melting will seem to have begun at some higher temperature. About 20% of the bulk of a sample in a melting
point capillary tube must usually melt before a separate liquid phase is visible.
ºC
|
|
|
|
|
0% B
100% A
|
|
|
|
|
Composition
(mole % B)
|
100% B
0% A
Figure 2
The composition of the eutectic mixture is characteristic of the two components in the mixture and can't be predicted. Figure 2 shows a diagram
for a system in which the eutectic mixture contains largely one of the two components.
The solid which separates from a liquid at the eutectic point is usually a mixture of separate crystals of each pure component. The individual
crystals can be seen under a microscope. If by some coincidence an impure sample happens to have precisely the eutectic composition, the entire
sample would melt sharply at the eutectic temperature giving the appearance, on the basis of the usual generalizations, of being a pure compound.
Additional Comments
Melting diagrams are a reasonable approximation to many situations actually encountered in the laboratory, but there are differences that should
be pointed out.
The diagrams represent equilibrium behavior rarely obtained in practice. Phase transitions take time and don't always take place spontaneously.
Many liquids can be cooled far below their freezing temperatures without crystallization taking place. This can happen if a suitable nucleus for
crystal formation is not available in the sample. The supercooled liquid can remain indefinitely in the metastable state if it is left undisturbed.
Organic solids encountered in actual lab work often contain more than one impurity, so that a binary melting diagram no longer applies. The
diagrams reproduced above are for mixtures of two components whose liquid phases are miscible in all proportions. The diagram doesn't apply if
two separate liquid phases are present. Also, the diagram represents mixtures whose components crystallize from the liquid as separate, pure
crystals. If a binary mixture can form crystals containing both components in the lattice, a different behavior is observed.
18
With reference to mixture melting point determinations, it is important that both components in the sample prepared be finely pulverized and
intimately mixed. Unless there is a large area of surface exposed and the crystals are in close contact, the expected melting at the eutectic
temperature won't be observed.
Guidelines for the Melting Points Report
Follow the general guidelines as described in "Format for Organic Chemistry Lab Reports". Note the following as they relate to this experiment:
1. Assume that your reader (in reality, this might be a lab supervisor or a research director) has some familiarity with the technique, so it isn't
necessary to give a lengthy introduction. It is also inappropriate to describe details such as how you filled the capillaries, attached them to the
thermometer, etc. However, you should make reference to the lab manual and to FFF as the source of the procedures you carried out.
2. The "Results and Discussion" part of your report should be divided clearly into two sections, corresponding to Part A and Part B as given in the
experimental procedures. Note: these should not be labeled as Part A and Part B in your report but should be given descriptive headings. Present
the data for Part A and Part B in the form of a table in each part. Indicate the apparatus used for each mp determination, and distinguish between
your values and class averages. Include "literature" values where appropriate (include the reference to the literature values). Use the customary
abbreviation, mp, for melting points. Also, graph the data from Part A (see below), discuss the general appearance of the graph, and indicate the
probable values of the eutectic composition and eutectic temperature.
Note:"Chemical literature" refers to the body of chemical information recorded in journals, handbooks, and similar sources. For these labs, the
Merck Index and the CRC Handbook of Chemistry and Physics are the most useful. The Merck Index is easiest to use because the listings are
alphabetical and there is a cross index. To use the CRC Handbook (section on physical properties of organic compounds), you need to find out
how the functional group is organized in the table; this information is given at the beginning under “Explanation of Table”.
A copy of both of these sources is in the lab, and can also be found in the reference section of the science library (5th floor). A more extensive
tabulation of physical properties is also given in the following available references: 1) Dictionary of Organic Compounds (a multi-volume series).
2) Physical Properties of Organic Compounds (CD ROM).
3. Plot the melting-point diagram for the mixtures of naphthalene and biphenyl as shown on p. 41 (FFF), showing the actual data points (which
are ranges). You can plot the range as a vertical bar or line, but draw the best-fit curve through the maximum of each range. (See the discussion
section to convince yourself that this is appropriate.) The mixtures used in the lab were based on weight percent; these should be converted to
mole percent (naphthalene has MW = 128, biphenyl has MW = 154). To convert weight percent to mole percent: 1) Assume you have 100g of
mixture, 2) note the grams of each component in 100g of the mixture, 3) convert grams of each component into moles of each component, 4) find
the total moles and 5) determine the mole percent of each.
Note: You are going to have to use your imagination in coming up with the best-fit curve for your melting-point diagram, because you have very
few data points and they are somewhat unreliable and scattered. Imagine what the curve would look like if you had 100 data points (different
compositions), each the average of three determinations! The curve would look like that shown in FFF. The two ends of the curve would slope
smoothly downward to meet at some eutectic point. The eutectic composition and the eutectic temperature should be noted and reported in the
main body of the report. You cannot assume that the lowest melting point among your data points represents the eutectic mixture.
Questions
Attach to your report answers to the following questions. These are not properly part of the report, so they should be attached at the end, after
everything else.
1. a)
A broad melting range is usually indicative of an impure substance. Give an example of a situation in
which a pure substance could give a broad melting range (assume that the mp technique and equipment are
OK).
b) A sharp melting range is usually indicative of a pure substance. Give an example of a situation in which an
impure substance melts sharply. (3 pts for a and b)
2. Define eutectic mixture without using the work “eutectic” as part of the definition. (2 pts)
What effect would the following impurities have on the melting behavior (range, appearance of the melt, etc.)
of benzoic acid and why? (4 pts)
a. Fragments of crushed glass
b. Residual recrystallization solvent
c. Filter paper fibers
d. Particles of ceiling plaster that fell into the sample
4. FFF, 2.8, p 48. (2 pts)
5. A number of students in a lab observed a considerably higher mp for benzoic acid than the known value (121-
19
126 oC as opposed to the known 122 oC). When the determination was repeated using a sample which had been
more finely pulverized, a more reasonable value was obtained. Explain. (2 pts)
Suppose you isolate an unknown compound, which you observe to have mp 188 oC. Subsequent investigation
leads you to suspect that your unknown is either hippuric acid or succinic acid, both of which melt at 188 oC.
Assuming that pure samples of both are available from the stockroom, outline a procedure, based on melting
points, which you could use to verify the identity of your unknown. State what you could expect to observe and
the conclusion you could make from your observations. (2 pts)
20
Experiment 2: The Purity and Purification of Solids -- Part II Recrystallization
Introduction
In Part I of this series of experiments, you encountered the use of the melting behavior as a criterion of purity for a solid. In this section, you will
deal with an important technique for handling solids that aren't pure. The technique is called recrystallization (some chemists call it
crystallization), and it is the most widely used routine procedure for purifying solids. You will use it again and again throughout the laboratory
course. The goal for this experiment is to learn the technique so that you can use it whenever you need it without having a specific set of
directions. Later procedures will direct you simply to "recrystallize the impure sample from ethanol" (for example), and you are expected to know
what this means and how to do it with no further instructions. An effective recrystallization is one in which 1) a high degree of purity is achieved
as well as 2) a maximum recovery of sample.
Recrystallization has a basic limitation in that it is generally effective only for substances that are already substantially enriched in one
component. A mixture containing several or many components in similar proportions can't usually be purified by recrystallization, but needs to be
subjected to other separation procedures before recrystallization can be carried out on the desired component(s). The most powerful and widely
applicable separation techniques for complex mixtures are variations of a method called chromatography. You will be introduced to these
techniques later in the course.
In this experiment, you will gain familiarity with the basic recrystallization technique using standard apparatus and a specified procedure.
Reference
More complete background information and general procedures are given in FFF (Technique 1). Stepwise directions for standard apparatus are
also given in the Appendix B.
Solubility
The effectiveness of recrystallization depends on differences in solubility at different temperatures, so it is important to have a clear idea of what
is meant by solubility. It is one of the physical properties of a substance, and refers to the tendency of that substance to dissolve in a liquid. It is of
course different for different liquids. This tendency can be described in a qualitative way by saying that the substance is highly soluble, highly
insoluble, or anything in between. The liquid in these circumstances is called the solvent, and the dissolved substance is the solute.
The solubility of a substance can also be expressed quantitatively by measuring the maximum amount that will dissolve in a given volume of
solvent at a particular temperature. The units are typically grams of solute per 100 mL of the solvent used (g/100 mL). A solution that contains
this maximum amount is said to be saturated. Most, but not all, substances are more soluble at higher temperatures. For example, the water
solubility of the adipic acid used in this experiment is 1.4 g/100 mL at room temperature, and 160 g/100 mL at the boiling point of water.
Note that the word concentration does not have the same meaning as solubility, although the units can be similar. Concentration refers to the
actual amount of solute dissolved per unit volume of solvent, rather than the maximum amount that can be dissolved.
One can also describe the solubility of one liquid in another. The word miscible is often used to describe liquids that are mutually soluble, and
immiscible refers to those that aren’t. There are of course different degrees of miscibility and immiscibility.
21
The Basic Technique - Its Pristine Simplicity
Once you understand recrystallization, it will seem obvious what to do and why the methods work; but generations of organic lab instructors will
attest that the method is for some reason difficult for many to grasp. Therefore the simplest possible recrystallization procedure will be described
here before variations on the method are introduced. The method depends on the fact, mentioned above, that most solids are more soluble in a hot
solvent than they are in the same solvent at a lower temperature. The procedure is carried out as follows (see diagram):
add more
solvent in
small portions
add some
solvent to
make a slurry
heat to
boiling
o
x o o
o oo x oo x
contine heating
at the boiling point
o xo
o
o x ox
o
o ooo x oxx
impure solid
x = impurity
some solid still
undissolved in the
boiling solvent
ooo oo
oo
to
vacuum
x
x ox
suction filter,
wash crystals
solids dissolve
to form a hot
saturated solution
cool
pure
crystals
filtrate with
impurities; some
desired compound
remains dissolved
xo x x
o oooo ooo
pure crystals form,
impurities stay
dissolved
The solid sample is placed in a container (generally an Erlenmeyer flask (not a beaker) because a flask contains the hot vapors more effectively)
prevents surface cooling of the solution, and can be swirled without loss of sample. Enough unheated solvent is added to the solid to make a
slurry. The solvent need not be measured. At this stage you want less solvent than the amount eventually needed to dissolve the sample hot.
The slurry is placed on a hot plate or other heat source and the solvent is brought to a boil with more or less continuous swirling. Some solid will
remain undissolved because not enough solvent is present at this stage to dissolve the sample.
Small portions of solvent are then added to the hot mixture while the solution is swirled and kept boiling. Addition of small portions of solvent is
continued until all of the solid dissolves, giving a little time between each addition for the dissolution process to take place. This procedure
creates a hot saturated solution of the substance being recrystallized.
The solution is set aside to cool slowly. At the lower temperatures, the solvent can no longer dissolve as much sample, so the excess separates as
crystals. This is the stage at which purification actually takes place. The impurities don't crystallize because they were present in relatively small
amount. The hot solution was not saturated with respect to impurities, and they also remain dissolved at lower temperatures because their
concentration in the solution is small.
The crystals of pure substance are separated from the supernatant solution (called the "mother liquor") by suction filtration, washed with a little
cold solvent then allowed to dry. The impurities are carried off in the mother liquor (filtrate).
To summarize, a hot saturated solution of the sample is prepared as indicated above by using only enough solvent to dissolve the sample at the
boiling point of the solution. When the solution is cooled, the pure major component separates as crystals because it is not as soluble at the lower
temperatures. Impurities don’t crystallize because they are present in relatively small amount to begin with, and remain soluble in the cold
solvent. (If the original sample contained a high proportion of impurities, the chances are slim that the impurities would stay soluble in the cold
solvent.)
22
Note that a small amount of the desired sample will still be soluble in the cold solvent, so that sample loss is inevitable even with the best
recrystallization technique. To minimize this loss, it is common to chill the recrystallized mixture in ice before filtration.
Variations of the Method
Sometimes the overall recrystallization process involves additional steps, as follows, but these are carried out only if necessary, because they
usually lead to further material loss.
Insoluble Impurities If the sample contains debris or other extremely insoluble material (sometimes called "mechanical impurities") this has to be
separated from the hot solution or it will re-contaminate the recrystallized sample. Insoluble impurities are removed by filtering the hot solution
through filter paper creased in such a way (fluted) that contact between the filter paper and the walls of the funnel is minimized so that filtration
occurs much more rapidly. (Rapid filtration is essential, to prevent premature crystallization in the funnel.) The filtered solution is then set aside
to cool and form crystals as before. The decision about whether or not to carry out a hot filtration is made after inspecting the hot solution to see
if insoluble material is actually present.
Colored Impurities Trace amount of highly colored substances, in particular a ubiquitous yellow-brown, are frequently observed to contaminate
organic substances. These impurities are not usually removed in an ordinary recrystallization, but they can often be removed by adding activated
charcoal. (Activation refers to the process of heating the charcoal strongly to drive off surface-adsorbed material.) The colored impurities have a
strong tendency to become adsorbed on the activated charcoal, and they are removed with the charcoal during the hot filtration. (Some of the
desired component may also be adsorbed on the charcoal.) The amount of charcoal used is about 1–5% of the weight of the sample being
recrystallized.
Experimental Procedure -- Recrystallization of Adipic Acid from Water
In this experiment, you will crystallize impure adipic acid synthesized from the oxidation of cyclohexene. Note that in general, you would use an
unspecified minimal amount of solvent, as described in the introduction, for recrystallization. Too much solvent will reduce the weight of sample
recovered. On the other hand, too little solvent will also cause problems of premature crystallization during the hot filtration step.
