Chemistry 323: Main Group Chemistry Lab Manual
Introduction
Safety in the Undergraduate Teaching Laboratories
Laboratory Procedures
Research Notebooks
Laboratory Reports
Grading Scheme
Order of Experiments
ii
iv
v
vi
vi
vii
Note: Students are to perform all experiments (within each group there may be some choice).
Students will work in pairs, but must submit separate reports and keep separate
lab books.
Group 13
Part 1: Synthesis of an aluminum coordination complex, aluminum
acetylacetonate, Al(C5H7O2)3.
Part 2: Synthesis of a covalent boron compound (pick 1 of two)
Group 14
Part 1: Silicone polymers
Part 2: Transformation of lead compounds
Group 15
Part 1: Synthesis and complexation of diphos
Part 2: Synthesis of Group 15-based complex ions
Group 16
Part 1: Synthesis of Potassium Peroxydisulfate
Part 2: Synthesis of a Heavy Chalcogen Compound (pick 1 of 3)
Group 17
Part 1: Synthesis of a polyhalogen anion containing iodine (pick 1 of 2)
Part 2: Analysis of iodine content by redox titration
i
ii
INTRODUCTION
This course is designed to introduce the student to research techniques in inorganic chemistry. The
experiments to be carried out involve the synthesis of various types of compounds by diverse
experimental techniques. Instrumental methods will be used to characterize the products. The
entrance into these new areas of research requires a greater appreciation of safety hazards related not
only to the chemical properties of reactants but also to the dangers presented by unfamiliar
apparatuses. New fundamental techniques will be encountered, and these must become as second
nature as weighing and pipetting were in prior courses. Finally, the research orientation of the course
requires careful record-keeping by the researcher. This first chapter will expand upon certain aspects
of (1) safety, (2) basic laboratory procedures, and (3) methods of keeping a research notebook. It
is important that you become well acquainted with this material before beginning the experiments.
SAFETY IN THE UNDERGRADUATE TEACHING LABORATORIES
These notes are directed to all users of undergraduate chemistry teaching laboratories.
General Guidelines for Safety
1.
No undergraduate may perform an experiment to which the student has not been specifically
assigned. Other than in project courses, no undergraduate experiment of any kind may be
performed in the absence of an instructor, demonstrator or technician.
2.
Learn the location of escape routes and of all safety equipment (showers, eye wash station,
fire extinguishers, fire alarm, telephone, etc.) before you start to work in any room. Know
how to use the equipment.
3.
Smoking, eating or drinking is not permitted. Nothing should be placed in the mouth.
Pipetting by mouth is absolutely forbidden.
4.
Regard all chemicals as potentially hazardous. Treat with special caution those chemicals that
the laboratory manual cites as toxic, poisonous or otherwise dangerous. Do not attempt to
clean up any spills yourself - inform the demonstrator of the problem as soon as possible.
5.
Compressed gas cylinders should always be securely anchored to a wall or heavy bench. If
a large cylinder tips over and the valve snaps off, the cylinder becomes a jet-propelled missile
which has sufficient power to penetrate a brick laboratory wall.
6.
If you are in doubt as to the safety of a procedure, don't do it until you have sought
professional advice.
7.
All accidents, however minor, must be reported to the person in charge of the lab section
immediately.
iii
8.
Practice good housekeeping - a clean work space is much safer than a messy one. The
dangers of spilled acids and chemicals and broken glassware created by thoughtless actions
are too great to be tolerated. Clean up your work space, including wiping the surface and
putting away all chemicals and equipment, at the end of the laboratory period.
Some Additional Good Practice Rules
1.
Carefully read the experiment before coming to the laboratory. An unprepared student is a
hazard to everyone in the room.
2.
Dispose of excess reagents as directed by your instructor or laboratory demonstrator.
Never return reagent to bottles.
3.
Always pour acids into water when mixing. Otherwise the acid can spatter, often quite
violently.
4.
Avoid breathing fumes of any kind.
(a)
To test the smell of a vapour, collect some in a cupped hand.
(b)
Work in a hood if there is the possibility that noxious or poisonous vapours may be
produced.
5.
Be careful when heating liquids. Flammable liquids such as ethers, hydrocarbons, alcohols,
acetone, and carbon disulfide must never be heated over an open flame.
6.
Test tubes being heated or containing reacting mixtures should never be pointed at anyone.
If you observe this practice in a neighbour speak to them or the instructor.
7.
Do not force rubber stoppers onto glass tubing or thermometers. Lubricate the tubing and the
stopper with glycerol or water.
8.
Finally, and most importantly, THINK about what you’re doing. Plan ahead. If you give no
thought to what you are doing, you predispose yourself to an accident.
Personal Protective Equipment
1.
Eye protection must be worn at all times in the undergraduate teaching labs. Adequate eye
protection consists of impact resistant safety glasses or goggles which have side shields and
a top flange in contact with the forehead. People who normally wear prescription glasses will
be required to purchase detachable side shields to be worn in the lab. If the prescription
glasses are deemed to be inadequate for satisfactory eye protection (due to small lens size),
safety glasses or goggles which can be worn over the prescription glasses will be required.
2.
Contact lenses MUST NOT be worn into a chemistry lab under any circumstances.
iv
3.
Adequate footwear which fully covers the feet must be worn. Sandals and open-toed shoes
are not acceptable. Wearing high heels or shoes with no treads is discouraged.
4.
Lab coats are required when working in the chemistry labs to prevent exposure to corrosive
or otherwise harmful chemicals. Lab coats should not be worn outside of the labs, especially
in areas where food is prepared or consumed, since they could contaminate areas not normally
exposed to chemicals.
