Organic Chemistry: Techniques and Transformations
LABORATORY 4
Fractional Distillation
Concept goals: Role of fractionating column and when such a column is necessary,
refraction of light by molecules.
Operational goal: Develop the skill to perform a distillation effectively, use refractive
indices to determine the identity of compounds.
Introduction
This laboratory provides practice in separating a homogeneous mixture, or solution, into
its pure components. This separation uses a technique called fractional distillation. This
separation is a physical method, which depends on differences in boiling points of the
two or more components. Fractional distillation is a common practice in industry and in
the laboratory. On a large scale it is used to refine petroleum into the “fractions” used
for gasoline, heating oil, etc. In the laboratory it is employed whenever a mixture of
liquids must be separated. By noting the boiling temperature of the collected fractions
and other physical properties, fractional distillation can also be used to identify the
components in a solution of unknown composition.
You will be assigned an unknown solution containing two pure, liquid components
whose boiling points differ by at least 20 °C. Be sure to record the name or number of
this fractional distillation unknown in both your laboratory notebook and in your final
report. After fractionally distilling the unknown mixture, the data you collect will be
graphed using a microcomputer with appropriate software, e.g., EXCEL™.
You will use the data you collect to plot a graph of Head Temperature in °C vs. Volume
of Distillate collected in mL. Head temperature is the boiling point of the liquid that
condenses at the top of the fractionating column and enters the side arm of the
condenser as a liquid. In a mixture of two components, the low boiling component will
rise to the top of the column first and eventually be displaced by the higher boiling
component. If the fractionating column provides good separation, the low boiling
component will come off the column at a steady, or constant, temperature. Then the
head temperature will rise abruptly, producing a “break” in the curve, and the second,
higher-boiling component will come off the column at a constant temperature.
Each period of steady, constant boiling produces a nearly horizontal “plateau” in the
graph. Projecting this plateau to the Head Temperature axis of the graph gives a good
approximation of the boiling point of that particular component. Thus, you can identify
both the low boiling and high boiling components in your mixture. The position of the
“break” between low and high boiling points can tell you the approximate composition
(relative amounts of each component) of the original mixture.
You will also be asked to determine the refractive indices of the fractions you collect.
These data will help you to better identify the components in your unknown mixture.
Organic Chemistry: Techniques and Transformations
Carefully follow your particular instructor’s requirements for preparing a final report of
your work in the laboratory. Ask your instructor if you have any doubt about his/her
requirements for the report.
Reading and Pre-Lab Assignments
Read the following sections in The Organic Chem Lab Survival Manual: A Student’s
Guide to Techniques by James W. Zubrick, John Wiley & Sons, Inc.
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•
•
•
•
•
Jointware and Clamps and Clamping. These chapters will familiarize you with the
equipment to be used in this experiment, and its care. We have clear glass
joints that do not need to be lubricated. We always use a drop of glycerol on the
rubber part of the thermometer adapter to make it easier and safer to insert and
adjust the thermometer.
Simple Distillation. See the apparatus set-up
Fractional Distillation. See the apparatus set-up
Theory of Distillation. This is not easy reading, however, you should be able to
understand the figures demonstrating the theory.
Refractometry. You will be determining the refractive index of your collected
fractions.
Before you come to the laboratory, do the Pre-Lab assignments for this laboratory as
assigned by your instructor.
Prepare your laboratory notebook as required by your instructor.
Procedure
Read the reading assignments in the Reading section above. Notice that fractional
distillation is just a modification of simple distillation. Read both assignments carefully.
Assemble and setup the fractional distillation apparatus as illustrated in the two figures
with the following modifications. It will not be necessary to use an ice-water bath or the
receiving flask. Instead, replace the receiving flask with a 15-mL graduated centrifuge
tube held in place with a clamp. In this way the volume of the distillate can be
measured in milliliters and different fractions may be easily obtained by changing
collecting tubes.
Begin the assembly of your apparatus by clamping the heating mantle to the large ring
stand about 5 cm from the bench top. Place the 100 mL round bottomed flask into the
well of the heating mantle and clamp it in place. This step is particularly important in
any heating operation. If it should be necessary to remove the heating source, the
mantle can be dropped away from the flask without the whole apparatus falling apart!
Lower the heating mantle and move it to the side. Add 20 ml of the binary liquid
unknown assigned to you into the round-bottomed flask. (Now is a perfect time to
record the number or letter of your unknown solution in your notebook. If you delay as
Organic Chemistry: Techniques and Transformations
long as 30 seconds, you may well have forgotten this important information.) Add 10-20
of the small boiling chips to the flask. Place the heating mantle back under the flask.
Continue with the assembly by adding the fractionating column and securing it with a
clamp. Place the three-way adapter on the top of the fractionating column. All glass
components of your apparatus should be in a straight, vertical line, parallel to the ring
stand rod. A Keck Clamp is used to hold the west condenser to the side arm of the
three way adapter and the condenser is gently clamped in place with a clamp placed
about 2/3 of the way down the condenser and attached to the small ring stand. This is
the tricky part. Be certain that all joints fit well without leaks or strain on the joints or on
the clamps.