The goal in a recrystallization is to obtain the maximum recovery and the highest purity.
The instructor will demonstrate the following:
1. Use of the balances.
2. Construction of a weighing boat. This is a piece of weighing paper folded in such a way as to create a rigid "boat"
that makes it easier to weigh, transport and transfer solids.
3. Fluted filter paper. (Successful hot filtration depends on a rigid, well-creased fluted filter paper, which promotes
rapid filtration.)
4. "Holding strap" for handling hot Erlenmeyer flasks.
5. Bending the spatula for use in suction filtration (to keep the "filter cake" flat).
A. Without Charcoal
Recrystallize 1.0 g of impure adipic acid from water, starting with step 1 of the "General Procedure for Recrystallization" (see Appendix B). Use
a 50-mL Erlenmeyer flask, and a total of 8 mL of solvent (end of step 5). Omit step 6 (charcoal) and proceed to step 7 (hot filtration). Continue as
indicated. Use no more than a total of 2 mL of additional solvent for "washing" in the hot filtration and final suction filtration. Begin the second
recrystallization while the first sample is cooling.
The suction filtration (step 12) is carried out with the small Hirsch funnel fitted with 1.5-cm filter paper.
B. With Charcoal
Recrystallize a second 1.0 g sample of impure adipic acid as above, this time using pelletized decolorizing charcoal (step 6). (Add more if you
can detect any color in the hot solution after the charcoal has been added.) (Use about 0.1 g of charcoal. Pelletized charcoal works slowly, so give
it time.)
Weigh both recrystallized samples when they are dry (allow at least 24 hr), calculate their percent recovery and determine their melting points
simultaneously along with a sample of the original impure adipic acid.
Clean-up and Waste Disposal
Clean equipment with lab detergent and water. Back-flush the Hirsch funnel with a little solvent to remove anything in the frit.
23
Guidelines for the Recrystallization Report
For general considerations, see the previous guidelines for journal format. There is no need to include details of the steps if appropriate reference
is made to the lab text(s), but be specific about compounds and weights used, solvents, any change in procedures, difficulties, etc.
The data can be organized in brief tables (one for each recrystallization) that include original sample weights, purified sample weights, percent
recovery, mp of purified and impure, sample appearance before and after. Include literature values of mp if available.
Questions
Prepare a pictorial representation of the adipic acid recrystallization, in which charcoal was used, illustrating all
of the steps (including charcoal treatment) and showing clearly what happens to all of the molecules at every
stage of the recrystallization. Start with a picture of the impure dry solid and end with a picture of the pure dry
solid. Symbolize the molecules as A for adipic acid, B for a soluble organic impurity, C for traces of soluble
highly colored impurity, I for insoluble debris. (5 pts)
Suggest a reason why recrystallization is generally not an effective purification method for a solid which is
highly impure; for example 40-50% impurity. (2 pts)
3. Usually the recrystallization mixture is chilled in ice just before suction filtration. (2 pts)
a. Explain why this might be advantageous.
b. Under what circumstances might it be detrimental? Explain.
4. FFF, 1.2 (2 pts)
5. FFF, 1.8 (2 pts)
6. Assume that you are an experienced organic chemist with perfect recrystallization technique. You recrystallize
10 g of impure naphthalene from ethanol in Portland, OR (elevation; sea level) and recover 7.4 g of pure
naphthalene. (2 pts)
a. If you were to repeat the experiment with 10 g of sample in Mexico City (elevation: 7700 ft) what change, if
any, would there be in the amount of boiling ethanol needed to dissolve the impure naphthalene? Explain
your answer clearly.
What change, if any, would there be in the amount of pure naphthalene recovered? Explain your answer clearly.
(Assume that the temperature of the mixture just before filtration is the same in both labs.)
24
Experiment 3: The Purity and Purification of Liquids
Part I - Boiling Points
Unfortunately, there is no simple measurement comparable to the melting point of a solid that can be used for checking the purity of liquids.
Anything liquid at room temperature can, of course, be solidified with sufficient cooling and the melting point of the resultant solid determined,
but this is a nuisance experimentally and seldom performed on a routine basis.
It is rather easy to stick a thermometer into a boiling liquid to measure its boiling point, but this gives no information at all about the purity of the
liquid. You would need to let the entire sample boil away, watching the temperature as it did so. A pure liquid would have a sharp or narrow
boiling range (e.g., bp 63.2 - 64.1˚C), and an impure liquid would have a large difference in temperature between the start and end of boiling
away (e.g., bp 91.0 - 108.2˚C).
A narrow boiling range, however, is not a reliable indicator of purity, because it is not uncommon to find mixtures that behave as if they were
pure liquids. For example, a mixture of two liquids that happened to have the same bp could boil at that same temperature (there is nothing
comparable to the depression one observes in the melting of solids). Also, there are numerous mixtures of liquids that behave like pure liquids
when boiled, even though the components of the mixture have different boiling points (these are called azeotropic mixtures).
In summary, boiling points (usually a range) do not have quite the same signification as melting points in establishing purity, and further testing
is often needed. In this lab you will encounter the most widely used supplementary method for testing liquid purity, an instrumental technique
called gas chromatography.
The most common method for separating and purifying liquids involves some form of distillation. (Gas chromatography can also be used,
especially on a micro scale.)
It is useful to be aware of the meanings of the following terms, as commonly used:
pressure - The pressure of a gas is a measure of its concentration and the force exerted by the gas on its container or surroundings. For a given
volume of gas, it is proportional to the moles of gas per unit volume as well as the absolute temperature, and is typically expressed in chemistry
as millimeters of mercury (torr). Concentration units such as moles per liter are usually not used for gases.
partial pressure - This term is used when dealing with mixtures of gases. The contribution of each gas to the total pressure is called the partial
pressure of that particular gas. The sum of the partial pressures necessarily equals the total pressure.
vapor pressure - This term is used when the gas and liquid phases of a substance are in contact. It can be confusing, because it can mean two
different things. In one usage, it simply refers to the pressure in the vapor phase that is in contact with a liquid that may or may not be pure. In the
more common usage, vapor pressure refers to a physical property of a pure liquid, namely the tendency of the liquid to become a gas.
Qualitatively, a liquid that has a relatively high vapor pressure is said to be relatively volatile. One with a low vapor pressure is said to be nonvolatile. The vapor pressure of a liquid can be determined quantitatively by letting the liquid equilibrate with its vapor, then measuring the
pressure of the gas in contact with the liquid at equilibrium. The vapor pressure of a liquid increases with temperature, but it is independent of the
amount of liquid present, and is also independent of the presence of other gases in the vapor phase. The vapor pressures at various temperatures
of many liquids (and solids) have been measured and tabulated.
vaporization - any conversion of a liquid (or solid) into a gas
condensation - any conversion of a gas into a liquid (or solid)
evaporation - more or less uncontrolled vaporization, in which the vapor is allowed to dissipate into the atmosphere
distillation - controlled vaporization of a liquid, in which the vapor is condensed by cooling and collected.
Part II - Simple Distillation
A distillation that involves only one vaporization/condensation stage is called a simple distillation. It is carried out in an apparatus known as a
still, an example of which is shown in FFF, p. 85. The diagram below actually shows the fractional distillation still which you will assemble using
the kit in your drawer. A simple distillation still would use the same components, except that the vertical fractionating column would not be used.
This experiment will not involve simple distillation, but it will be used in other experiments, typically to remove the bulk of the solvent from a
solution containing a solid or high boiling liquid. It can also be used to separate a mixture of two liquids if one liquid has a considerably lower
boiling point than the other. In other cases it is ineffective for purification, and the modification called fractional distillation is employed.
The components of the still below include a still pot (the vessel holding the original liquid), attached to a fractionating column connected to a still
head holding the thermometer, connected to a water-cooled condenser to which is connected a take-off adapter and a receiver to collect the
distilled liquid. (For this experiment, the receiver is a 10-mL graduated cylinder checked out from the stockroom.) The heat source is an
electrically heated device called a heating mantle. The amount of heating is controlled by plugging the heating mantle into a variable transformer.
25
Part III - Fractional Distillation
Fractional distillation refers to a purification process in which a mixture of liquids is distilled to yield separate fractions, each with a different
boiling point (range). Getting a fraction that is mainly one pure component requires that the distillation process involve multiple, successive
vaporization/condensation stages. The reason why the method works is rather straightforward, and depends on the following fact (true only for
so-called "ideal" mixtures of liquids): If you boil a mixture of two liquids, the vapor above the boiling liquid will contain a higher proportion of
the more volatile (lower boiling) component.
To understand how this can lead to purification of a sample, consider a 50/50 mixture of ethyl acetate (EA, bp 77 ˚C) and toluene (TOL, bp 110
˚C). This mixture boils at about 90 ˚C, and the vapor above it is about 70/30 EA/TOL; that is, the vapor is enriched in the lower boiling
component. If this vapor is condensed, removed and re-boiled (see the diagram), it would boil at about 83 ˚C and the vapor above it would be
further enriched, 88/12 EA/TOL. The 88/12 vapor, if condensed and re-boiled (about 80 ˚C), would yield a new vapor that was about 95% EA,
5% TOL. So the vapor becomes more nearly pure ethyl acetate with each successive vaporization/condensation stage.
70/30 EA/TOL
88/12 EA/TOL
95/5 EA/TOL
etc.
50/50 EA/TOL
(bp - 90º C)
70/30 EA/TOL
(bp - 83º C)
88/12 EA/TOL
(bp - 80º C)
The still used in a fractional distillation has a "fractionating column" placed between the still pot and the still head. This column provides the
surface on which the successive vaporization/condensation stages can occur. The column may contain a series of perforated plates or any other
surface that allows for upward flow of vapor, downward flow of condensate, and intimate contact between vapor and liquid. In this experiment,
you will carry out a fractional distillation using metal sponge as the column "packing".
Experimental Procedure - Fractional Distillation of a Mixture of Ethyl Acetate and Toluene
In this experiment, you will separate a mixture of ethyl acetate and toluene into three fractions as follows:
Fraction 1 - ethyl acetate fraction collect from the starting bp (first drop) to about 82 ºC.
Fraction 2 - a small intermediate fraction (about 82-106 ºC).
Fraction 3 - toluene fraction; collect from about 106 ºC to whatever temperature registers when you stop the distillation.
26
Don't take the above temperature ranges too literally. If the temperature stays fairly constant at 82 ºC, keep collecting fraction 1. Start collecting
fraction 2 when the rate of temperature rise indicates that the ethyl acetate is nearly gone.
In addition, you will record the temperature at regular intervals to obtain data for the distillation curve for this distillation (plot of head
temperature versus volume collected). The compositions of the three fractions will be determined later using two independent techniques:
refractive index and gas chromatography.
For this experiment, you will need to check out a 10-mL graduated cylinder.
Packing the Fractionating Column
Packing the fractionating column carefully is an important requirement for achieving a successful separation. When you are done, the packing
should be uniformly distributed inside the column, with no knots or empty spaces.
The air condenser in your kit packed with copper sponge will serve as the fractionating column. Select a piece of copper sponge that has been
used before as a packing; if none is available, use fresh copper sponge. Make sure the packing weighs about 1.2 grams. Roll it into a sausage
shape with your hands then, work out any knotted areas with your fingertips to ensure that the packing has a uniform density throughout. Insert
the packing into your larger condenser using a metal rod (the instructor will demonstrate). Take care not to break off the three nipples near one
end of the inside tube. (This renders the condenser totally useless.) Manipulate the packing with the rod so that it is uniformly dense the length of
the column. (Check this by holding the packed column up to the light.)
Setting Up the Still (see Figure 1, page 25)
Follow the steps below carefully. You will need all the glass wear shown in figure one. Assemble the still in the following sequence.
1. Place 18 mL of 50/50 (vol.) ethyl acetate/toluene in a 25-mL round-bottom flask along with 2-3 boiling chips. (A distilling pot should be no
more than about 2/3 full at the beginning, to allow room for frothing and boiling.) Connect the air condenser (with the Cu sponge loaded) to the
25-mL round bottom flask.
2. Use a 3 fingers clamp, clasp the air condenser to the ring stand. Make sure the round bottom flask is resting in the sand bath on top of a
hotplate. *Important* The air condenser needs to be as vertical as you can make it. If it isn't vertical, the condensate will drain to one side of
the column and will not contact the rising vapor as effectively.
3. Pre-assemble the top of the still (except for the thermometer) by attaching the still head (3-way connector), the water condenser (with tubes)
and drying tube. Use you second ring stand with a clamp to loosely clasp the water condenser . The secondary clamp is to relieve the stress at
the still head.
4. Mount the 10-mL graduated cylinder (receiver) using a small 3-finger clamp.
5. Once the distillation glass wear is assembled, mount the thermometer in the still head after first inserting it into the compression cap with an Oring. (Always use caution when inserting glass into the O-ring. Ask your TA if you are unsure of how to do this safely.) Adjust the height of the
thermometer so that the top of the mercury/alcohol bulb is a little below the side arm (see Figure 2 page 25). If the thermometer is too high, it will
give an incorrect reading because it won't be fully immersed in the hot vapor stream. Thermometers are usually added last at they are the easiest
to break and the hardest to clean up.