5.
Long hair and bangs must be tied back.
LABORATORY PROCEDURES
Use of time
Plan your experiments so that you will profitably use time that would otherwise be spent
watching, e.g., a distillation, a sublimination, or a nonhazardous reaction that need not be
attended. This course allows some latitude in the planning of experiments, and you should
be looking for opportunities to use the available time effectively.
Cleanliness
Since most of the experiments will involve the use of equipment that other students will use
sometime during the course, it is essential that all equipment be left in good condition at the
end of each period. Any equipment that is broken should be reported to the instructor
immediately so that a replacement may be found in time for the next class.
Glassware that is difficult to clean with a detergent are more conveniently cleaned by pouring
a few milliliters of concentrated H2SO4 into the soiled flask, followed by an equal volume of
30 per cent H2O2 and then by swirling the mixture. This is a very strong oxidizing mixture,
which rapidly cleans almost any glassware. Needless to say, the H2SO4-H2O2 solution
dissolves clothing and produces severe skin burns; it should be handled with rubber or
polyethylene gloves.
Performance of experiments
The laboratory programme is a very important component of the Chemistry 322 course and
it is imperative that students be well prepared prior to coming into the laboratory. Many
experiments will require more than one week to complete and it will be necessary to find
appropriate places in the procedures where experiments can be left safely until the next
laboratory period.
Experiments should be signed out a week in advance on the sheet provided in the laboratory.
Note that it will be necessary to check the sign-up sheet to find out which experiments will
be available. Some experiments may require the student to come in outside the lab time for
a brief period. Those will be set up on an appointment basis agreeable to both the student and
the lab instructor.
v
Often times, in a research laboratory, quality is more important than quantity. For this reason,
students are expected to show their products to the demonstrator for inspection. As many
of these products are air sensitive, the sooner after isolation it is seen, the better.
Additionally, some labs call for recystallisation of the product. A trick that has saved
numerous students is to set aside a small portion of the unrecrystallised product in case this
step goes awry. Occasionally, recrystallisation actually give an inferior looking product!
Except in exceptional circumstances (e.g., illness), attendance at the laboratory sessions is
mandatory (until all experiments are completed).
RESEARCH NOTEBOOK
The communication of scientific facts and experimental results is an important duty of the scientist.
Without it, little would be gained from the scientist's efforts. The first step in the communication
chain is the accurate and detailed recording of experimental facts in a bound notebook. The purpose
of this record is to allow you or someone else to learn from what you did in the experiment and to
help you or them to repeat your success or avoid your failure. Detailed information about a synthesis
or measurement is much appreciated by someone wishing to repeat your experiment. Your notebook
records of your experiments should include more than enough detail to allow you or someone else
to repeat the experiment successfully. It is much better to be overly detailed than to overlook
observations that may be of use later.
Your notebook may contain drawings of experimental apparatus (or a reference to a figure in this
manual). It should also contain experimental observations such as colour changes, temperatures of
reaction mixtures, difficulties encountered, weighings, measurements, and cross references to spectra
(label spectra with notebook page number on which the compound preparation is given). All of these
experimental details should be recorded in the notebook at the time of the observation. Data are not
to be first written on loose paper. This rule was not instituted by a cranky teacher; it is simply a
waste of time to record observations and then recopy them into a notebook. Needless to say, if this
rule is followed, your research notebook will not be a work of art in neatness, but it should be
readable. Since water and acid may spill on the notebook, it is important that your records be kept
with permanent ink. Mistakes are simply crossed out.
Information specifically requested at the end of each experiment should also be included at the end
of the experimental observations. Each page of the notebook should be numbered and dated to
indicate the day that the experiment was performed. To facilitate referring to experiments, the first
two or three pages in the notebook should be left blank for a table of contents. As you complete an
experiment, its title and page should be recorded in this table of contents.
Your grade for the notebook will be based on completeness of experimental information, as well as
point-form answers to any experiment questions which you did not answer in a formal report.
vi
LABORATORY REPORTS
The lab due dates are listed on the class calendar, which will be handed out in class. Each person is
responsible for 2 formal reports. The writeups should follow the general form of a journal article.
As such, the following sections should be used:
Abstract – this is a specific statement of the nature of the experiment, stating what was learned in
a succinct manner. It should be no longer than 5-8 lines.
Introduction – this is a general introduction, which would include background theory.
Experimental – this should be in journal form, which assumes the reader has a fundamental
knowledge of equipment and techniques (e.g., the term “distillation” need not be defined, and
a distillation aparatus should not be drawn in the formal report). Therefore, it can be quite
brief, but must give all necessary information (masses, melting points, observed spectral lines
in IR, etc.). This could also go at the end (immediately before the references) of the paper.
Results and Discussion – this is the bulk of the paper, and can be divided up into subsections as
required. For example, there is often a section titled “synthesis,” which can expand on the
information in the experimental section. This is normally where explanations of unusual
techniques, sources of experimental error, etc. would be discussed. Any questions that are
to be answered as listed in the lab manual would generally go in this section (or the intro) –
these should be worked into the discussion and not written in point form.
References – all literature material must be referenced. They can either be reported as footnotes or
endnotes.
The labs will be marked out of 20 points, with the following breakdown:
Abstract: 2 points
Intro/Discussion (including answers to the questions): 10 points
Experimental: 4 points
Overall presentation (including references, grammer, etc.): 4 points
GRADING SCHEME
The lab is worth 25% of the total mark in the course. While the information in the labs will not be
specifically tested on exams, the underlying theory ties in with the course work.
