Experiment 22: Addition and Elimination Reactions:
Synthesis of Diphenylacetylene from trans-Stilbene
Certain acetylenes are synthesized readily by an addition-elimination
sequence starting with the corresponding alkene. You will employ this method
to prepare diphenylacetylene from trans-stilbene.
On mechanistic grounds, which stereoisomeric dibromide do you expect to
obtain from the addition reaction, the meso isomer or the racemic mixture of
enantiomeric R, R and S, S isomers? (The experiment will provide the
opportunity to test whether your expectation is correct, since the meso
dibromide melts at 238°C while the racemic mixture of dibromides melts at
114°C. )
You will use pyridinium bromide perbromide (also known as pyridinium
tribromide – structure above first arrow in scheme above and below, right) as
the brominating agent; it serves as an in situ source of bromine through the
following equilibrium:
Pyridinium tribromide is a stable, crystalline salt is more convenient and safer
to handle than bromine (safety: can be fatal if inhaled, or absorbed through
skin contact, lachrymator, causes extreme burns), and has been found to
possess far greater stereo-selectivity than bromine itself in the bromination of
alkenes such as cis- and trans-stilbenes. Note that each mole of the reagent
(MW 320) releases one mole of Br 2. Though it is a solid and much less
hazardous, you must still handle it with extreme care. Wear gloves, goggles,
use a spatula and alert your instructor if you have any spills. Do not try to
sample the smell of the compound. Treat it like it is bromine because it easily
generates bromine.
The second step requires a very high temperature in order to complete the
reaction in a reasonable time. This is accomplished by carrying out thisstep in
a solvent of very high boiling point and heating in an oil bath at 170°C
Caution is necessary when working around such a hot bath.
Procedure
1. Place 0.0055 mole of trans-stilbene in 20 mL of acetic acid in a 50 mL
Erlenmeyer flask. Heat the mixture on a steam bath until all of the
stilbene dissolves. Then add 0.0063 mole of pyridinium bromide
perbromide (Caution: corrosive; wear gloves) and mix the reactants by
swirling, If necessary, use an additional small amount of acetic acid to
rinse into solution any crystals of the reagent that have adhered to the
walls of the flask. Continue heating on the steam bath for approximately
ten minutes. The solution does not normally change much over the
course of the reaction. It normally is orange throughout the reaction
though in principle the orange color should diminish as the bromine is
consumed.
This procedure can be scaled down according to the amount of material
you actually have. When scaling to half scale, for example, cut all the
quantities in half and use a smaller flask (why?). When scaling down
though, you do not cut the reaction times down (why?).
2. Cool the reaction mixture in cold water (do not use an ice bath or the
acetic acid will freeze). Collect the product by vacuum filtration and wash
it with a small amount of cold methanol to remove all or most of the
yellowish color. Save a small amount of this stilbene dibromide to take a
melting point after it is dry. Weigh the remainder of your product now
and continue with the next step. If you have less than 0.5 g, consult your
TA or instructor.
3. Please note that the remaining procedure is already scaled. The
following quantities take into account the reduction of material from the
bromination step. Do not scale down these quantities if you have 0.5
grams or more of material. If you have less than 0.5 grams consult your
TA or instructor about scaling the procedure. In a large test tube which
you obtain from your TA place the remainder of your stilbene dibromide
(at least 0.5 gram) along with 3 pellets of potassium hydroxide. (These
consist of about 85% KOH and 15% water. Three pellets weigh about 0.5
g; it is not necessary to weigh them since you are using an excess.) If you
have more than 1.5 g stilbene dibromide, use 4 KOH pellets. Caution:
Potassium hydroxide is very hygroscopic and very caustic; measure it out
quickly and recap the bottle immediately. Clean up any spills with a damp
paper towel. Do not pick up the pellets with your hands (gloved or
otherwise). Use a spatula.
4. Add 4 mL of triethylene glycol.
5. As a heat source for the dehydrobromination reaction, use a sand bath
that has been regulated to 170-180°C. To prepare your sand bath, fill your
heating mantle with sand. Set the step control at 60 volts early in the lab
and work on getting it regulated. It is helpful to stir the bath with a glass
rod (not the thermometer). Do not over- heat it or leave it unattended, If
the sand gets too hot, the thermometer can explode. Thermometers break
very easily if used as a stir rod.
6. When the temperature is regulated, use a clamp to mount the test tube
containing your reaction mixture in the sand bath, occasionally mixing
the contents of the test tube with a glass stirring rod while maintaining the
sand bath at 170-180°C. Caution: Hot sand can cause painful burns!!!
Heat the reaction for about twenty minutes. It should turn a fairly
translucent reddish- brown. Remove the test tube from the sand bath
(Reminder: the test tube is hot!) and cool it to room temperature in room
temperature water. Then, place the test tube in an ice bath.
7. Add 20 mL of cold water to the cooled reaction mixture, allow the
reaction to cool for ten or fifteen minutes and collect the resulting nearly
colorless diphenylacetylene by vacuum filtration. Do not rush this step.
Diphenyl acetylene is a low melting solid and if contaminated, can
separate as an oil (melting point depression). Give it time to crystallize.
Isolate the crude produce using an appropriately small vacuum filtration
apparatus. Allow the crude product to dry and measure its melting
point. If the melting point is within five degrees of the literature value
(see below) , do not recrystallize the material.
8. If the melting point is not close to the literature value, it can be
recrystallized directly from 95% ethanol in the hood. Review the
procedure for recrystallization. Essentially, you dissolve your product in
the minimum necessary volume of hot ethanol, carry out a hot gravity
filtration, and let the hot ethanol solution stand undisturbed while it cools
slowly; this should lead to the formation of large colorless crystals. After
the first crop of crystals has been collected by vacuum filtration,
concentrate the mother liquor by boiling it to obtain a second crop of
pure material, which is also collected by vacuum filtration. Combine the
two crops of crystals, and after allowing the product to dry determine the
yield and melting point, and submit the sample to your TA. The reported
melting point of diphenylacetylene is 61°C.
Your Lab Report should include answers to the following questions:
1. What results would have been obtained in this experiment if cisstilbene had been used instead of trans-stilbene as the starting
material? That is, would the intermediate or final products have been
different, and if so, how?
2. How could you determine whether the elimination step occurs by
the E1, or the E2 mechanism?
3. In this synthesis why do we first add two bromine atoms to transstilbene if we are just going to take them off again?
4. Why does the double elimination step require such a high
temperature to proceed? To figure this out, work carefully through
the anti-stereochemistry of the successive bimolecular eliminations.