6. Before turning on the water, make sure the water outlet tube will drain into a sink. Gently turn on the cold water
and adjust the flow you see a gentle, steady stream from the hose. At this point have the TA check your setup.
Do not heat your system until it is checked!!
7. After being check, insulate the exposed glass between the top of the column packing up to and including the area around the side-arm. Use
cotton pads for this purpose. Also, insulate the upper exposed part of the still pot and where it connects to the fractionating column. Keep the
cotton pads out of the space between the pot and the heating surface. Much of the heating occurs by radiation rather than convection or thermal
contact. Be sure that the standard taper joints are seated properly so that they don't leak vapors. Turn the hot plate on to aproximatly 60-70%.
This may need to be adjusted according to individual hotplates.
Label the three screw-capped vials in preparation for receiving the fractions. Take a few minutes to bring your notebook "up to date". Describe
what you did up to now, and prepare the table headings for recording volumes and boiling points (see below; record the head temperature at the
1st drop (0 mL) and about every 1 mL thereafter). (You can work on your notebook while the apparatus is heating up.)
Carrying Out the Distillation
27
Remember that you have two goals in carrying out this distillation, and two sets of data to record. Be prepared for both. They are the following:
1. to separate the original mixture into a fraction containing mainly ethyl acetate, one containing mainly toluene, and a small intermediate
fraction containing both. Also the volume and the boiling range of each fraction are to be recorded. (This is a reflection of the purity of the
fraction.)
2. to monitor the volume of distillate, recording the head temperature about every 1 mL (the 1st drop is 0 mL). The data will be used later to
construct a distillation curve (graph) for the report. Recording the data is easiest if you start from zero again when you start collecting a new
fraction. You can add up the volume later.
Turn on the power to the hot plate and set it to 60-70%. When the liquid starts to boiling, You may have to adjust the heat vapor level continues
to rise. When the vapor level reaches the still head (the glass will get warm), be prepared for a rapid temperature rise and watch for the 1st drop of
distillate to enter the receiver. Note the temperature and record this as the bp at 0 ml.
Adjust the heat input (on the hot plate) so that the distillation rate is steady at about one drop every five seconds. (Faster distillation will give less
pure fractions.) Record the temperature every 1 mL. It will stay nearly constant or rise slowly as the lower boiling ethyl acetate distills. When the
head temperature indicates that most of the ethyl acetate has distilled (about 82˚C), rotate the curved take-off adapter so that the drops collect in
it, note the volume of the distillate in the graduated cylinder and quickly transfer this fraction to the storage vial. Remount the graduated cylinder
after rotating the curved adapter back to its original "drip" position, allowing the small amount of collected distillate to drain into the receiver.
In a similar fashion, collect fraction 2 above 82 ˚C to the point at which it starts to level out again meaning that mainly toluene is coming over
(about 106 ˚C). Then collect fraction 3 until the end of the distillation. Record the head temperature just before you stop. (Depending on your
thermometer, the temperature at which you change fractions may vary a little from those suggested.)
The intermediate fraction is normally quite small (this is desirable), whereas the first and last are large, containing mainly ethyl acetate or mainly
toluene. (Sometimes the head temperature drops when most of the ethyl acetate has distilled. You will need more heat to bring over the toluene.)
Stop the distillation by turning the hot plate off when about 3-4 mL remains in the pot. A general safety rule in distillation is "Don't Distill to
Dryness", because the excessive heat buildup when there is no more liquid to boil can cause the decomposition of pot residues, sometimes
leading to the explosion of unstable materials..
Check to see that your notebook record clearly identifies the volume and bp (range) of each fraction. Cap the labeled samples tightly and set the
three vials in a beaker for storage. (They tip easily. The ethyl acetate will evaporate if it is not tightly sealed). You will later analyze each fraction.
Clean-up and Waste Disposal
Flammable waste
Allow the still to cool somewhat, then disassemble it starting from the top (reverse order of assembly). Dry the pieces with compressed air; there
is no need to wash them with water, because they have contacted only clean, distilled, volatile liquid. Pull the metal sponge from the fractionating
column, and leave it on a sheet of paper in the hood for drying and then put it back on the shelves to re-use later.
Transfer the pot residue to the appropriate waste container. The distilling flask can be rinsed with a little acetone and dried in an air stream. If the
flask has a yellowish discoloration, blow the flask dry after draining the pot residue, then scrub out the discoloration with water and detergent.
Dry the flask with a little acetone followed by an air stream. After performing refractive index and GC analysis dispose of fractions in the
“Flammable Waste”
Refractive Index
Determine the refractive index of each of your three fractions as well as pure toluene, pure ethyl acetate and a 50/50 mixture. Directions for using
the refractometer are given in Appendix F. Record the temperature at which you made the measurements and correct all values to 20 ˚C.
Prepare a calibration curve for use in determining the composition of your fractions. This is a graph of refractive index versus composition
(weight percent). Plot the three points for pure ethyl acetate, pure toluene and the 50/50 mixture. Connect them by a straight line. This represents
a linear relationship between refractive index and composition, which is a pretty good assumption. (A more accurate calibration curve would be
based on refractive indexes measured for several known compositions.)
Determine the composition of each fraction using the calibration curve and the observed refractive index of the fraction.
Gas Chromatography
See Appendix G for the general directions. The TA will demonstrated by your. It is useful to find out from the person ahead of you on an
instrument what sample sizes worked best for each fraction.
28
Analyze each of your fractions (as well as either pure ethyl acetate or pure toluene) using the non-polar column. For each analysis, record the
following:
A. Sample identity; B. the column used (the non-polar column contains 20% DC-200 silicone oil as the stationary phase; the polar column
contains 20% Carbowax 20M); C. the Oven temperature; D. Attenuator setting; E. Sample size; F. Retention time for each peak (the chart
speed is 3 cm/minute.) G. Peak identities
Note that the column temperature, the attenuation and sample size may be different from the settings posted beside the instruments. You should
record the exact amount of sample you used, and the column temperature and attenuation you observed from the instrument when the sample was
taken.
Cut the chart paper into the individual chromatograms, and tape each, properly labeled, in your notebook. Make photocopies for the report.
Determine the approximate composition of each fraction (weight percent) by making use of the relative areas under the peaks as described in
FFF, p. 150. We will not use the internal standard method, but will assume that the detector responds identically to each component (this is not
necessarily true, but is a useful first approximation). Calculate the composition from peak areas. For example, to compute the weight percent of
toluene in the sample:
Wt % toluene = toluene peak area x 100% (toluene peak area + ethyl acetate peak area)
As part of your lab report, you will compare the compositions that you obtained by the two methods of analysis you used. (You have no way
knowing without further information that is more accurate.)
Guidelines for Distillation Report
Use the standard journal format. Include in appropriate sections such items as initial volumes of material, volumes of fractions, boiling range of
each fraction, illustrations of equipment, etc. The data should be tabulated, and presented graphically where appropriate. In this section, you
should discuss the graphs and the main conclusions derived from them and from the experiment as a whole. Include literature references. Discuss
the GC results and compare them with the results obtained by refractive index. Present this comparison in the form of a table. Include
photocopies of the properly labeled chromatograms, as well as conditions for the GC analyses.
Prepare two graphs, the distillation curve (head temperature versus volume distilled) as well as the calibration curve (refractive index versus
composition). See the guidelines for the accepted format for graphical presentation. On the graph showing your distillation curve, in another color
draw the "ideal" curve for this particular distillation (perfect separation into ethyl acetate and toluene). Also draw a curve for the expected pot
temperature during the distillation. (This won’t be obvious. Think about what is happening to the composition of the liquid in the pot as the
distillation progresses, and how this change will affect the boiling point of the liquid.)
Correct all refractive indexes to 20 ˚C before you use them further. The calibration curve should contain only two experimental points, the
refractive indexes of pure toluene and pure ethyl acetate. These are to be connected with a straight line. (It's an assumption that you can do this,
but it turns out to be OK.) Then read off the composition of each fraction from its refractive index.
Questions
Prepare an enlarged sketch of your distilling head, showing the proper placement of the thermometer bulb and also showing the flow
of vapor through the head. What would be the specific consequence of having the bulb too high? (2 pts)
2. FFF, 5.8. (4 pts)
The blah taste of many processed orange juice brands results in part from the fact that the juice must be pasteurized by heating.
Frozen orange juice concentrate, however, has not been pasteurized and yields a drink that has a much more fresh, natural flavor.
Explain how the water is removed from the juice in preparing the concentrate without subjecting it to flavor-destroying heat. (1 pts)
4. FFF, 7.2. (1 pt)
5. Prepare a rough sketch of a distilling apparatus that contains a fractionating column with two plates (assume
theoretical). Assume that a mixture of A and B corresponding to Fig. 5.5, p 83 in FFF is being fractionally
distilled. At one point in the distillation, the liquid in the pot contains 20 mole percent A and 80 percent B.
Refer to Fig. 5.5 in answering the following: (5 pts)
a. Indicate the composition of the liquid on each plate as well as the liquid collecting in the condenser.
b. Indicate the composition of the vapor above the pot liquid as well as above the liquid on each plate.
c. Indicate the temperature of the pot liquid as well as the vapor above it. Do the same for the liquid and vapor
at each plate.
6. a. What is meant by an azeotropic mixture ? (1 pt)
b. Would you expect that the components of an azeotropic mixture could be separated by a very efficient
fractionating column? Explain. (1 pt)
29
Experiment 4: Isolation of Eugenol from Cloves Using Steam Distillation
Introduction
This experiment is an example of "natural product" isolation. It will introduce four important techniques widely used in handling organic
compounds, listed below with page references to FFF.
Steam distillation
Solvent extraction (liquid/liquid extraction)
Drying organic liquids
Infrared spectroscopy
-
p. 107-110
p. 62-63
p. 71-74
p. 163-177
The volatile flavor and odor components in cloves ("oil of cloves") are to be isolated from whole crushed cloves by steam distillation. One of the
major components in oil of cloves, eugenol, will be separated from the others by a variation of solvent extraction called chemically active
extraction. This will eventually yield a pentane solution of eugenol, which will be freed of dispersed water using a solid drying agent. The
eugenol will be isolated by evaporating the pentane, and will be characterized by infrared spectroscopy.
Eugenol is used in perfumery; it is an insect attractant, and a moderately effective pain killer when rubbed into the gums.
Steam Distillation
Recall, from the previous distillation experiments, that a mixture of two liquids usually has a boiling point that is somewhere in between that of
the two pure liquids. You used a mixture of ethyl acetate (bp 77 ºC) and toluene (bp 110 ºC). (The exceptions are mixtures of liquids called
azeotropes).
In this experiment, you will distill a mixture of water (bp 100 ºC) and a liquid which contains mainly eugenol (bp 255 ºC) and other high boiling
compounds, and you will notice that the mixture distills at a temperature that is below 100 ºC. How is it possible that a mixture containing such a
high boiling compound can be distilled at such a low temperature? It is a consequence of the fact that water and oil of cloves are virtually
insoluble in each another. The details of what is going on are best explained with specific data. The explanation will use as an example a mixture
of water and bromobenzene. These two liquids are also insoluble in each other.
Pure water boils at 100 ˚C at atmospheric pressure (760 torr). Pure bromobenzene (C6H5Br) under the same conditions boils a lot higher, at 155
˚C. These two liquids are immiscible (insoluble in each other) because water is very polar and bromobenzene is non-polar, so, they form two
liquid layers. If you were to boil a mixture of these two immiscible liquids in any proportion, both water and bromobenzene would distill
together, and the mixture would boil at about 95 ˚C, lower than the boiling point of either of the pure liquids. How is this possible?
A liquid boils when the vapor pressure of the liquid equals the pressure above it. This is about 760 torr if the liquid is exposed to the atmosphere.
If the liquid is composed of two immiscible substances, each contributes molecules to the vapor independently of the presence of the other, as if
the other were not there. So, at 95 ˚C, water is known to have a vapor pressure of 634 torr and bromobenzene a vapor pressure of only 126 torr,
but their vapors combine to give a total of 760 torr. Therefore the sample boils at 95 ˚C.
In other words,
total pressure of the vapor = pressure contributed by water + pressure contributed by bromobenzene
= partial pressure of H2O + partial pressure of C6H5Br
= vapor pressure of pure H2O + vapor pressure of pure C6H5Br
Expressed symbolically,
Ptotal = PoH2O + PoC6H5Br
where P˚ is the vapor pressure of the pure component.
In general, then, the total vapor pressure above a mixture of immiscible liquids is equal to the sum of the vapor pressures of the pure liquids. This
differs from the case of miscible liquids, such as ethyl acetate and toluene, where the intermingling of one component "cuts" the contribution of
the other to the total vapor pressure. The contribution of each component to the vapor turns out to be proportional to the relative number of
molecule of each in the liquid, which is proportional to the mole fraction of each. For example, if only 35% of the molecules in a liquid are
toluene (which also means 35 mole percent toluene), the pressure contributed by toluene would be 0.35 times the vapor pressure of pure toluene.
30
In other words,
total pressure in the vapor = pressure contributed by component A + pressure contributed by component B
= partial pressure of A + partial pressure of B
= (vapor pressure of pure A * mole fraction of A) + (vapor pressure of pure B * mole
fraction of B)
Expressed symbolically,
Ptotal = PoAXA + PoBXB
where X is the mole fraction of a particular component. This is known as Raoult's law and describes the behavior of most miscible liquids (except
azeotropes) as discussed in the fractional distillation experiment.