20%
5%
formal lab reports (two, 10% each)
research notebook, including answers to all discussion questions not formally reported.
NOTE: Although the experiments will be done in pairs, each student must submit their own formal
reports and lab book. It is expected that these reports will be done independently.
ORDER OF EXPERIMENTS
These experiments do not directly follow the course material, but are instead designed to give a
vii
flavour of experimental techniques. As such, the experiments can be done in any order. There will
be a sign-up sheet in the lab. Since there is a limited number of sets of apparatus, you will have to
book the week before. In addition, this booking system will ensure that all the chemicals and
glassware you need will be available at the start of the laboratory period.
The experimental groups represent the groups on the periodic table. Some groups have choice, while
others do not.
Compounds of the Nobel Gases are highly explosive and usually require highly specialised equipment
to synthesise (i.e., stainless steel reactor vessels, very high pressures, etc.), so we will not be making
such compounds in this course. In addition, the s-block elements will not be specifically investigated,
but are normally present as cations in many of the compounds that will be synthesised, and thus will
be studied incidentally.
The Chemistry of Group 13
Boron is the only true non-metal in Group 13. Aluminum is often considered a metalloid, but has
most of the classical characteristics of a metal (conductivity, maleability, lustre). The heavier
elements are undoubtedly metals (gallium through thallium), but they have some significant
differences to the transition metals, most noticably the stability (in In and Tl) of the +1 oxidation
state.
The lighter metals (Ga and Al) invariably exist in the +3 oxidation state. Boron, on the other hand,
has access to both the +3 and the -1 formal oxidation states, although these are normally in covalent
compounds, not ionic. The most obvious characteristic of the lighter Group 13 elements is the low
valency - neutral compounds would contain only 3 bonds to the Group 13 element. Thus, most
simple, binary compounds of this group are Lewis acids, seeking to fulfill an octet about the central
atom by binding to an electron pair of a Lewis base. In the case of boron, this has led to a very rich
and diverse cluster chemistry.
Aluminum and galluim can often form simple anions such as AlF4- and GaCl4-, and also some
hypervalent or more complex ions such as AlF63- and Ga2Cl7-.
In this experiment, you will synthesise an aluminum coordination compound and a covalent
(molecular) boron compound, to emphasise the difference in reactivity between B and Al. The
heavier elements of Group 13 are either too expensive (Ga) or highly toxic (In, Tl), and will not be
used in this laboratory.
Part 1. Synthesis of an aluminum coordination complex, aluminum acetylacetonate,
Al(C5H7O2)3.
Preamble: Acetylacetonate, commonly abbreciated “acac,” is one of the most common
inorganic ligands. Acetylacetone has one acidic proton, which is removed on addition of base
to give the monoanionic acac ligand, C5H7O2-. In this synthesis, you will demonstrate the
metallic properties of aluminum by synthesising a classic coordination complex.
To 20 mL of water, add 3.0 g of acetylacetone. This will form a bilayer. Add 6 N NH3
solution dropwise (hood), with swirling, until the acetylacetone dissolves (this forms acac).
This solution is added to a solution of 3.0 g of aluminum sulfate heptadecahydrate,
Al2(SO4)3.17H2O, in 30 mL of water. The product should precipitate immediately from the
neutral (pH 7) solution.
Suction filter the crude product, washing with water. Dissolve the crude material in toluene,
and reprecipitate with addition of petroleum ether. Suction filter and wash with pet. ether.
Check the purity of your product by taking its melting point.
13-1
Questions:
1.
Draw the structure of acetylacetone and point out the most acidic hydrogen.
2.
Report your melting point and compare with the literature value.
3.
Suggest a method to further purify the product.
Reference:
Young, R.C. Inorg. Synth. 1946, 2, 25.
Part 2. Synthesis of a Boron Compound
In this part of the experiment, you will synthesise a molecular compound containing boron. Do either
Part A or Part B.
Part A. Synthesis of 1,3-dimethyl-1,3-diaza-2-boracyclopentane
Preamble: N,N’-dimethylethylenediamine is also a ligand for the formation of metal
complexes. When reacted with boron compounds, however, the result is a covalent bond
between N and B.
Synthesis: Set up a 2-neck 100 mL round-bottomed flask with a reflux condenser attached
to one neck (hood). Attach a drying tube to the top of the condenser, and a nitrogen line to
the other neck of the rbf. Dry the apparatus by heating with a bunson burner (with N2 flowing
through the system). While continuing to pass N2 gas, add 3.6 g of trimethylamine-borane
adduct, 4.3 g of N,N’-dimethylethylenediamine, and a few boiling chips. Reflux for the rest
of the lab period (at least 2 hours). Store the crude material in a sealed vessel under nitrogen
In the next lab period, flame dry a distillation apparatus and attach a drying tube (hood).
Under a static pressure of N2, distill the oil (around 90°C). Run an infrared spectrum on the
resultant oil to check the purity.
Questions:
1.
Draw the structure of 1,3-dimethyl-1,3-diaza-2-boracyclopentane. Are there any
possible resonance structures wherein the octet of B is satisfied?
2.
Compare your IR spectrum with that in the literature. Comment on the purity of your
product.
Reference:
Merriam, J.S.; Niedenzu, K. Inorg. Synth. 1977, 17, 164.
13-2
Part B. Synthesis of borane-t-butylamine complex
Preamble: One of the most common reactions of boron to the reaction between the Lewis
base BR3 and a Lewis acid such as NR’3 or PR’3. As many compounds BR3 are gaseous and
poisonous (and therefore difficult to work with), this experiment will use a Brønsted-Lowry
acid-base reaction between an ammonium acid and a borane base to form a R3B-N3 adduct.
