IN a 500-cc. Pyrex distilling flask are placed 150 g.
of potassium hydroxide. The mouth of the flask is provided with a
one-hole stopper holding a dropping funnel; the side tube of the flask
is connected with a condenser set for downward distillation.
The b-bromostyrene (100 g.) is placed in the dropping funnel.
The distilling flask is gradually heated in an oil bath until
the temperature of the bath is 200'0, and the bromostyrene is
then dropped in upon the molten potassium hydroxide, at the rate
of somewhat less than a drop a second. Since the boiling point of
phenylacetylene is 142-143'0, and that of bromostyrene is 218-220'0,
the phenylacetylene distils away from the unchanged bromostyrene.
While the bromostyrene is being dropped in, the temperature
of the oil bath is raised very gradually to 215-220'0, and is kept
at this temperature until all the bromostyrene has been added.
Finally the temperature is raised to 230'0, and is held there
until no more distillate comes over. The distillate is colorless;
it consists of two layers, the lower one being water.
The upper layer is separated and dried with solid potassium hydroxide.
It is then distilled. The yield of the distilled phenylacetylene,
boiling at 142-144'0, is 37 g. (67 per cent of the theoretical
amount). 2. Notes
Toward the end of the reaction, a crust of potassium bromide
may tend to cover the melted potassium hydroxide. One can break
the crust by shaking the distilling flask gently, or by using
a glass rod inserted through a second hole in the stopper holding
the dropping funnel.
It is convenient to have such a rod or stirrer passing through
a mercury seal in the stopper of the flask. An occasional turn
of this stirrer breaks the crust and facilitates the operation.
Mechanical stirring should not be employed, as it reduces
the yield tremendously. Apparently this is because it facilitates
the solution of bromostyrene in the tarry by-products and thus causes
it to polymerize instead of reacting with the potassium hydroxide.
A single Pyrex flask can be used for only three or four runs.
The flask should be emptied while still very hot.
The yield of material can be somewhat increased by working with small lots
(25 g. of bromostyrene).
The use of steel or copper vessels in place of a glass flask seems
to diminish the yield slightly.