If one of the immiscible liquids in a two-phase distillation is water, the process is usually known as steam distillation. It can be carried out either
by running live steam into the apparatus, or by having an excess of water to begin with. Using steam distillation, it is possible to distill high
boiling materials without subjecting them to damaging high heat, because the temperature stays below 100 ˚C. The water in the distillate is later
separated from the desired compounds.
Steam distillation is widely used in commerce and in research. Mint oil, for example, is obtained from mint leaves by passing steam through the
harvested leaves and condensing what comes over. (Mint is grown in large quantities near Madras, OR). In this experiment, "oil of cloves" will
be obtained by the steam distillation of cloves; the oil of cloves will be separated from the water in the distillate by extraction with pentane. The
principal flavor component
in oil of cloves, eugenol, would boil if pure at 255 ˚C.
Drying (Appendix H)
The pentane solution is dried over anhydrous sodium sulfate to remove traces of dispersed water, then the pentane is evaporated to obtain the
eugenol. The solid drying agent works by incorporating the water molecules in its crystal structure to form a solid hydrate, which is then
separated from the solution.
Na2SO4
Anhydrous solid
+
H2O
Na2SO4 • nH2O
hydrated solid
The amount of water in the hydrate is variable, depending on the identity of the drying agent as well as the amount of water present.
Experimental Procedure
Add 1.0 g of freshly crushed cloves (mortar and pestle), 10 mL of H2O and a magnetic stir vane to your 25 mL round-bottom flask (figure 3). A
Claisen head fitted with a compression cap is then attached to the round bottom flask with the threaded adaptor. Using a three prong clamp,
attached to a ring stand, clasp your apparatus so the round bottom flask can rest in a sand bath on top of a hotplate. A Hickman still fitted with a
compression cap and a water condenser is then attached to the claisen head. To ensure the apparatus does not tip over, loosely clasp the water
condenser to the ring stand with a three prong clamp. Connect the water tubing to the water condenser, gently turn on the water and ensure there
are no leaks. Have the TA check you set-up before you start heating!!!
Once checked, turn the hot plate on and warm the sand bath to approximately 160-180 oC or until the misture is boiling and the distillate starts
collecting in the hickman still. As the distillation progress, you will need to maintain approximately 10 mL of water in the round-bottom flask.
Water will be added to round bottom flask through the compression cap, on the claisen head, via a syringe. While the distillation is occurring you
will need to collect approximately 5-6 mL of distillate in your centrifuge tube. This is accomplished by periodically opening the compression cap
on the hickman still and removing the distillate with a glass Pasteur pipette. You will want to premark your centerfuge tube to 5 mL with a
Sharpie.
After collecting the 5-6 mL of distillate in the centrifuge tube, turn the heat and the cooling water off. Add 2 g of solid NaCl and 3 ml of
pentane to the centrifuge tube, cap the tube and thoroughly mix (extract) by shaking. *** Remember to keep your thumb on the cap.*** Vent the
cap, being careful not to point the cap at yourself or lab partners. Allow the mixtures to separate and remove the pentane layer with a pasture
piper (figure 4). Drip the pentane/ eugenol mixture through a ≈ 5 cm plug of anhydrous Na2SO4 collecting the eluent in a pre-weighed 25 mL
beaker. The TA will demonstrate how to make a pasture pipette filter.
Re-extract the aqueous layer two additional times with ≈ 3-4 mL of pentane. Remember to run each pentane mixture through the anhydrous
Na2SO4 plug. Rinse the Na2SO4 plug with ≈ 3-4 mL of pentane. Gently swirl the beaker on a warm hot plate to remove the pentane. *** DO
NOT use a “HOT” hot plate. If the plate is too hot the mixture could bump or catch on fire. The hot plate should be barely warm to the touch but
not hot. ***
After all the pentane has been removed, weigh the cooled beaker. Compute the weight of eugenol, as well as the percent yield from the original
weight of cloves.
31
It is imperative at this point to have an absolutely DRY sample of eugenol. Water will destroy the salt plates used for measuring the IR spectra of
the sample.
Record the infrared spectrum of eugenol as a neat liquid (pure, no solvent) on sodium chloride plates. Check out the plates from the stockroom.
See Appendix D for directions. Record the exact positions for the characteristic broad O-H stretching absorption at about 3500 cm-1, as well as
the C=C stretch at about 1600 cm-1. Clean the plates after use by wiping off the eugenol with the tissue provided (Kimwipes) then, rinsing with
acetone and dry in the air. Do not wash the salt plates with water! Tape the IR spectrum in your notebook and submit a photocopy with the
report. For further information about IR spectra, appendix D and the correct section in your lecture book.
H2O out
Clamp
H2O in
Hickman
still
Clamp
Claisen adapter
25 mL RBF
Sand bath
Hot Plate
Figure 3
Figure 4
Clean-up and Waste Disposal
The residue in the round-bottom flask from the steam distillation can be put down the drain after removing the cloves and boiling chips, which
can be put in the waste basket. Clean the flask with soap and water, with a final acetone rinse (make sure to rinse the glassware in the hood not
the sink. Clean the remaining still parts with a little acetone and invert to dry all glassware. Put extra acetone in “acetone waste”
Guidelines for Report on Isolation of Eugenol
32
Use the standard journal format. Include an appropriate sections such items as initial volumes of material, boiling of each fraction, illustrations of
equipment, etc.
Conclusions:
Note your overall yield and observations. Comment on the consistency of the spectra with the known structures. Be sure to identify the –OH and
alkene stretching frequencies in eugenol. Be sure to attach the IR spectra to the report
Questions (attached to the end of the report, after the IR spectra, etc.)
1. FFF, 8.1 (8 pts)
2. Briefly explain the efficiency of a liquid/liquid extraction using 9 mL of pentane in one portion versus three
successive extractions with 3 mL portions of pentane. (4 pts)
3. FFF, 3.4 a,b. (6 pts)
4. Explain what is meant by "venting" an extraction. Why is it important to do this? (3 pt)
5. Several students in the lab neglected to put the cap back on the bottle of anhydrous Na2SO4 after they weighed
some out for use in the eugenol isolation. Explain why this isn't a good idea. (3 pt)
6.
FFF, 4.3. (4 pt)
7. FFF 8.6 (6 pts)
8. Suppose a reaction mixture containing 30 g of malononitrile, CH2(CN)2 is dissolved in 300 mL of water. The
malononitrile is to be isolated by extraction with ether. The solubility of malononitrile in ether at room
temperature is 20.0 g per 100 mL and that in water is 13.3 g per 100 mL. What weight of malononitrile would
be recovered by extraction with (a) three 100-mL portions of ether; (b) one 300-mL portion of ether? (6 pts)
33
Experiment 5: ISOLATION OF CAFFEINE FROM TEA LEAVES
Introduction
Obtaining pure caffeine from tea leaves represents an example of "natural product" isolation. The principal techniques involved in the isolation
and purification are given below (FFF references where available):
Solid/liquid extraction
Liquid/liquid extraction Technique 3
Simple distillation
Technique 5
Sublimation
Technique 9
Caffeine is one example of a large class of organic natural products known as alkaloids ("like an alkali"). Alkaloids are loosely defined as weakly
basic, nitrogen-containing plant products that have a marked physiological activity when administered to animals. Their effects can range from
pain relief (morphine, for example) to extreme toxicity (strychnine). Caffeine has been described as a cardiac, respiratory and psychic stimulant,
as well as a diuretic (increases the excretion of urine). The molecule contains two fused heterocyclic rings. (Heterocyclic means that the ring
contains one or more atoms that are not carbon; in this case, the "heteroatoms" are nitrogen.)
O
CH 3
O
CH 3
N
N
N
N
CH 3
Caffeine (mp 238˚C)
Caffeine occurs naturally in tea, coffee beans, cola nuts, guarana paste and maté leaves. It is added to some drugs and soft drinks. In this
experiment, it will be extracted from tea leaves with hot water (an example of solid-liquid extraction) then, the caffeine will be removed from the
water extract and separated from other tea components using liquid/liquid extraction with dichloromethane as the organic solvent. The
dichloromethane will be removed from the caffeine by simple distillation; finally, the crude caffeine will be purified by vacuum sublimation.
Sublimation refers to the direct conversion of a solid to a gas without going through a liquid phase. The vapor can be re-condensed directly back
to a solid. Sublimation can be a useful technique for purification of a solid if the solid has a sufficiently high vapor pressure. It is most useful for
small samples (large samples are cumbersome), and has the advantages over recrystallization of speed and minimal sample loss. It can be carried
out at atmospheric pressure or under vacuum. (Caffeine can be sublimed under both conditions.) Reduced pressure usually leads to a faster
sublimation at a lower temperature (minimal sample decomposition).
Check out the following for this experiment:
1. Microdistillation apparatus (including 2 condenser hoses)
2. Vacuum sublimator (Check this out just before you use it.)
Experimental Procedure
Heat 100 mL of water in a 250 mL beaker to 85-90˚C on a hot plate. Suspend two tea bags1 in the water then continue heating to maintain a
gentle boil for approximately 20 minutes. During this period, gently press the tea bag several times with the flat side of a spatula. After 20
minutes or so, move the beaker to the desk top, remove the tea bag, place it in a 50-mL beaker and squeeze it with the bent flat end of the spatula;
add the squeezings to the bulk of the extract. Repeat the squeezing. Allow the hot solution to cool considerably, then stir about 1 gram of sodium
carbonate monohydrate into it.2
The rest of the isolation of crude caffeine is carried out in the hood. Set up a ring stand equipped with a small iron ring and a dry separatory
funnel.3 Place 10 mL of methylene chloride (dichloromethane, CH2Cl2) in the separatory funnel, and, when the tea extract has cooled to room
temperature (the "tea" should be cool; otherwise it will vaporize the CH2Cl2 (bp 39.8˚C).), carefully add it to the funnel. Gently swirl the
separatory funnel for 5 minutes, being cautious to avoid getting the aqueous phase into the bottom of the funnel.4 Allow the phases to separate.
Then SLOWLY drain most of the lower phase (methylene chloride + caffeine) directly into a micro distillation apparatus.5
34
Repeat the extraction with a second 10-mL portion of methylene chloride, and combine this with the first extract in the microstill. Toss in 2-3
boiling chips, connect the cooling water, and remove the methylene chloride by distillation. The caffeine stays behind in the still pot. Keep track
of the boiling range. Although you are not interested in the CH2Cl2 (bp 39.8˚C) except to get rid of it, it is wise to check things as you go along.
Stop heating (lower the heating mantle) when whitish caffeine crystals first appear on the sides of the still pot. Place the distillate in the bottle
labeled "Halogenated waste” .
When the distillation apparatus has cooled to room temperature, use a dropper to transfer the caffeine solution to the outer jacket of a sublimation
apparatus. (See the diagram on the next page.) Rinse the still pot with 1 mL of fresh methylene chloride and add this to the sublimation apparatus.
Now evaporate the methylene chloride in a stream of air, warming the container with your hand or the residual heat in the mantle. This step must
be done in the hood and away from any source of open flame.
Near the end of the evaporation, be sure that all of the caffeine is in the bottom of the sublimator.
_______________________________________________________________________________________________
1Orange Pekoe-Pekoe cut black tea works.
2The sodium carbonate, a base, is added to convert acidic, polymeric, phenolic substances, known as tannins, to phenoxide salts. The phenoxides
will remain in the aqueous phase during the extraction of the caffeine by dichloromethane. If the base were not added, the tannins would be
carried along with the caffeine and complicate the sublimation process.
Phenolic tannins ( acids )
+
Na 2CO 3 (base )
(soluble in H 2O and CH 2Cl 2)
sodium phenoxides ( salts )
(soluble in H 2O only)
3The purpose of starting with a dry separatory funnel, adding CH2Cl2 first, and carefully adding the aqueous extract is to avoid getting water
trapped underneath the methylene chloride layer. Then it will not be necessary to dry and filter the methylene chloride phase.
4Normally one shakes a separatory funnel vigorously to obtain intimate mixing of the two layers. In this case, vigorous shaking results in an
emulsion that is difficult to break up. We will depend on the increase in surface area of the vortex to yield intimate mixing of the two layers. If
you inadvertently create an emulsion, it can be "broken" by centrifuging the emulsified material.
5You may want to use a short stem funnel to avoid allowing any solution to escape via the side arm. Alternatively, the microstill can be held at an
angle so that the solution is added on the wall opposite the side arm.
Clean-up and Waste Disposal
The recovered methylene chloride and residual "tea" solution should be placed in "halogenated waste”.
Sublimation
H 2O in
H 2O out
(to vacuum)
sublimed caffeine
collects here
crude
caffeine
Vacuum Sublimator
Note: When preparing the apparatus for sublimation, don’t turn on the water to the cold-finger condenser until the apparatus is under full
vacuum. Otherwise, the cold water (especially in Fall ) can cause atmospheric moisture to condense on the cold finger and contaminate the
sublimed caffeine.
35
The sublimation can be carried out using the aspirator vacuum and trap bottle system at your desk. Note: The most important factor for a
successful sublimation is a strong vacuum. Check to see that you are getting a good vacuum before you begin. If you aren’t, find the leak or move
to a different aspirator. Heat the bottom of apparatus cautiously, with a small flame or a microburner. The sublimation is finished when no more
solid collects on the cold finger. (It's a little hard to tell. Any caffeine that collects on the upper part of the outer jacket instead of the cold finger
can be resublimed by heating that area.) Allow the apparatus to cool, release the vacuum, remove the cold finger cautiously, and scrape the
sublimed caffeine onto a weighing paper. Determine the weight of the caffeine, and store it in a vial.