Synthesis of tert-butylammonium chloride, (CH3)3CNH3+Cl-: Dissolve 2.5 mL of tertbutylamine in 15 mL of anhydrous diethyl ether (hood). Add 2M HCl in diethyl ether
gradually until precipitation is complete. Suction filter the product, wash with a little ether,
and air dry on the frit. If storing for longer than a couple of hours, seal it in a bottle filled with
N2 gas.
Synthesis of the tert-butlyamine-boron adduct, (CH3)3CNH2-BH3: To a round-bottomed flask
add 1.3 g of your tert-butylammonium chloride and 15 mL of dry tetrahydrofuran (THF). Stir
the suspension with a magnetic stirrer, and add 0.20 mL of sodium borohydride, NaBH4
(hood). Place a drying tube over the neck of the flask. If the solution stops stirring, add a
further 10-15 mL of THF. Note: H2 gas is released! Continue stirring for 2 hours.
After the reaction has completed, suction filter the solution to remove NaCl and any
unreacted (CH3)3CNH3+Cl-. Rotovap the THF solution to dryness to isolate your adduct
product. Recrystallise the crude material by dissolving in a minimum of toluene (1-2 mL) and
adding hexane until it precipitates. Determine the melting point.
Questions:
1.
Report your melting point. Comment on the purity of your product based on the
melting point.
2.
Using valence bond theory (Lewis structure), determine the direction of polarity of
the N-B bond. Carry out the same analysis using molecular orbital theory. Do the
two theories give you the same answer?
Reference:
Angelici, Robert J. Synthesis and Technique in Inorganic Chemistry. 2nd edition, p. 202.
Philadelphia: Saunders. 1977.
13-3
The Chemistry of Group 14
Carbon is well known for its ability to catenate; that is, to form long chains with many carbon-carbon
bonds. The other non-metals in Group 14, also have this ability. Although the expense of germanium
has limited its investigation, molecules containing catenated silicon are common. Even more common
are the silicone materials, made up of polymers of chains of Si-O units. In Part 1 of this lab, you will
synthesise and characterise two silicon polymers, one with a linear chain that is later converted into
the second polymer, a cross-linked one.
The metallic elements of Group 14, tin and lead, are now best known for their toxicity. The wealth
of toxicity data arises from the fact that both metals have highly useful properties and are easy to
isolate. They were readily available to even rudimentary civilisations. Tin compounds are often more
dangerous, due to their higher volatility. Thus, in Part 2 of this experiment, you will investigate some
lead compounds, on a small scale so as to limit waste.
Part 1: Synthesis and Characterisation of Two Silicone Polymers
Preamble: Dichlorodimethylsilane is toxic and volatile, so care must be taken when using this
material. In addition, the compound readily hydrolyses in the air with the formation of (highly
favourable) Si-O bonds, releasing HCl gas. In fact, we will use this reaction to form our
silicon compounds, but we do not want to waste material in this way by leaving the cap off
the bottle! The silicone oil is a linear polymer and is a liquid. The cross-linked polymer is
much more viscous and has different properties than linear polymer
Synthesis of silicone oil: Mix 20 mL of dichlorodimethylsilane with 40 mL of diethyl ether.
Add 40 mL of water (dropwise) to the solution with stirring. Allow the layers to separate and
wash the ether layer (containing your product) with dilute (10%) sodium bicarbonate solution
until the washes have neutral pH. Then wash once with water and allow it to dry over
magnesium sulfate. Decant and distill off the ether (rotovap) and record the yield and IR of
the resultant oil.
Synthesis of cross-linked silicone, “silly putty”: After performing all the necessary
experiments above, place the oil in an evaporating dish (7 cm) with 1-5% by weight boric
oxide. You may also add some celite (no more that 50% by weight) for filler. Heat this
mixture, stirring occasionally with a glass rod, to 200°C for 20 minutes. Use a heating
mantle, not a bunsen burner.
Questions:
1.
Report the spectrum of your oil and silly putty. Use a KBr disc for the silly putty, not
nujol. Point out any differences between them.
2.
Why can’t you use nujol when running the infrared spectrum of a silicone polymer?
14-1
Part 2: Some Lead Chemistry
Preamble: This experiment is cyclical, in that at the end, you will regenerate the starting
material. Thus the expense (and environmental difficulties) of disposing of lead-contaminated
waste is minimised. Both lead and formic acid are toxic substances, so this Part should be
performed entirely in the fumehood. Be sure to dispose of all waste products (including
decanted solutions, washings, etc.) in the lead waste container.
Preparation and characterisation of lead(II) formate: In a medium-sized test tube, place 0.5
g of lead(II) acetate, Pb(CH3CO2)2.3H2O, and 3 mL of water. Immerse in a warm water bath
(60-80°C) and stir with a glass rod until the solid dissolves. The clear solution is treated with
1 mL of 99% formic acid. A white precipitate should form immediately. Cool the solution
in an ice bath for at least 3 minutes, to allow full crystallisation of the lead formate. Decant
off the mother liquor (dispose in the lead waste) and wash the crystals twice with 3 mL of
acetone, decanting off the wash each time. Be sure to stir the mixture with your stir rod while
washing to ensure full removal of residual acid or water. Place in an oven (60-80°C) for 15
minutes to dry. Record your yield.
Test the product using drop tests. Test for formate ion with permanganate solution. Place
2 drops of acetic acid and 1 drop of 0.1 M KMnO4 on a watch glass. Addition of a small
amount of solid lead formate should decolourise the solution upon mixing. Try the test of
lead(II) acetate as well.