Use the last clamp on the small ring stand to position one of the 15 ml centrifuge tubes
just below the exit opening of the condenser. Attach the rubber condenser tubing to
the west condenser so that the water flows in the bottom and out the top.
All lockers should now have a screw cap/ O-ring type of thermometer adapter. Place
the thermometer adapter on the top of the three-way adapter and adjust the
thermometer so that the entire bulb is below the sidearm of the adapter. (If you have
one of the older adapters with a neoprene rubber cap with a hole in it, you will need to
lubricate the thermometer. Place a small drop of glycerol (glycerine) on the rubber
portion of the thermometer adapter and carefully insert the thermometer, keeping your
hands as close together as possible. The glycerol provides an excellent glass-rubber
lubrication and should enable you to adjust the height of the thermometer safely.)
Consult with your instructor about any doubts or confusion you may have.
Recheck all joints to insure a tight fit. When you are satisfied that your setup is correct,
have it approved by your instructor or teaching assistant before you continue.
Plug the powerstat into the outlet. Turn the powerstat on. Adjust the voltage to 45 to
50 volts. Distill the solution at a rate of no more than 5-10 drops of distillate per minute
coming over into centrifuge tube. A slow equilibration is essential to a successful
fractional distillation. For this reason, keep the voltage low at the beginning and increase
it as necessary to maintain a slow, steady distillation. Record the head temperature of
the distillate in °C for about every 0.5 mL of distillate.
The first material to distill will be the low boiling component of your mixture. You
should have a good idea of its identity by noting its distillation temperature. Collect
about 4 to 5 mL of this material as your first fraction. Change centrifuge tubes and
collect a second 5 mL fraction. During this second fraction you should see the
temperature rise somewhat as a mixture of the two components distills. When the
entire lower boiling component has distilled, the temperature will rise to the boiling point
of the second component. You may need to adjust the voltage to 70 volts or higher in
order to provide enough energy to distill the higher boiling material. When the
temperature has stabilized at the boiling point of the higher boiling component, change
sample tubes and collect a third fraction. This should be mostly the higher boiling
component. After you have collected about 15 ml of total distillate, turn the power off,
Organic Chemistry: Techniques and Transformations
lower the heating mantle away from the flask, and allow the apparatus to cool before
taking it apart.
Determine the refractive index of the first and third fractions (the lower and higher
boiling components of your binary mixture). Record your data directly into your
notebook (it is good science to have a “no data on scraps of paper” rule). As a reminder,
use only plastic pipettes in applying sample to the easily scratched prism of the
refractometer. Clean the refractometer between samples by wiping with a Chem wipe.
Use a computer with a program such as Excel™ to generate a fractional distillation curve
for your data. Be certain to use the SCATTER PLOT option so that you have
independent variables for both axes. Plot the Head Temperature (Y-axis) vs. Volume of
Distillate (X-axis) and use the graph to determine the boiling points of the low-boiling
and high-boiling components in the mixture. Before printing your graph be certain that it
has a meaningful title and clearly labeled and appropriately scaled axes.
Compare your measurements with the list below. The list gives the published values for
the boiling points and refractive indices of the known compounds that were used to
make the fractional distillation mixtures. Decide which of the known compounds in the
list have boiling points that most accurately agree with the boiling points of the two
unknown components of your mixture. The refractive index determinations of your low
and high boiling fractions should refine your choices. Follow your instructor’s guidelines
for a laboratory report and be certain that your report includes (1) the letter or number
of your unknown mixture, (2) your measured boiling points and refractive indices of
your unknown components, and (3) the names, structures, literature boiling points, and
refractive indices of the known compounds that most clearly agree with your measured
boiling points.
Table 2.1 – Fractional Distillation Knowns
Known Compounds
Methyl acetate
Acetone
Methanol
2-propanol
1-propanol
2-butanol
2-methyl-1-propanol
1-butanol
Boiling Point, °C Refractive Index, n
56.9
1.3614
58.1
1.3591
64.7
1.3292
78.4
1.3770
97.2
1.3840
98
1.3970
108
1.3960
118
1.3990
20
D
Some Thoughts on the Graphical Presentation of Data
Graphical communication of data is one of those important skills that scientists must
master. In Experiment 2, Fractional Distillation, you collected data, which will help you
to both identify the unknown liquids in your mixture and to see trends in the way a
Organic Chemistry: Techniques and Transformations
fractional distillation proceeds. Examination of these data in a table only reveals part of
the story. A well drawn graph will help you and your audience to see the whole picture.