CAUTION! A potential problem in the sublimation is suck-back of water from the aspirator into the sublimator. This can occur when there is a
drop in water pressure, as when someone else turns on water. Make sure that a trap bottle is connected to your aspirator, and be alert to release the
vacuum if necessary. For the same reason, it is important to release the vacuum before turning off your aspirator.
This method can yield about 50 mg of white product, although student yields are usually considerably less. Determine the melting point of your
product as well as that of authentic caffeine. Also carry out a mixture melting point with authentic caffeine. The melting points should be
determined using a sealed capillary tube (consult with your instructor for details). (Why use a sealed capillary? Make a guess.) Obtain the infrared
spectrum if requested by your instructor. Hand in the product with the usual label.
Clean-up and Waste Disposal
The dark residue at the bottom of the sublimator can be removed with lab detergent and steel wool. Push a piece of steel wool to the bottom with
a test tube brush or spatula then rotate the brush and steel to scour the sublimator.
Product should be placed in “solid waste”
Guidelines for Report on Isolation of Caffeine
Use the standard journal format.
Attach a photocopy of the IR spectrum of your purified caffeine to the report. Comment on the consistency of the spectra with the known
structures; identify the carbonyl stretching frequency in caffeine.
Questions (attached to the end of the report, after the IR spectra, etc.)
1. In the caffeine experiment, some students got a cloudy distillate when they removed the CH2Cl2 prior to sublimation. The cloudiness was
actually due to the presence of water in the distillate, and what was happening was an example of "azeotropic drying". Explain what was going
on. (5 pts)
2. FFF, 9.1. (5 pts)
3. FFF, 9.3. (5 pts)
36
Experiment 6: Cyclohexene from Cyclohexanol
You will use the Lab Report Form for this experiment.
Introduction
Dehydration of cyclohexanol to cyclohexene can be accomplished by heating the cyclic secondary alcohol with an acid catalyst at a moderate
temperature or by distillation over alumina or silica gel. The procedure selected for this experiment involves catalysis by phosphoric acid
containing a little sulfuric acid. (Phosphoric acid by itself takes a long time because it is a weak acid, and sulfuric acid by itself causes charring
with liberation of sulfur dioxide fumes.)
OH
H 3PO 4/H 2SO 4
Cyclohexanol
bp 161ºC, mp 25ºC
d = 0.96 g/mL
+
H 2O
Cyclohexene
bp 83ºC
d = 0.81 g/mL
When a mixture of cyclohexanol and catalyst is heated in a flask equipped with a fractionating column, the formation of water is soon evident. On
further heating, the water and the cyclohexene formed distil together by the principle of steam distillation, and any high-boiling cyclohexanol that
may volatilize is returned to the flask.
However, after dehydration is complete and the bulk of the product has distilled, the column remains saturated with water-cyclohexene that
merely refluxes and does not distil. Hence, for recovery of otherwise lost reaction product, a "chaser" solvent is added and distillation is
continued. A suitable chaser solvent is water-immiscible technical-grade xylene,* boiling point about 140˚C; as it steam-distils it carries over the
more volatile cyclohexene. When the total water-insoluble layer is separated, dried, and redistilled through the dried column, the chaser again
drives the cyclohexene from the column; the difference in boiling points is such that a sharp separation is possible. The holdup in the metalsponge-packed column is so great that if a chaser solvent is not used in the procedure, the yield will be only about one-third that reported in the
literature.
The mechanism of this reaction involves three steps: 1) initial rapid protonation of the hydroxyl group by the acid (either sulfuric or phosphoric),
2) loss of water to form a carbocation, followed by 3) deprotonation of the carbocation to create the alkene.
H
OH
+
H–OSO 3H
O
step 1
..
H
step 2
H 2O
OSO 3H
H
H
+
H
OSO 3H
step 3
H
* Technical-grade xylene is a mixture of o-, m-, and p-dimethylbenzenes
+
H
H 2SO 4
37
Experimental Procedure
Introduce 2.0 g of cyclohexanol, 10 drops of 20:1 H3PO4/H2SO4, and a boiling chip into a 5-mL flask equipped for fractional distillation. (See
below. Use about 0.8 g of copper sponge loosely packed in the fractionating column, and chill the receiver in ice. You may need to check out a
thermometer that fits the opening.) Mix the layers thoroughly; note the evolution of heat. Heat the mixture gently using a heating block on a
hotplate; after some time the product starts to form. (If no product forms after heating for some time cool the apparatus, add 1-2 drops of
concentrated sulfuric acid to the vial, and resume heating.)
Distil until the residue in the flask has a volume of about 0.5-1.0 mL and very little distillate is being formed; note the temperature range. Let the
assembly cool a little by removing it from the heat, remove the thermometer briefly, and add 2 mL of xylene (the chaser solvent) into the top of
the column using a dropper. Note the amount of the upper layer in the boiling flask and distill again until the volume of the layer has been
reduced by about half, or until the temperature rises abruptly.
Centrifuge the distillate briefly to cause the layers to separate then remove any visible water layer with a dropper. Add sufficient anhydrous
sodium sulfate so that it does not clump together. Shake the solution with the drying agent and let it dry for at least fifteen minutes. While this is
taking place, clean the distilling apparatus with ethanol, and finally a little acetone followed by an air stream. It is absolutely essential that the
apparatus be completely dry; otherwise the product will be contaminated with whatever solvent is left in the apparatus.
Using a dry dropper, transfer the dry cyclohexene solution to the distilling flask, rinse the drying agent with xylene, and add more xylene to give
a total volume of about 3 mL in the flask. Add a boiling chip, and distill the product. Record the bp (range) of the distillate. Stop the distillation
when the head temperature starts to rise significantly (near 85˚C) to avoid contamination of the cyclohexene with xylene. A typical yield of this
volatile alkene is about 1 g.
Transfer the sample to a tared microvial and determine its weight. Be sure that the Teflon (hard) side of the cap is down and the silicone (soft)
side up. (Cyclohexene will turn the silicone into jelly.) Cyclohexene is very volatile, and will disappear if it isn't properly sealed.
Please note: A condenser needs
to be in place here.
chill
in ice
insulate column
and still head
with glass wool
Run the infrared spectrum and interpret it for purity. Look for peaks due to starting material and xylene.
Clean-up and Waste Disposal
1. Dispose of the residue in the original reaction flask (contains acids, xylene, etc.) in ”Flammable with Corrosive Waste”
Product is disposed in “Flammable Waste”
2. Leave the column packing on a piece of paper in the hood to evaporate adhering organic liquids the put back on shelf. The still components
probably need only to be dried in an air stream without washing.
3. Allow the used sodium sulfate to dry out in the hood then place it in ”Solid Waste”.
38
Guidelines for the Cyclohexene Report
Turn in the completed report form, including answers to the questions below. Please note that the bp of your product should be the entire range,
from the temperature at which the first drop collected to the temperature at which the distillation was stopped. Submit your product in a properly
labeled container to the lab instructor for inspection.
Questions
1. The crude product in the chilled receiver after the beginning of work-up probably contained the following:
cyclohexene
unreacted cyclohexanol
traces of dicyclohexyl ether (by-product)
water
xylene
Prepare a schematic diagram (see next page) of the remaining work-up and purification, showing clearly how, and at what stage, each
contaminant is removed from the cyclohexene. (4 pts)
2. Write a mechanism for the dehydration of the alcohol shown. Use the electron-pushing formalism and show each step clearly. (5 pts)
CH 3
CH 3 C
CH
CH 3
CH 3
H 2SO 4
CH 3
+
C C
CH 3 OH
CH 3
H 2O
CH 3
3. Give the principal product(s) of the following: (6 pts)
CH 3
OH
CH 3
H 2SO 4
a)
b)
(trans)
CH 3 CH
CH
OH
CH 2 CH 3
H 2SO 4
39
Guidelines for Preparing ‘Schematic Diagram’ of Work-up and Purification.
Background
If you find that the procedure for carrying out the synthesis of an organic compound seems like a blur to you and you don’t understand what is
happening at every stage in the process, welcome to the club. This is normal at this time. Most novices in the laboratory are unclear about what
the various ingredients in the procedure are for, and whether they should be called reactants, catalysts, solvents, etc. In addition, they have a
shaky concept of what might be called ‘material flow’, which refers to where the different things end up when certain operations are carried out,
such as separation of layers in a separatory funnel, distillation, filtration, etc.
The first page of the report form, previously discussed, is designed to gradually strengthen your understanding of the basic stages in most
syntheses, as well as the names and functions of the various ingredients. To get you thinking about material flow, there will be a question for
every synthesis report that asks you for a schematic diagram of the work-up and purification procedure. This is a type of flow chart that starts
with all of the things present in the crude reaction mixture at the beginning of work-up, and shows where everything eventually ends up during
work-up and purification. After you have done this a few times, you will begin to feel a lot more confident that you understand what you are
doing in the lab.
Preparing the diagram (flow chart)
One straightforward way of preparing the diagram is illustrated on the next page with the sequence of connected boxes. This particular diagram is
appropriate for the cyclohexene synthesis, and you can simply fill in the information and turn it in with your report. The first steps are done for
you. Note that the top box contains a list of the materials present, and at the line to the next box, you indicate some operation that you carried out.
Sometimes the operation involves a separation process, such as filtration or separation of layers with a separatory funnel; then the connecting line
will branch to two boxes, which are identified and the contents noted. At the end of the whole process will be a box representing the compound
you are preparing. An inspection of the diagram should reveal what happened to everything else that was there at the start.
An important consideration in deciding where things go during a separation process is solubility differences. The concept of ‘like dissolves like’
applies, which means that polar compounds tend to dissolve in polar solvents, and non-polar compounds in non-polar solvents. Most organic
compounds are relatively insoluble in water and soluble in organic solvents. Exceptions are low-molecular-weight polar compounds such as
alcohols, ketones, etc., which tend to be water soluble. Ionic compounds tend to be soluble in water and insoluble in organic solvents. Keep this
in mind when you are deciding where things go during a separation process. Ionic compounds are also relatively non-volatile, which is the reason
that drinking water can be prepared from sea water by distilling off the water.
Also keep in mind that chemical changes are sometimes involved during work-up. The reactions should be shown in the diagram. You then have
to deal with these reaction products (and usually an excess of some reagent used) and show where they end up.
Schematic diagram for cyclohexene work-up and purification
40
Crude product (distillate)
cyclohexene
cyclohexanol
dicyclohexyl ether
water
xylene
Separate layers
aqueous layer
organic layer
cyclohexene
Balanced equations for any reactions occurring during work-up:
41
Experiment 3. Cholesterol from Human Gallstones
Introduction
Cholesterol
Cholesterol (below) is an unsaturated alcohol having the formula C27H46O. It possesses the characteristic fused ring system of a steroid, making
it a steroidal alcohol (also called a sterol).
Cholesterol is among the most abundant steroids. It is present in almost all human and animal tissue, particularly in the brain, spinal chord and
nerves. An average healthy adult human body has about 200 - 300 grams of cholesterol. Although its function in the body is not completely
understood, it is known to be a precursor to steroidal hormones and bile acids.
Cholesterol is insoluble in water and blood plasma, and is transported in the bloodstream bound to lipoproteins, which are proteins attached to
lipids (fats). Research has divided these lipoproteins, when centrifuged, into two broad classes - high-density (HDL) and low-density (LDL)
lipoproteins. A relatively high concentration of HDL bound to cholesterol seems to cause no health problems and in fact is beneficial; but a high
ratio of LDL-cholesterol leads to the deposition of cholesterol both in the gall bladder (resulting in gallstones) and also on the walls of the arteries
(causing a plaque that cuts off blood flow and hastens hardening of the arteries or atherosclerosis).
In this experiment, cholesterol will be isolated from human gallstones by heating crushed gallstones with an organic solvent (2-butanone),
filtering out insoluble residue, then inducing crystallization by adding methanol and water. The cholesterol obtained in this way invariably
contains trace amounts of impurities that can't be removed by ordinary recrystallization. They are left behind, however, if the cholesterol is
converted to a highly insoluble dibromide, followed by debromination back to cholesterol using zinc dust. The basic chemistry is:
Br
C
C
cholesterol
+
Br 2
C
C
Br
cholesterol dibromide
Zn
C
C
+
ZnBr 2
cholesterol
Gallstones
The gall bladder is attached to the undersurface of the liver just below the rib cage. It retains bile produced by the liver and feeds it into the upper
part of the small intestine as needed for digestion. Bile consists primarily of bile acids, which are carboxylic acids closely resembling cholesterol
and which aid in the digestion of fats by functioning as emulsifying agents. The gall bladder also harbors free cholesterol. If the concentration of
cholesterol in the bile exceeds a certain critical level, it will come out of solution and agglomerate into particles that grow to form gallstones. An
amateur geologist given a bottle of gallstones to identify once labeled them a "riverbed conglomerate" - and indeed they do resemble stones in
color, texture, and hardness. They come in a variety of shapes and colors and can be up to an inch in diameter.