Test for the presence of lead using the lead iodide test. Dissolve 10 mg of lead formate in 1
mL of water and add a few drops of 1 M sodium iodide (make this solution yourself, but as
little as possible - 1 mL is more than enough). A colour change and precipitate indicates the
presence of lead. Repeat the test with 1 M H2SO4 to further confirm the presence of lead.
Preparation of elemental lead from lead(II) formate: Take half of your remaining lead
acetate (after doing the tests above), which should be at least 100 mg, and place in a test tube.
The other half will be used below (these two parts, preparation of Pb and preparation of PbO,
below, should be done simultaneously). Be sure to record the mass of the tube and solid
together. Place a glass wool plug over the open end of the tube. Then, place the tube in the
tube furnace for 5 minutes (no longer!) at 300°C. Remove from the furnace, allow to cool,
remove the glass wool plug, and reweigh the tube. Be sure to note the colour and consistency
of the solid product.
Preparation of lead(II) oxide from lead(II) formate: Take the other half of your lead(II)
formate (that not used above) and place in another test tube. Be sure to measure the mass
of both the tube and the material together. Heat this tube for at least 60 minutes in the tube
furnace at 300°C. Be sure to record the appearance of the product at the end of the heating
time.
14-2
Reconversion of lead to lead(II) acetate starting material: Return your test tube containing
elemental lead to the furnace, leaving it in for at least 60 minutes. Combine this solid with the
solid obtained from the lead(II) oxide formation. Dissolve completely in 4 M acetic acid (this
will require heating). Allow the acetic acid to evaporate (by leaving on a watch glass over
a week in the fumehood). Test that you have recovered lead(II) acetate by comparing the IR
spectra of it with the commercial acetate starting material.
Questions:
1.
What are the substances created in the two tests for lead that are performed on the
lead(II) formate you produced? Why do these materials precipitate?
2.
Why do you need glass wool covering the tube mouth in the preparation of elemental
lead?
3.
Commonly, in the preparation of elemental Pb, the measured yield exceeds the
calculated yield. Suggest a reason for this.
4.
Report the IR of commercial Pb(CH3CO2)2 (recall it’s a trihydrate) and the
Pb(CH3CO2)2 that you synthesised. Comment on any differences.
Reference:
Arnàiz, F.J.; Pedrosa, M.R. J. Chem. Educ. 1999, 76, 1687.
14-3
The Chemistry of Group 15
The Group 15 elements can be known collectively as the pnicogens (from the Greek word meaning
to choke or stifle, referring to the fact that nitrogen is an inert asphyxiant). This term is not IUPAC
approved, but is sometimes used in the chemical literature.
Because of the odd number of electrons in the neutral atom, the pnicogens often have odd numbered
oxidation states (+3 and +5 being the most common for P and lower). Additionally, many compound
ions exist, such as NH4+, PF6-, and SbF6-. In the neutral compounds, the most common valencies are
coordination numbers 3 and 5. In the case of C.N. 3, there will also be a lone pair; thus, many of
these compounds are Lewis bases.
Part 1 of this experiment is the synthesis of a common ligand in transition metal chemistry. Because
of the lone pairs on phosphorus in this compound, they can act in a Lewis base sense by binding to
a positive metal to make a metal complex. In this course, we are interested in the ligand primarily.
In Part 2, the syntheses of a number of salts containing ions of Group 15 are given. A complete
experiment will consist of completion of Part 1 and one of Compounds A or B in Part 2.
Part 1: Synthesis of Diphos and a Nickel Complex of Diphos
Preamble: This synthesis involves the use of liquid ammonia as a solvent and also to solvate
electrons. The intense blue colour is consistent with electrons in solution. Free electrons are
required to cleave a phenyl group from PPh2 to create PPh2-, which then reacts with
dichloroethane to create the desired product, diphos, as shown,
2 e- + PPh3 6 PPh2- + Ph2 PPh2 + ClCH2CH2Cl 6 Ph2PCH2CH2PPh2 + 2 Cl-
There are two sites of binding on diphos (i.e., it is dibasic, because of the lone pairs on the
phosphorus atoms), and usually both these phosphorus centres would bind to the same atom,
as in the case of the nickel complex you will synthesise
Synthesis of 1,2-bis(diphenylphosphino)ethane, diphos, Ph 2P-CH2CH2-PPh2: Weigh out 11.5 g of sodium metal into a beaker containing 30 mL of hexane. The purpose of the hexane
is to wash off the mineral oil from the sodium. Also weigh out 5.7 g of triphenylphosphine.
Set up the ammonia condensor in the fumehood as directed by the lab demonstrator. Be sure
you add a magnetic stirrer to the flask! Condense in 75 mL of ammonia. Cut up the sodium
into 10-20 small chunks and add slowly to the ammonia while stirring the solution. Do not
add it too fast or it will boil over. Once all the sodium is added, add the triphenylphosphine,
again taking care not to boil over.
Make a solution of 6 g of dichloroethane in 1 mL of ether. Add this dropwise to the stirred
15-1
solution. Once all the dichloroethane is added, remove the ammonia condensor and allow the
ammonia to evaporate. This could take some time (say, half an hour).
Once the flask reaches room temperature, add 25 mL of water and shake the flask. Filter the
solid in a Büchner funnel and wash with a further 25 mL of water and 4 times with 2 mL of
methanol. Recrystallise the crude product in 150 mL of 1-propanol. Filter and air-dry on a
Büchner funnel.