An EXCEL plot of the data from a distillation experiment is reproduced below. The Excel
plot wizard was used to construct the plot. A scatter plot with points connected by lines
was chosen. Whenever you have variable x and y data, the scatter plot is the only
proper choice. This graph was cut and pasted into a word processing document. You
should be able to do this with your report.
experiment
2
120
100
80
60
Series1
40
20
0
0
2
4
6
8
10
12
14
16
While the graph above does represent a plot of the data, it is badly flawed in several
ways. A graph must always have a meaningful title. The x and y axes must be clearly
labeled. In addition, the graph can be greatly improved by paying attention to matters
of form as well as of substance. Double clicking on various parts of the graph will
enable you to modify the form. In the plot below, the grey background was eliminated,
the data range, was adjusted, and the major and minor ticks and the axes were
improved. The graph was re-sized to emphasize the importance of the y axis.
Fractional Distilation of Binary Mixture BP-9
110
105
100
95
90
85
80
75
70
65
60
55
50
0
1
2
3
4
5
6
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Volume of Distillate, (mL)
12
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16
Organic Chemistry: Techniques and Transformations
Results, Discussion and Conclusion
Write your results, discussion of results and your conclusion. Complete any post-lab
questions.
Abstract
This part should be filled in after the completion of the experiment and analysis of all
data. When submitting the report, the abstract should appear at the beginning of the
report.
Report
Adhere to the format required by your instructor and submit the report on time.
Summary
1. Fractional distillation is a separation technique used to separate two or more liquids
that have boiling points closer than 20oC.
2. There is a difference in temperature between the liquid in the round-bottomed flask
and the vapor on top of the fractionating column (know which has a higher
temperature, the vapor on top of the fractionating column or the liquid in the flask).
3. Refraction is the bending of light rays from the normal path when passing from one
medium to another medium of different optical density.
4. The refractive index is the ratio of the sine of angle of incidence to the sine of angle
of refraction; refractive index, being a ratio has no units.
5. The refractive index can only be determined for a pure compound
Questions
1.
Name one piece of apparatus that is not used in a simple distillation but is used in
a fractional distillation.
2. What is the purpose of a condenser? How does a condenser operate?
3.
What is a boiling chip? How does a boiling chip regulate boiling? When should a
boiling chip be used?
4.
Why should the bulb of the thermometer be placed below the side arm of the
three-way adapter?
5.
What type of glass is used in the laboratory? What property of this glass is useful
for laboratory work?
6.
The temperature in the pot (the round bottom flask) and the head (the three-way
adapter) are different during a distillation of a binary mixture. Is this correct? Why?
7.
Why did your lower boiling point substance “come off” the column prior to your
higher boiling substance? Discuss this in terms of volatility and/or vapor pressure.
8.
Define “azeotrope” and give an example of an azeotrope.
9.
Sketch a graph showing the distillation of an azeotrope containing two pure
substances. Plot the Head Temperature (Y-axis) vs. Volume of Distillate (X-axis).
10. In this digital age, the ability to read a scale is becoming a lost art. Our Abbé
refractometers use just such an old fashioned scale. With a line of people behind you
waiting to use the instrument, you will need to be able to quickly and accurately read
your refractive index. What are the refractive indices, to four decimal places, of the
three samples below?
1.390
1.400
1.370
1.380
1.360
Organic Chemistry: Techniques and Transformations
Temperature/ Composition phase diagrams are important aids in the understanding of
the theory of distillation. Answer these questions after you have read Zubrick’s chapter
on the theory of distillation.
11. Consider an 80:20 mixture of A and B.
140
(a) What would be the boiling point of this mixture?
(b) What would be the composition of the vapors just
above the boiling liquid?
(c) What would be the composition of the initial
distillate collected from a fractional distillation of
this mixture if the apparatus had a separation
efficiency of three theoretical plates?
(d) How many theoretical plates would be needed
in the distillation to be able to separate pure B
from this mixture?
Temperature °C
130
120
110
100
90
80
70
60
%A
0 10 20 30 40 50 60 70 80 90 100
B 100 90 80 70 60 50 40 30 20 10 0
Mole % A and B
12. Below are some real student data for Distillations of 40 mL of mixtures of
cyclohexene (bp 81 °C) and toluene (bp 111 °C) performed in this laboratory in 1997.
ABBY
BRAD
CINDY
DOUG
Which student performed a simple distillation of these substances?
Which student distilled these substances, but had the thermometer placed too high
in the 3-way adapter?
(c) Which student carried out a careful fractional distillation of a mixture of 30 mL
cyclohexene and 10 ml toluene?
(d) Which student carried out a careful fractional distillation of a mixture of 20 mL
cyclohexane and 20 mL toluene?
(a)
(b)
13. One of the important lessons of this experiment (and the mp experiment) is that it is
often necessary to collect multiple types of data before the identification of an unknown
substance can be made with certainty. Consider the real case of Heather who carried
out the fractional distillation experiment in 2003. Heather measured the boiling point of
her third fraction to be between 97 and 98°C. She then took the refractive index and
found it to be 1.3900. It was only when she ran the infrared spectrum of her sample
(see appendix) and found a characteristic C-O-H stretching band at 1120 cm-1 that she
identified her unknown. What was Heather’s high boiling component?