As gallstones collect, they irritate the lining of the gall bladder causing severe pain, nausea, and vomiting. The stones can block the bile duct and
lead to fatal complications. The remedy is surgery, although recently gallstones have been dissolved right in the gall bladder, not with boiling
alcohol but with methyl tert-butyl ether. The ether is injected into the gall bladder through a 1.7-mm tube, which pierces the skin and liver on its
way to the gall bladder. Depending on the number and size of the gallstones, the dissolution process can take from 7 to 18 hours.
The average American woman at age 75 has a 50% chance of developing gallstones, while for a man of the same age the chance is only half as
great. Gallstones and coronary heart disease are also much more common in overweight individuals. Almost 70% of the women in certain Native
American tribes get gallstones before the age of 30, whereas only 10% of black women are afflicted. Swedes and Finns have gallstones more
often than Americans do; the problem is almost unknown among the Masai people of East Africa.
The most common treatment at present for gallstones is surgical removal, an operation performed 500,000 times each year in the United States.
This is the source of the gallstones used in the present experiment. They were kindly provided by a former PSU student who is now a pathologist
at a local hospital.
42
Purification
Cholesterol isolated from natural sources contains small amounts (0.1-3%) of 3ß-cholestanol,
7-cholesten-3ß-ol, and 5,7-cholestadien-3ß-ol. These are so similar to cholesterol that their removal by crystallization doesn’t work. However,
complete purification can be accomplished through the sparingly soluble dibromo derivative, 5,6ß-dibromocholestan-3ß-ol. 3ß-Cholestanol is
saturated and does not react with bromine; thus it remains in the mother liquor. 7-Cholesten-3ß-ol and 5,7-cholestadien-3ß-ol are dehydrogenated
by bromine to dienes and trienes, respectively, that likewise remain in the mother liquor and are eliminated along with colored by-products.
The cholesterol dibromide that crystallizes from the reaction solution is collected, washed free of the impurities or their dehydrogenation
products, and debrominated with zinc dust, with regeneration of cholesterol in pure form. Specific color tests can differentiate between pure
cholesterol and tissue cholesterol purified by ordinary methods.
Experimental Procedures
Physical Properties of Reactants and Products
2-Butanone
Diethyl ether
MW
72.11
74.12
bp (˚C)
80
34.6
Part 1. Extraction of Cholesterol from Gallstones
Before beginning the extraction, set up a suction filtration apparatus consisting of a Hirsch funnel seated in a 6-inch side-arm test tube (stockroom
check-out) into which a 15 x 125 mm rimless test tube (check out) has been placed to collect filtrate. Use a cotton plug if necessary, to raise the
test tube high enough to catch the filtrate. In a large test tube containing a boiling chip, dissolve ~300 mg of crushed human gallstones in 4.5 mL
of 2-butanone by gentle heating on an aluminum block or steam bath. In another test tube also containing a boiling chip, heat 2 mL of 2-butanone
to boiling.
As soon as the gallstones have disintegrated and the cholesterol has dissolved, filter the hot solution with gentle suction on the Hirsch funnel into
the rimless test tube. Wash out the original test tube with the hot 2-butanone, and rinse the Hirsch funnel, as well. The brown residue removed by
filtration is primarily bile pigment, bilirubin (a metabolite of hemoglobin). (Structure provided on following page.) Allow the bilirubin to dry, and
record its weight.
Under a stream of air (in the hood) evaporate the solution to 1.9 cm height in the rimless test tube, dilute the solution with 1.5 mL of methanol,
heat the mixture to boiling (boiling chip), then add water drop-wise until a very faint cloudiness persists. Cholesterol is completely insoluble in
water and not very soluble in methanol. Addition of water will produce a solution saturated with cholesterol at the boiling point. This is the
critical part of this experiment. Add a drop of water, re-dissolve any precipitated cholesterol with heat, and continue adding water until the
solution is saturated while hot (faint, persistent cloudiness).
(NOTE: Bumping is a severe problem here, even with a boiling chip. Constant agitation with a spatula is advised.)
Cork the tube, wrap it with some insulating material, and allow the solution to cool slowly without disturbance to room temperature; then cool it
in ice. Slow cooling produces large crystals. Collect the product on the Hirsch funnel, wash it with a little ice-cold methanol then draw air
through the sample for a few minutes to dry it. Determine the weight and melting point of the cholesterol and calculate the yield from gallstones.
Clean-up and Waste Disposal for Part 1
The filtrate from the suction filtration as well as any excess 2-butanone or methanol should be placed in the container marked "Cholesterol
Extraction, Waste Filtrate". Bilirubin can be placed in the wastebasket.
43
Part 2. Bromination of Cholesterol
Be extremely careful to avoid getting the solution of bromine and acetic acid on the skin! Carry out the reaction in the hood and wear disposable
gloves for maximum safety. (The brominating agent is prepared from 4.5 mL of bromine and 0.4 g anhydrous sodium acetate for every 50 mL of
glacial acetic acid. It is dispensed from a buret.)
In a 10 x 75 mm test tube dissolve 100 mg of gallstone cholesterol or of commercial cholesterol (content of 7-cholesten-3ß-ol about 0.6%) in 0.7
mL of ether by gentle warming, and then add 0.5 mL of a solution of bromine and sodium acetate in acetic acid. Cholesterol dibromide begins to
crystallize in a minute or two. Cool in an ice bath and stir the crystalline paste with a stirring rod for about 10 minutes to ensure complete
crystallization, and at the same time cool a mixture of 0.3 mL of ether and 0.7 mL of glacial acetic acid in ice. Then collect the crystals on the
Hirsch funnel and wash with the iced ether- glacial acetic acid solution to remove the yellow mother liquor. This material is not easy to filter on a
very small scale because the crystals are very small. Finally wash the crystals on the filter with a few drops of ethanol, continuing to apply
suction, and transfer the white solid without drying it (dry weight 120 mg) to a test tube. Clean-up and waste disposal, see below.
Part 3. Zinc Dust Debromination
Add 2 mL of ether, 0.5 mL of glacial acetic acid, and 20 mg of Zn dust, and mix the contents. (Note the approximate volume at this stage.) In
about 3 minutes the dibromide dissolves; after 5-10 minutes of mixing, zinc acetate usually separates to form a white precipitate (the dilution
sometimes is such that no separation occurs). (If the reaction is slow add more zinc dust. The amount specified is adequate if material is taken
from a freshly opened bottle, but zinc dust deteriorates on exposure to air.) Stir for 5 minutes more and then add more ether if necessary to
restore the mixture to its original volume.
Part 4. Liquid-Liquid Extraction
Begin a liquid-liquid extraction by decanting the solution from the zinc into a centriguge tube, wash the ether solution twice with 2 mL of water
(to ‘wash’ a solution means to subject it to liquid/liquid extraction in the usual way, at this point it is important that you begin shaking the mixture
to improve the extraction results) and then with 1 mL of 10% sodium hydroxide and finally with 1 mL of saturated sodium chloride solution. If at
any point you do not see multiple layers, add more of the volatile component, i.e. the ether. Then shake the ether solution with anhydrous sodium
sulfate, pipette off the dry solution, wash the drying agent with a little ether, add 1 mL of methanol (and a boiling chip), and boil the solution to
the point where most of the ether is removed and the purified cholesterol begins to crystallize. Remove the solution from the heat, let
crystallization proceed at room temperature and then in an ice bath, collect the crystals, and wash them with ice-cold methanol.
Weigh the dried crystals (typically 60-70 mg), determine their melting point and record the IR spectrum using the KBr pellet method. Calculate
the percent recovery of cholesterol in the bromination/debromination sequence.
Clean-up and Waste Disposal for Parts 2, 3 and 4
The unreacted zinc dust should be discarded in its labeled waste container after it has been allowed to dry and then sit exposed to the air for about
one-half hour on a watch glass. Sometimes the zinc dust at the end of this reaction will get quite hot as it air oxidizes.
The aqueous solutions can be flushed down the drain.
The organic filtrates should be placed in the container labeled "Waste Organic Filtrates; Cholesterol Bromination/Debromination".
After the solvent has evaporated from the sodium sulfate (hood), it can be placed in the wastebasket.
Guidelines for the Cholesterol Report
Use the standard "journal" format for this report. Turn in your sample of cholesterol with the report.
Questions
44
1. a) (4 points) Circle all of the chiral centers in the cholesterol structure.
b) (3 points) State the meaning of α and ß with regard to steroid structure, and indicate whether the hydroxyl group on ring A and the alkyl
group on ring D are  or ß.
2. a) (4 points) Explain why recrystallization, as a general technique, is usually effective at removing a few percent of impurity, even though the
solubility (g/mL) of the impurity might be similar to that of the main component. Where does the impurity end up? Why doesn't it crystallize
along with the main component?
b) (4 points) The bromination/debromination purification procedure was used in order to remove a few percent of impurities with structures
related to cholesterol. Normally, this amount of impurity is easily removed by simple recrystallization. For some reason, this is not effective here.
What might be going on in the cholesterol purification that could make simple recrystallization ineffective even though solubility characteristics
are similar? (Hint: Take a long look at the structures involved.)
45
APPENDIX A Composition and densities of Common Acids and Bases
This information applies to the common acids and bases found in standard glass-stoppered bottles, which are usually marked “concentrated” or
“dilute”.
Reagent
Molarity
% By Weight
Density (g/mL)*
conc. HCl
12 M
37%
1.18
conc. HNO3
16 M
71%
1.41
conc. H2SO4
18 M
95%
1.84
conc. H3PO4
18 M
85%
1.69
conc. acetic acid
(glacial acetic acid)
17.5 M
100%
1.05
conc. "NH4OH"
(conc. NH3)
15 M NH3
29% NH3
0.90
dilute HCl
6M
dilute HNO3
6M
dilute H2SO4
3M
dilute "NH4OH"
(dilute NH3)
6 M NH3
* Density refers to the reagent having the concentration indicated, not the pure material (except for acetic acid).
46
APPENDIX B GENERAL PROCEDURE FOR RECRYSTALLIZATION
These directions include charcoal treatment for removing colored impurities. If charcoal is not needed, omit step 6. If the hot solution at the end
of step 5 contains no insoluble debris and charcoal is not being used, omit the hot filtration and proceed directly from step 5 to step 10.
1. Be prepared. Assemble solvents, glassware, charcoal and equipment needed for the operations up to and including the hot filtration, step 8.
(The Buchner or Hirsch funnel can be set up later.) You don’t want to have your sample “cooking” on the hot plate unnecessarily as you run
around setting up apparatus.
2. Place the impure solid in an Erlenmeyer flask. (An Erlenmeyer flask is generally preferred for a recrystallization because it contains vapors
better, and can be swirled without loss of material. A beaker should not be used with organic solvents.)
3. Add solvent to just cover the solid (make a slurry). Add 2 or 3 boiling chips.
4. Heat to a gentle boil with swirling and stirring. (Make a holding strap out of a piece of paper.)
5. With continued heating, add solvent in small portion until the sample is completely dissolved. Swirl frequently during this operation. Add a
little extra solvent.
6. Remove the flask from the heat, and swirl until the solution is a little below its boiling point. Cautiously add a little charcoal and swirl again.
If there is no frothing or violent boil-up, add the rest of the charcoal and swirl thoroughly. Keep the mixture at or near its boiling point for a
minute or two, with swirling. Pelletized charcoal takes longer.
7. Filter the hot liquid through fluted filter paper (for rapid filtration, it is important that the filter paper be rigid and well creased), using a preheated stem-less funnel, into a second Erlenmeyer flask. (Support the funnel on an iron ring. To minimize premature crystallization in the funnel,
keep it rather full and cover it with a watch glass while the hot liquid drains.)
8. Rinse the original flask and filter paper with a little hot, fresh solvent.
9. If crystals have formed in the second Erlenmeyer flask during filtration, re-dissolve them by heating. (The rapid cooling when the hot liquid
hits the cold glass is considered to be not conducive to good crystal formation.)
10. Set the flask aside to cool to room temperature. During cooling, initiate crystal formation by scratching the glass under the liquid surface with
a spatula or stirring rod, or adding a “seed” crystal. (A very small amount of the crude material may be used as a seed.)
11. After the mixture has cooled to room temperature, chill it thoroughly in an ice bath. Also chill some fresh solvent in preparation for step 13.
12. Filter with suction (Buchner or Hirsch funnel). If needed, use some of the cold filtrate to help transfer crystals from the Erlenmeyer flask to
the filter.
13. Rinse the flask and wash the crystals with a little chilled, fresh solvent. (Too much will re-dissolve your sample.) The filter cake should be
kept flat using a bent spatula, to promote uniform draining of solvent. Draw air through the sample for a few minutes to remove most of the
solvent.
14. Transfer the crystals with the filter paper to a watch glass. Spread out the crystals and allow them to air dry (at least 24 hours if water was the
solvent). Remove the filter paper when the sample is dry. (Wet filter paper tends to shred.)
47
APPENDIX C
Pasteur pipet
This is a type of dropper with the end drawn to a capillary (they are often called capillary pipets). The ones in your lab drawer are 5 3/4 inches
long, hold about 2 mL, and are filled with a small rubber bulb. You will do many transfers using this device, often with a specified volume of
solvent. Most of the time, as in extractions and washings, the exact volume called for is not critical, and you can save time by approximating it
using the to-scale diagram shown. Select a pipet with the capillary end intact, and mark it at the appropriate places.