Synthesis of [1,2-bis(diphenylphosphino)ethane]nickel(II) chloride, [(Ph2P)2C2H4]NiCl2:
In 10 mL of warm 1-propanol, dissolve 0.16 g NiCl2.6H2O, adding methanol until the solid
goes into solution. Add 0.25 g of your recrystallised diphos. Filter the resultant crystals and
air dry on a Büchner funnel.
Questions:
1.
Record the IR of both diphos and the nickel complex. Point out any differences to
verify that the diphos has complexed with the nickel.
2.
What is the fate of the Ph- produced in the liquid ammonia reduction?
Part 2: Synthesis of Group 15 ions
In this part of the experiment, there is one compound with an ion for each of phosphorus, arsenic, and
antimony. Because Sb has more metallic character that As or P, it makes a cation instead of an anion.
Compound A: Orthoarsenic Acid and Ammonium Orthoarsenate, (NH4)3AsO4.3H2O
Synthesis of orthoarsenic acid. This experiment is carried out in a fume hood. Place 10 mL
of concentrated nitric acid in a dropping funnel, and add (dropwise) to 10 g of solid
arsenic(III) oxide. Evolution of nitrogen oxides will occur. Heat the solution (hotplate) with
stirring until the evolution of gas ceases. Decant off the solution and evaporate to dryness.
After evaporation, add a minimum of water (filtering if necessary, through a glass frit), and
heat the solution until the temperature reaches 130°C. Allow the solution to cool - it should
be supersaturated with a honeylike consistency. Store half in a desiccator in the fridge at least
overnight. The product will crystallise out during that time. The other half will be used in
the next step.
Synthesis of ammonium orthoarsenate. Bubble ammonia gas through the half of the solution
you saved from above. White crystals of the product should form immediately. Filter on a
glass frit.
Questions:
1.
It is possible to isolate other products from the saturated arsenic acid solution. What
are they and how are they isolated?
15-2
Reference:
Handbook of Preparative Inorganic Chemistry, Vol. 1, 2nd Edition, Georg Brauer, editor,
p. 602. New York: Academic Press. 1963.
Compound B: Antimony(III) Sulphate, Sb 2(SO4)3 and Antimony Oxysulfate, (SbO)2SO4
Synthesis of antimony(III) sulphate. Dissolve 10 g of antimony(III) oxide in 50 mL of hot,
concentrated sulfuric acid. When dissolved, allow to cool. Wash the crystals that precipitate
with xylene followed by ether.
Synthesis of antimony oxysulphate. Take a portion of the above product and add to cold
water. Stir and filter the solid. Dry at 100°C. Take an IR of both products to show that
conversion from the Sb3+ to SbO+ has occurred. Use a KBr pellet.
Questions:
1.
Would bismuth be more or less likely to form cationic salts than antimony? Why?
Reference:
Advanced Practical Inorganic Chemistry, D.M. Adams and J.B. Raynor, p. 42. London:
Wiley. 1965.
Note: This book is not owned by the Trent Library - I have photocopied the relevant pages
and made them available on reserve.
15-3
The Chemistry of Group 16
The elements of Group 16 are known as the chalcogens (from the Greek word for bronze, due to
the affinity of sulfur for soft metals such as copper). The four non-radioactive chalcogens (O, S,
Se, Te) are highly variable in their physical and chemical properties. In spite of this, consecutive
members of the group. Both O and S are strictly non-metals, with several allotropic forms. The
heavier Se and Te have considerable metallic character in certain situations, but will also form
small ions and molecules much the same as normal non-metals.
It is important to note that selenium and tellurium are highly toxic, both in the native form and
in low oxidation states, and must be treated as heavy metals such as mercury or cadmium.
Certain forms have the ability to penetrate the skin, and thus any spill on the body must be
immediately washed with copious amounts of soap and water. Higher oxidation states (say, +4
or greater) are much less toxic, but still should be handled with care. The penalty for
carelessness for a Te or Se chemist is a severely restricted social life for several months, as the
metabolites used by the body to eliminate these metals (through the sweat glands and
respiration) smell like rotting garlic.
All members of the chalcogens exist in multiple oxidation states, the most common being even
numbers (-2 to +6). All members except oxygen commonly exist with valencies up to 6.
In this experiment, you will synthesise a sulfur compound in Part 1, using an electrolytic cell. In
Part 2, you will synthesise a compound containing either tellurium or selenium depending on
your choice.
Part 1: Synthesis of Potassium Peroxydisulfate, K2S2O8
Preamble: This synthesis involves use of an electrolytic cell. This cell provides
electrons and an appropriate voltage to effect a redox reaction that would otherwise be
unfavourable. A common example of this type of synthesis is the isolation of NaOH, Cl2,
and H2 by the application of current to sea water.
The Electrolysis Cell: The cell has been prepared for you. It consists of an anode made
by sealing a platinum wire into 6 mm glass tubing. The length of anode that is in contact
with the solution is about 5 cm. The cathode is a Pt wire wound around the glass tubing.
The electrode assembly is inserted into a cork or rubber stopper that either contains a
hole or is loosely fitted into the approximately 2 x 20 cm test tube. These measures
allow gaseous reaction products to escape from the system. An adjustable power supply
conveniently provides the 1.0 amp/cm2 current density required for the K2S2O8
preparation. Note that this amperage level is dangerous, and all electrode connections
should be made with care.
Synthesis of the potassium peroxydisulfate: Prepare a saturated solution of KHSO4 by
saturating a solution of 150 mL of water and 60 mL of concentrated H2SO4 with K2SO4.
