STEP 1: Dissolve 74 g (0.5 mol) of cinnamic acid in 300 ml of hot dichloromethane in a 500-m1 flask and cool the solution in ice-water with shaking.
STEP 2: As soon as the solid begins to crystallise out, add a solution of 80 g (26 ml, 0.5 mol) of bromine (care in handling) in 50 ml of dichloromethane rapidly in three portions with vigorous shaking and cooling.
STEP 3: Allow the flask and contents to stand in an ice-bath for 30 minutes to allow complete crystallisation of the product; collect the latter by filtration.
STEP 4: Obtain a pure specimen of 2,3-dibromo-3- phenylpropanoic acid (m.p. 204 °C decomp.) by recrystallising a small sample of the crude product from aqueous ethanol.
STEP 5: Boil the bulk of the crude bromoacid under reflux with 750 ml of 10 per cent aqueous sodium carbonate solution, cool and separate the layer of crude fl-bromostyrene.
STEP 6: Extract the aqueous phase with two 75 ml portions of ether, combine this extract with the organic phase, dry over anhydrous calcium chloride and remove the ether on a rotary evaporator. About 65-70 g of crude fl-bromostyrene is obtained.
STEP 7: Place 100 g of potassium hydroxide pellets in a 500-m1 flask, moisten the pellets with about 2 ml of water and fit the flask with a still-head carrying a dropping funnel and a condenser set for downward distillation.
STEP 8: Heat the flask in an oil bath maintained at 200 °C and add the crude fi-bromostyrene drop-wise on to the molten alkali at a rate of about 1 drop per second.
STEP 9: Phenyl-acetylene begins to distil over; slowly raise the bath temperature to about 220 °C and keep it at this point until the addition is complete.
STEP 10: Then continue to heat at about 230 °C until no more product distils over.
STEP 11: Separate the upper layer of the distillate, dry it over potassium hydroxide pellets and redistil. Collect the phenylacetylene at 142-144 °C: the yield is 25 g (49%). The p.m.r. spectrum (CDC14, TMS) exhibits signals at 6 3.08 (s, 1H) and 7.35 (m, 5H).
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