0.5 mL
1.0 mL
1.5 mL
Pasteur Pipet (full-scale)
An Annoying Thing about Using a Pasteur Pipet
When you transfer a volatile solvent like ethyl ether or dichloromethane using a pipet, the liquid that is drawn up tends to squirt out again almost
immediately. This is due to the build-up of back pressure in the pipet from the solvent vapors, particularly if the pipet has been warmed by your
hand. You can alleviate this by handling the pipet carefully, and by drawing the solvent or solution in and out several times, which tends to
saturate the vapor space with solvent vapor.
Using a Pasteur Pipet as a Filtering Device
A Pasteur pipet can be modified to be used as a microscale filtration device in cases where the amount of material is inappropriate for ordinary
apparatus. One modification, which is recommended by some lab manuals, is called a Pasteur filter pipet. It uses a small cotton plug in the
capillary end of the pipet. Directions for its construction are given below. It takes some practice to be able to do this quickly and with the right
amount of cotton, and one of the claims made for the device seems a little improbable.
Pasteur filter pipet. This is constructed by taking a small cotton ball and placing it in the large open end of the standard Pasteur pipet. Hold the
pipet vertically and tap gently to position the cotton ball in the drawn section of the tube (a). Now form a plug in the capillary section by pushing
the cotton ball down the pipet with a piece of copper wire (b). Finish by seating the plug flush with the end of the capillary (c). The optimum size
plug will allow easy movement along the capillary while it is being positioned by the copper wire.
The compression of the cotton will build enough pressure against the walls of the capillary (once the plug is in position) to prevent plug slippage
while filling the pipet with liquid. If the ball is too big, it will wedge in the capillary before the end is reached, and wall pressure will be so great
that liquid flow will be shut off. Even some plugs that are loose enough to be positioned at the end of the capillary still will have developed
sufficient lateral pressure to make the filling rate unacceptably slow. With a little practice, however, theses plugs can be quickly and easily
inserted.
The reason for placing the cotton plug in the pipet is two-fold; first, a particular problem with the transfer of volatile liquids with the standard
Pasteur pipet is the rapid buildup of back-pressure from solvent vapors in the rubber bulb. This pressure quickly tends to force the liquid back out
of the pipet. The result can be valuable product dripping on the bench top. The cotton plug tends to resist this back-pressure and allows much
easier control of the solution once it is in the pipet. The time delay factor becomes particularly important when the Pasteur filter pipet is
employed as a microseparatory funnel.
a
b
c
48
Second, each time a transfer of material is made, the material is automatically filtered. This process removes dust and lint, which are a constant
problem when working at the microscale level with unfiltered room air. The debris tends to remain trapped in the cotton when the liquid is
expelled from the pipet.
Another way to use the pipet as a filter:
A simpler way to filter with a pipet is to lodge a piece of cotton in the barrel of the pipet where it meets the capillary end. Liquid is then
transferred to the open end using another pipet, and the filtrate drains out of the bottom. (See the diagram.) The cotton needs to tight enough to
retain solids, but not so tight as to impede the drainage of liquid.
49
APPENDIX D
DIRECTIONS FOR OBTAINING INFRARED SPECTRA
I. Sample Preparation
Liquid Samples - NaCl Windows
Caution. The NaCl windows, like common table salt, are water soluble. Keep water away!
1. Check out a NaCl plate and a plate holder from the chemistry stock room. Do not place your fingers on the surface of the windows or the oils
on your hand will appear on your spectrum. Your body moisture will also fog the plates.
2. Verify that the plates are clean. Dip the small end of of a Pasture piper into your sample, then gently touch the tip to middle of the NaCl plate
to transfer the sample. A drop or two is all you need. You may want to use the edge of the pipet to smear the sample on the center of the plate.
3. Carefully place the plate in the holder, oil side out, and obtain the spectrum.
After obtaining the spectrum, remove the windows and rinse them with acetone. Wipe off the windows as well as
the holder with a Kimwipe.
Solid Samples - KBr Pellet Method
The most satisfactory method for solid samples involves subjecting a mixture of the solid and potassium bromide (KBr) to high pressure, which,
ideally, fuses the mixture into a transparent or translucent pellet whose IR spectrum can be recorded. This is done by thoroughly grinding ~1 mg
of the solid sample with ~100 mg of dry KBr. Either a hydraulic press or a special die can be used to prepare the pellet.
Following this procedure for grinding the KBr mixture:
Be prepared. KBr is hygroscopic. It can absorb water from the atmosphere, leading to unwanted absorption in the O-H region. It is therefore
stored in a heated oven. To avoid exposing the KBr to atmospheric moisture, make sure that a KBr press is available before measuring out the
KBr.
Thoroughly grind about 1 mg of your sample in a spot plate, using a glass stirring rod or a small test tube. [Thorough grinding is important.
Otherwise, some high energy will be scattered from the faces of residual crystals in the pellet, particularly at the high energy end of the spectrum,
leading to increasing absorption ("ramping") of the baseline toward the left. ]
Use the scoop provided to transfer about 100 mg of KBr to the sample in the spot plate. (Tap the scoop lightly to get the KBr approximately even
with the top of the cavity.) Quickly mix the KBr with the sample using a spatula and grind the mixture. Repeat the mixing and grinding two more
times. Immediately transfer the mixture to the press, even it out by tapping the press.
The KBr pellet can be made from the die sets located in the IR room. Follow the directions posted near the instruments. A good KBr press should
be clear and transparent. After obtaining the spectrum, dispose the pellet and use a test tube brush to remove the residual powder from the press
or the threads.
Solid Samples - Cast Film Method
The cast film method involves depositing a solution of a solid on a salt plate, and allowing the solvent to evaporate. The resultant solid film often
yields a satisfactory IR spectrum. If the film is too crystalline, the spectrum may exhibit ramping at the high energy end because of scattering
from the crystal faces.
The IR spectrum is recorded without using a second salt plate over the film.
In a small test tube, dissolve about 1mg of solid in a volatile solvent such as diethyl ether.
Deposit the solution on a salt plate, and allow the solvent to evaporate. (Do this in the fume hood to avoid exposure to solvents.)
Mount the plate in the holder, and record the spectrum as usual.
If the peaks are too weak, deposit more sample and record the spectrum again. If the peaks are too strong (off scale), wash off the sample with the
solvent and try again with less sample on the plate.
Clean and dry the salt plate as usual.
50
II. Operation of IR Instrument
Directions are posted on the instrument.
51
III. Characteristic IR Absorptions
Alkanes
2840 - 3000
~1465
~1450
~1375
~720
C–H stretching
CH2 scissoring
CH3 asymmetric bending
CH3 symmetric bending
CH2 rocking (requires at least 4 connected CH2 groups)
Alkenes
3000 - 3100
C–H stretching
1610 - 1680
C=C stretching (often weak)
650 - 1000 =C–H bending (usually the strongest in the spectrum)
cis-disubstituted
vinyl group (R–CH=CH2)
1626 - 1662
C=C stretching
1638 - 1658
665 - 730 =C–H bending
905 - 915 & 985 - 995 =C–H bending
C=C stretching
trans-disubstituted
vinylidene group (R2C=CH2)
1668 – 1678 C=C stretching
1648 - 1658
C=C stretching
960 - 980
=C–H bending
885 - 895 =C–H bending
Alkynes
3260 - 3350
2100 - 2260
610 - 700
≡C–H stretching
C≡C stretching (often weak)
≡C–H bending (often strong and broad)
Aromatic Rings
3000 - 3100
1585 - 1600 & 1400 – 1500
1000 - 1300
C–H stretching
C-C ring stretching ('skeletal vibrations')
C-H in-plane bending
C–H out-of plane bending vibrations for substituted benzenes
671
benzene
690 - 710 & 730 - 770
monosubstituted
870
pentasubstituted
735 - 770
1,2-disubstituted
690 - 710 & 750 -810
1,3-disubstituted
805 - 825 & 870 - 885
1,2,4-trisubstituted
Alcohols
3500 - 3600
3200 - 3500
1000 - 1260
800 – 810
840 - 850
855 - 870
810 - 833
705 - 745 & 760 – 780
675 - 730 & 810 - 865
1,2,3,4-tetrasubstituted
1,2,3,5-tetrasubstituted
1,2,4,5-tetrasubstituted
1,4-disubstituted
1,2,3-trisubstituted
1,3,5-trisubstituted
O–H stretching (non H-bonded, sharp)
O–H stretching (H-bonded, very strong)
C-C-O asymmetric stretching
Ethers
1200 - 1275 & 1020 - 1075 mixed unsaturated/saturated C-O-C stretching
1085 - 1150
aliphatic C-O-C symmetric stretching
Carbonyl Compounds
1700 - 1750
C=O stretching, strong (saturated ketones, aldehydes, carboxylic acid, esters, amides)
Conjugation causes absorption at lower wavenumbers, while ring strain or the presence of electron-withdrawing groups causes absorption at
higher wavenumbers.
3000 - 2500
carboxylic acids, characteristic broad, complex band structure of O–H stretching and combination bands
1550 - 1650 & ~1400 carboxylate anion, asymmetric and symmetric O–C–O stretching, respectively
Amines
3200 - 3500
1580 - 1650
1260 - 1340
1020 - 1250
650 - 900
APPENDIX E
N–H stretching (broad if H-bonded, sharp otherwise)
NH2 scissoring
aromatic C–N stretching
saturated C–N stretching
N–H wagging (broad)
Recrystallization Using A Craig Tube
52
Introduction
One of the basic principles of microscale laboratory operations is to avoid transfers of material whenever possible, because transfers usually lead
to loss of sample. Microscale recrystallization can be carried out in a Craig tube, a device designed so that the crystallization, filtration, and
drying of the sample occur in the same vessel.
lift
wire
hanger
invert,
centrifuge
mother
liquor
A
B
C
D
The Craig tube consists of a lower section like a test tube except that the upper part is enlarged to hold a glass plug. The region where the plug
and lower tube touch is ground to a rough surface so that liquid can seep through but crystals cannot pass. Crystallization is carried out with the
Craig tube in an upright position (A). When crystallization is complete, the glass plug is inserted (B), and a wire hanger attached in such a way
that it can be used to lift the Craig tube at the end of centrifugation. A centrifuge tube is placed over the Craig tube and hanger (C), then the
whole assembly is inverted and centrifuged. The mother liquor (filtrate) is forced through the ground-glass "seal" while the crystals are held back
(D). The Craig tube is lifted from the outer centrifuge tube using the wire hanger, the glass plug is removed, and the crystals are allowed to air dry
in the tube.
Procedure
The steps below are essentially the same as those in any recrystallization that does not involve hot filtration or the use of charcoal. The main
difference is the method for separating the mother liquor from the recrystallized sample.
1. Tare the lower section of the empty Craig tube. (To "tare" a container means to determine and record its weight for later use).
2. Place the sample in the Craig tube.
3. Add solvent to cover the solid. (Make a slurry.)
4. Heat the mixture to a gentle boil, stirring the mixture continuously. (A melting point capillary makes a convenient stirring rod.)
5. With continued heating and mixing, add more solvent in small increments until the sample is completely dissolved.
6. Set the solution aside to cool. After it has cooled a bit, try to induce crystallization by "scratching" or adding a seed crystal. If no or only
small amounts of crystals appear, gently heat at boiling once more to concentrate the solution and let cool again.
7. When the solution has cooled to room temperature and no more crystals appear to be forming, insert the upper glass plug and chill the sample
thoroughly (10-15 min) in an ice bath.
8. Just before centrifuging, wrap a piece of copper wire around the glass plug as shown in the diagram (C), slide an inverted centrifuge tube over
the top of the assembled Craig tube, then turn the whole assembly upside down.
9. Centrifuge immediately (otherwise it will warm up again) using as a counterweight another centrifuge tube nearly full of water. Centrifuge for
about 1 minute.
Note. If the centrifuge rattles noisily, turn it off immediately and adjust the amount of water in the counterweight.
10.
Remove the assembly from the centrifuge, and lift out the Craig tube using the wire hanger.
53
11.
Remove the plug, tilt the Craig tube on its side, and spread out the crystals so that they will dry thoroughly. (Check with your
instructor about using the oven for drying a particular sample.)
12. When the crystals are dry, reweigh the Craig tube and compute the weight of recrystallized sample.
NOTE:
1. The Craig tube does not allow the use of charcoal or hot filtration to remove insoluble impurities. If these are necessary, they should be carried
out in a different container. See you instructor.
2. If the melting point of a sample is below the boiling point of the recrystallization solvent, the mixture should not be heated to boiling but
should at all times be kept under the melting point of the solid. Otherwise the sample tends to oil out (separable as a second liquid layer) and
reabsorb impurities.
54
APPENDIX F
REFRACTIVE INDEX
Introduction
The refractive index, symbolized by the letter n, is defined for any medium as the ratio of the velocity of light in a vacuum to the velocity in
that medium. It is always greater than 1 because light has its maximum velocity in a vacuum; organic liquids typically have values between
1.3400 and 1.5600. Refractive index is a physical constant characteristic of any particular substance, in the same way that melting point or boiling
point is characteristic. It can be measured with high accuracy and is a useful criterion for judging the purity of a liquid sample or helping verify
the identity of an unknown liquid. (This assumes, of course, that a pure sample of the substance has already been prepared and its refractive index
recorded.)