About 40 g of K2SO4 will be required to prepare the solution. Cool the solution to 0°C in
16-1
an ice bath to ensure that precipitation of excess K2SO4 is complete. Pour the supernatant
solution into the electrolysis cell and immerse the cell in an ice bath. Turn on the power
supply (record the time and amperage) and adjust the amperage until the anode current
density is 1 amp/cm2. The amperage required will be determined by the area of the
anode, as noted in the earlier discussion. Use the amperage that is required by the area of
your anode to achieve a 1 amp/cm2 current density. Allow the current to flow for 30 to
45 minutes, during which time white crystals of K2S2O8 collect on the bottom of the tube.
The reaction will slow considerably toward the end of this period owing to depletion of
HSO4-. The resistance of the solution to the current will generate sufficient heat to
require replenishing the ice in the bath during electrolysis.
After the reaction period, turn off the power supply and record the time. Suction-filter
the K2S2O8 crystals and wash them on the frit, first with 95 per cent ethanol and finally
with diethyl ether. In each washing stage, stop the suction, fill the fritted funnel with
solvent, stir the contents thoroughly, then suction filter. This will ensure that all H2O is
removed from the product. Determine the yield.
Questions:
1.
Write a balanced chemical equation for this reaction and calculate the percent
yield.
2.
From the amperage and time, calculate the current efficiency.
3.
Draw a Lewis diagram (dot formula) for the S2O82- ion.
4.
What is overvoltage? Use this concept to explain why the oxidation of water does
not occur in preference to oxidation of the hydrogensulfate anion at the anode.
Reference:
Angelici, Robert J. Synthesis and Technique in Inorganic Chemistry. 2nd edition, p. 162.
Philadelphia: Saunders. 1977.
Part 2: Synthesis of a Selenium- or Tellurium-Based Anion
Synthesise one of compounds A, B, or C.
Compound A: Sodium Selenopentathionate, Na2SeS4O6.3H2O
Prepare a salt-ice bath. Dissolve 1.7 g of SeO2 in 2 mL of water and 10 mL of glacial
acetic acid. Cool in the salt-ice bath to a temperature below 0°C.
Dissolve 13 g of Na2S2O3.5H2O in 4 mL of water (this will probably require heating the
solution to dissolve all the thiosulphate; if so, cool to room temperature after dissolution
before proceeding). Add this solution to a dropping funnel and add over the course of 20
minutes, with stirring, to the SeO2 solution. Do not allow the reaction mixture to exceed
0°C!!! Do not allow the addition to proceed any more quickly, as the product will
16-2
decompose in a local excess of thiosulfate anion. After the addition is complete, add 15
mL of ethanol. Once crystallisation starts, add 5 mL of ether and cool on an ice bath for
15 minutes. Suction filter the crude product.
To recrystallise, dissolve in a minimum (~5 mL) of 0.2 M HCl, and gravity filter to
remove any insoluble particulates (your product will go into solution). Then add 10 mL
of methanol and cool on ice to crystallise.
Questions:
1.
Write a balanced chemical reaction and calculate the percent yeild.
2.
How would you remove the water of crystalisation if you wanted anhydrous
sodium selenopentathionate?
Reference:
Handbook of Preparative Inorganic Chemistry, Vol. 1, 2nd Edition, Georg Brauer, editor,
p. 434. New York: Academic Press. 1963.
Compound B: Sodium Telluropentathionate, Na2TeS4O6.2H2O
Prepare a salt-ice bath. Dissolve 1.9 g of TeO2 in 7.5 mL of conc. HCl and 7.5 mL of
glacial acetic acid. Cool in the salt-ice bath to a temperature below 0°C.
Dissolve 11 g of Na2S2O3.5H2O in 6 mL of water (this will probably require heating the
solution to dissolve all the thiosulphate; if so, cool to room temperature after dissolution
before proceeding). Add this solution to a dropping funnel and add over the course of 15
minutes, with stirring, to the TeO2 solution. Do not allow the reaction mixture to exceed
0°C!!! Do not allow the addition to proceed any more quickly, as the product will
decompose in a local excess of thiosulfate anion. After the addition is complete, add 15
mL of ethanol and cool on an ice bath for 15 minutes. Suction filter the crude product.
To recrystallise, dissolve in a minimum (~6 mL) of 0.2 M HCl, and gravity filter to
remove any insoluble particulates (your product will go into solution). Then add 10 mL
of methanol and cool on ice to crystallise.
Questions:
1.
Write a balanced chemical reaction and calculate the percent yeild.
2.
How would you remove the water of crystalisation if you wanted anhydrous
sodium telluropentathionate?
Reference:
Handbook of Preparative Inorganic Chemistry, Vol. 1, 2nd Edition, Georg Brauer, editor,
p. 454. New York: Academic Press. 1963.
16-3
Compound C: Telluric Acid, H6TeO6
Dissolve 1.25 g of KMnO4 in 35 mL of water. To a 250 mL Erlenmeyer flask, add 25
mL of water and 10 mL of conc. nitric acid. Dissolve 2.5 g of TeO2 in this mixture (this
will require boiling the solution in the fume hood). While maintaining a gentle boil, add
the KMnO4 solution in several portions. Once the addition is complete you should have a
cloudy solution (due to suspended MnO2). Remove from the heat and add 30% H2O2
dropwise (CAREFULLY!) until the solution is clear (the peroxide reacts with the MnO2).
Evaporate the solution to 12 mL total volume, then add 2.5 mL of conc. nitric acid.
Leave this solution to crystallise at least overnight.
Suction filter the crystals. If you want to recrystallise them, dissolve in a minimum of
hot water, add 3 drops of conc. nitric acid, and let stand uncovered until your next lab
period.
Questions:
1.
Write a balanced chemical reaction and calculate the percent yield.
2.
Draw a Lewis structure for telluric acid. Based on the structure, determine the
number of acidic protons the compound contains.