An instrument for measuring refractive index is called a refractometer. The particular type available in the laboratory is a Fisher brand
refractometer. The principle of its operation can't be explained in a few sentences, but to relate the refractive index to physical quantities more
accessible than the velocity of light, it can be mentioned that n is also equal to , where i and r are the angles of incidence and refraction of a beam
of light passing through the reference medium (vacuum, or usually air in actual practice) into the sample whose refractive index is being
measured.
Description of the Instrument
The instrument employs a sample holder and an illuminated dual-reference scale. The scale can be read directly in refractive index units from
nD=1.30-1.90 and interpolated to an accuracy of ±0.002 units. A second scale on the right reads directly in percent sucrose from 0 to 80% which
can be interpolated to an accuracy of 1%.
In operation, a lamp casts a beam of light through a slit in the scale and then through a 2 mm aperture to the eye. Over this aperture is a fixed
glass plate with plane parallel faces. A small glass prism with a beveled edge is mounted on the glass plate that covers the aperture, forming a vshaped well. A drop of the sample placed in this well covers part of the aperture so that the user sees a clear image of the scale and also a virtual,
or secondary image caused by refraction of the light passing through the sample and prism. Solutions having a refractive index less than the glass
will cause the light to bend downward and the virtual image will be seen above the clear slit in the scale. If, however, the refractive index of the
solution is greater than that of glass, the light beam will be bent upward, and the virtual image will be seen below the clear slit in the scale.
Measuring the Refractive Index
NOTE: Before you use the refractometer, your instructor will make sure that the prism is clean and properly installed on the eyepiece. The prism
is made of soft glass and its polished surface is easily scratched. If it becomes scratched, the readings will be less accurate. Observe the
precautions regarding sample application and handling.
With a dropper, place a drop of liquid sample into the v-shaped well such that the liquid is drawn into the apex. Do not touch the prism with the
glass pipet tip.
Depress the light switch at the rear end of the instrument and read the refractive index through the eyepiece, estimating the third decimal.
Record the temperature on the thermometer that is close to the instrument.
Note that there is a white band at around 1.517. This is the slit on the scale that allows the beam of light to come through. Your index reading will
be above this bright white slit, either as a narrow white image or as a multicolored band. If a multicolored band is observed, take the reading of
the yellow portion of the spectral band.
55
To take additional measurements, use the hose that is connected to the compressed air to blow-dry the sample area before applying the sample. I
think the students don’t need to air dry the samples they are very volatile and will evaporate quickly. Also this instruction makes the students
think that they need to remove the prism and air dry it in the hood because no air hose is present in the GC room where the instrument is located.
Additional Comments
25
1. The usual notation for refractive index is shown in the following example: nD 1.3842. The superscript gives the temperature, and the
subscript gives the wavelength of light used in the measurement. The refractive index is affected by both of these variables. The D refers to the D
line in the emission spectrum of sodium vapor, a closely spaced doublet at 5890 and 5896 angstrom units. You will note that the source of
illumination is actually an ordinary light bulb, which produces white light (a mixture of wavelengths). A light source of this type is much more
convenient to use than a sodium lamp. The presence of the other wavelengths in the light beam is corrected for by the compensator, a system of
prisms.
The sensitivity of refractive index to changes in temperature depend on the particular compound being examined. Two extremes are water and
carbon disulfide, the former having a temperature coefficient of 0.0001 refractive index units per degree Celsius and the latter 0.0008. A value of
0.00045 is considered typical for an organic compound. Hence temperature control is necessary if reproducible values are to be obtained. The
observed value is "corrected" to a standard temperature, usually 20˚C, by adding or subtracting 0.00045 for each degree difference in
temperatures. The refractive index becomes smaller with increasing temperature.
2. You will note that on the instrument, there is an additional scale on the right that reads form 0 to 80. This is a convenience to the sugar
industry, which uses refractive index to monitor the concentrations of aqueous sugar solutions. When the measurement is made at 20˚C, this scale
reads directly the percent of dissolved sugar.
56
APPENDIX G The Use of Gas Chromatography
Upon First Approach:
Open Helium Gas Cylinder. It should be flowing at 20-25 psi for correct flow-through (set at 23 psi). When first opening, the delivery pressure
gauge will slowly drop into the right setting. Also make sure that the bottle pressure actually reports a number. Alert the stockroom for changing
the tank when it gets less than 200psi.
Check that the Helium flow is getting through the GC. Attach the soap bubble meter with water in the bulb to Port B Outlet of the GC- located
on the left side panel of the GC. This should be fairly high flow, so a lot of bubbles will be created. If not, alert the Stockroom. Do not make any
changes without us there.
Check the Septa. If the machine is off, remove the port cap from Port B Inlet of the GC-upper right front. Check the blue Septa for punchthrough due to too many injections. This could be a cause for No Flow coming out of the Port B Outlet (from step 2). The Stockroom will change
this regularly, but if it is in need of a new one, let us know. If the GC is on, grab a glove to remove the Septa because it will be extremely hot!
Turn on the GC. This is the Red Power button. Allow at least 45-60 minutes to reach temperature.
Once Temperature has been reached, switch on the detector. Please Switch off the detector when not in use, or at the end of class.
Do not flip any other switches on the GC, or make any changes to any of the dials or settings Please!
Turn on the Breadboard/Gain circuit on the middle shelf of the GC Cart. It is only one switch, located on the upper right hand corner of the
Analog/Digital Trainer Pad.
Turn on the Computer. Make sure it is connected to an Ethernet data port.
Ready for Data Collection
Log in to the Computer.
Open GC Control.vi from the Desktop.
Run Program by Clicking on the Run Arrow in the top left of the Labview Screen. If the Screen looks like it has Graph Paper for the Background,
then it is NOT in “Run” or “Play” mode. There are also helpful instructions at the top of the screen.
Enter your name and Sample ID (i.e. “Joe Labview 5050sample”) in the Input Prompt. Do not use characters of this type ‘/, ? : ;+ =’ in your
sample name.
Make Sure That the Helium Tank is flowing; GC Power, GC Detector and Breadboard Circuit must also be “ON” (see “First Approach” above).
Prepare your GC Sample for Injection.
Pick up the GC Syringe (5ul) and make sure that it works and is not broken and that the plunger slides smoothly. Take note of where the plunger
measures the amount of sample.
Choose a Standard 50:50::Toluene:Ethyl Acetate for your first run. These should be on the GC Cart.
Pierce the septa with the GC Syringe and extract the plunger to 3.5 ul.
Make Absolutely Sure that you do not pull the plunger past the 5.0uL mark. This will unseat the very thin and delicate wire that sits in the GC
Needle. It is nearly impossible to put back in once it’s out.
Write down in your notes what amount you take and make this a standard sample size for the rest of your data collection. Let us know when we
need to replace the septa for the standards provided.
Inject the sample into Port B of the GC. You do not need to push the needle until it hits something, just far enough to pass the septa. This will
keep you from bending/breaking the Syringe.
Press the Green Start button on the Labview Program when the Plunger has been depressed with your sample.
Let the Program Run until the peak or peaks have collected and returned to the baseline. Stop the program with the Stop button. A collection run
should not last longer than 90 seconds if everything is running smoothly. Alert the Stockroom if it is not. The Run Length has a maximum of 5
minutes before it automatically stops.
Now you are ready for your next run. Enter a new sample name in the Group Name Field.
Repeat until samples have been recorded.
50:50::Toluene:Ethyl Acetate (standard)
Either Pure Toluene OR Pure Ethyl Acetate (standard)
Fraction 1
Fraction 2
Fraction 3
Take note of the Data Path field right above the Chromatographic Field. This is where your data is saved. The beginning of the file name is a
Date/Time stamp followed by the sample name you had given it. This will most likely place your sample at the end of the file directory.
Minimize the Labview Screen and follow the Shortcut to the Data. Scroll to the bottom for your data. It is saved as a .csv file. E-mail yourself a
copy or transfer it to a thumb drive for processing with Excel.
Press exit to Quit the Labview Program and close all other programs and log out of the computer when you are done.
N. Meier
1/19/2011
APPENDIX H
DRYING
57
Drying Solids
The word "dry" is carelessly used in chemistry, sometimes to mean free of solvent, sometimes more precisely to mean free of water. Most solids
that are non-hygroscopic (do not pick up moisture from the air) can be "dried" after crystallization (that is, can be freed of solvents such as
hexane, alcohol, etc.) simply by allowing them to stand in air. The atmosphere is an almost perfect vacuum for such solvents; any molecules that
volatilize are irreversibly lost. Drying solids which have been recrystallized from water is slower, since the atmosphere is moist; solids which are
actually hygroscopic must be dried in a desiccator where an artificially dry atmosphere is maintained by keeping some water-absorbing substance
(e.g., P2O5) in a separate part of the desiccator from the sample. It is useless to try to dry solids that have been recrystallized from organic
solvents in a desiccator containing the usual desiccants. These are usually effective only for water and will not efficiently absorb other solvents.
The time it takes for a recrystallized solid to dry depends on the volatility of the solvent used. Samples recrystallized from low boiling solvents
such as hexane or chloroform may be considered dry after about half an hour if the crystal mass has been adequately exposed to the atmosphere.
Samples recrystallized from water may take several hours to dry. If a sample has to be dried in a hurry, it can be transferred to a round-bottom
flask and placed under vacuum with the flask heated gently on the steam bath or a warm water bath (depending on the melting point of the solid).
Drying Liquids (Solvents or Solutions)
Impure liquids or solutions, and especially solutions that have been in contact with water (as in extraction procedures), may contain dissolved or
dispersed water. The most common method of drying solutions is to add to them an anhydrous, insoluble inorganic salt that adsorbs water to form
a stable, solid hydrate. The hydrated drying agent is then removed by filtration or decantation from the dry solution. Table I lists a number of
useful drying agents; for the reasons listed in the table, sodium and magnesium sulfates are the most useful. Table I gives the formulas of the fully
hydrated salts.
Some important points to consider when using drying agents are:
1. Solute may be adsorbed on the drying agent, and the agent may catalyze reactions (see Table) or in some cases react itself with the solute.
Never leave a solution over a drying agent longer than necessary, and always wash the residual cake of hydrated salt with several small portions
of fresh, dry solvent.
2. The time required for drying a solution depends on the tenacity with which the solvent and solute retain water. It is seless to dry a solution
containing a solvent miscible with water over sodium sulfate or magnesium sulfate. Ether and ethyl acetate, which will dissolve small amounts of
water, should be dried for 10 to 30 minutes; hydrocarbons or chlorinated hydrocarbons can be dried more quickly. Magnesium sulfate changes
from a fluffy powder to a heavy cake on adsorbing water; if fluffy agent persists after a solution has been allowed to stand for ten minutes, it may
be considered dry.
If the drying agent turns into a gummy, semi-liquid mass, this means that the liquid contained a lot of water and more drying agent is needed. If
necessary, the liquid can be decanted onto a fresh portion of drying agent in a new container.
3. Sodium and magnesium sulfate lose their effectiveness when heated much above room temperature because the hydrates are no longer stable
with respect to their components.
58
TABLE I. Drying Agents Usable with Organic Liquids
Agent
Speed
Anhydrous
Sodium Sulfate
Slow
high
weak
Fast
high
weak
Perhaps the best compromise between rapid, efficient drying action
and inertness. Shows a mildly acidic reaction and may occasionally
catalyze the hydrolysis of sensitive compounds. Forms MgSO4.7H2O.
Fast
high
medium
Better drying agent than the above compounds, but moderately acidic
and prone to form complexes with amines and alcohols.
Forms CuSO4.5H2O.
Fast
low
high
medium
high
medium
medium
high
high
Anhydrous
Magnesium
Sulfate
Anhydrous
Cupric Sulfate
Drierite
(Anhydrous
Calcium Sulfate)
Calcium
Chloride
Calcium Oxide,
Barium Oxide
Fast
Sodium Carbonat
Potassium
Carbonate
Fast
Calcium
Hydride
Molecular Sieves
(synthetic
metaloalumino
silicates)
Strength
Comments
The mildest available drying agent, neutral reaction. Forms Na2SO4.10H2O.
Useful, but may catalyze reactions or form complexes. Forms CaSO4.2H2O.
Sodium Hydroxide,
Potassium
Hydroxide
Phosphorous
Pentoxide,
Sulfuric Acid
Capacity
very high
high
high
medium
very fast
High
very high
medium
high
very high
high
high
Fast
Use of this reagent is limited to hydrocarbons and alkyl halides, since
it shows a strong tendency to complex with most oxygen- or
nitrogen-containing organic compounds. Forms CaCl2.6H2O.
May be used only with substances that are stable to strong base. React with
H2O to form Ca(OH)2 or Ba(OH)2.
These are among the most powerful drying agents commonly
available. They can be used only with a few inert solutions that are
stable to base and in which the hydroxides are insoluble.
The remark made for calcium and barium hydroxide applies to a lesser
extent to the carbonates. Form NaCO3.10H2O or K2CO3.2H2O.
These acidic reagents, like the alkali hydroxides, can only be used in
a few isolated cases. They should never be used unless the solution in
question has been shown to be inert to them.
This substance is a very powerful chemical drying agent. It actually
reacts with any active hydrogen (e.g., with that in water, alcohols) liberating
hydrogen gas and forming a calcium salt. It has limited applicability.
These substances have interior channels that will admit and absorb water
molecules but not large substances. They are extremely effective drying
agents for nearly pure solvents. When used with solutions, there is some
danger that the solute may clog the openings to the channels, lowering
drying efficiency.