Reference:
Handbook of Preparative Inorganic Chemistry, Vol. 1, 2nd Edition, Georg Brauer, editor,
p. 451. New York: Academic Press. 1963.
16-4
The Chemistry of Group 17
One normally thinks of the halogen elements in terms of either their coloured elemental forms or as
simple anions X-. There is a much wider chemistry of these elements, especially for the brave chemist
who is willing to work with fluorine. As the most electronegative element, fluorine can form
compounds with elements that are normally unreactive, including the nobel gas xenon. In addition,
positive oxidation states in normally electronegative metals can be accessed, such as O2+ in OF2, and
Cl3+ in ClF3. Unfortunately, fluorine and binary fluorine compounds (like HF) are notoriously difficult
to work with as F etches glass (thus stainless steel equipment is required) and is highly toxic.
Chlorine can often serve the same purpose as fluorine with a sufficiently electropositive element, such
as iodine. Thus, interhalogen ions such as the anionic ICl4-, with I in the +3 oxidation state, can be
isolated.
Iodine itself can for a number of different anions, as it has a limited catenating ability. Thus, the
complex ions I3- and I5- are well established. In fact, there is considerable evidence for a large series
of such anions, including I7-, I9-, I182-, etc. Structural analysis implies that these complex polyiodides
are in fact sets of I2 and I- linked together by weak bonds. For example, I7- could be considered as
3 I2 + I-. This is also the way these polyiodides behave in solution (i.e., they dissociate).
In this lab, you will synthesise a compound that contains a polyhalogen anion, either Ix- or ICl4-. You
will then analyse the compound for iodine content using a redox titration.
Part 1: Synthesis of a Polyhalogen Anion
Synthesise one of compounds A, B, or C. Note that both compounds A and B contain the Ix- anion,
but the preparative details are different, leading to different values of x.
Compound A: Preparation of a polyiodide salt, NMe4Ix
Dissolve 2.5 g of I2 in 30 mL of cold methanol. This will require stirring for some time and
crushing the iodine crystals, but do not heat the solution. Once dissolved, add 1.0 g of finely
powdered (mortar and pestle) tetramethylammonium iodide, Me4NI. As you stir in the
ammonium salt, metallic green plates should start to appear in the solution. Once all the
Me4NI has disappeared, allow the solution to stand for at least an hour (it will keep for a
week if covered with parafilm and left in the dark). Filter the product and wash with 5 mL
of cold methanol.
Reference:
Popov, Alexander I., Buckles, Robert E. Inorg. Synth. 1957, 5, 167.
Compound B: Preparation of a polyiodide salt, NMe4Iy
17-1
Dissolve 2.5 g of I2 in 30 mL of hot methanol (hood). Add 0.5 g of tetramethylammonium
iodide, Me4NI. Stir the solution, maintain the heat, but do not allow the solution to boil, until
all the Me4NI has disappeared. Allow the solution to stand and cool undisturbed for at least
90 minutes (it will keep for a week if covered with parafilm and left in the dark). Decant off
any remaining solvent, wash with 5 mL of cold methanol, and decant off the wash. Allow the
crystals to air dry (no suction).
Reference:
Popov, Alexander I., Buckles, Robert E. Inorg. Synth. 1957, 5, 167.
Results for Part 1
Be sure to record the yield of your product, and to hand in a small sample of your crystals for
grading.
Part 2: Analysis of Product
Preamble: Iodine can be analysed by a process called iodometry. By this method, iodine
atoms in any sort of iodine-based molecule or ion can by released by reaction with a suitable
compound. This converts all the iodine to elemental iodine (I2) or to iodide (I-), which is then
titrated by a standard method. You must standardise and run your samples all in the same lab
period, as thiosulfate solutions only last a few hours.
Preparation and Standardisation of Thiosulfate Solution: You must prepare and standardise
a solution of sodium thiosulfate. Dissolve approximately 12.5 g of sodium thiosulfate
hydrate, Na2S2O32-.nH2O (n . 5), in 500 mL of water. This will give an approximately 0.1 M
solution. Standardise the solution using dry potassium iodate, KIO3. Weigh out three 0.05
g samples of iodate in each of 3 Erlenmeyer flasks. Just before titrating each flask, add 10
mL of distilled water, excess (1 g - need not be accurate) KI, and, once all the KI is dissolved,
10 mL of 0.5 M HCl. The resultant solution will be dark brown. Titrate immediately with
the thiosulfate solution until it turns straw coloured, then add a few drops of starch to give
the solution a blue colour. Continue titrating to the starch end point (colourless). Repeat for
the other two flasks.
The standardisation reaction is shown in Reactions 1 and 2.
1
2
Analysis of Compounds A and B: Weigh out accurately two 0.2 g samples into Erlenmeyer
flasks. Add 0.25 g of KI, dissolve in 20 mL of methanol, then add 20 mL of distilled water.
Titrate to a starch endpoint as with the standardisation run.
17-2
Questions:
1.
2.
3.
4.
5.
Balance Reactions 1 and 2.
Report the average molarity of your thiosulfate solution.
Report the value of x or y if you synthesised one of compounds A or B. If you
synthesised compound C, determine how close you are to the theoretical value of 1.
Draw the Lewis structure of S2O32-. Which resonance structure is most likely?
Explain your reasoning.
How does the addition of KI to a titration of Ix- improve the accuracy of your
titration?
Reference:
Vogel, Arthur I. A Textbook of Quantitative Inorganic Analysis. 3rd edition, p. 343. London:
Longmans, Green, and Co. Ltd. 1961.
17-3