Organic Syntheses, CV 1, 438
Submitted by John C. Hessler
Checked by J. B. Conant and G. R. Barrett.
1. Procedure
In a
500-cc. Pyrex distilling flask (Note
1) is placed
150 g. (2.7 moles) of potassium hydroxide. The mouth of the flask is provided with a
one-holed stopper holding a
dropping funnel; the side tube of the flask is connected with a
condenser set for downward distillation.
While the
bromostyrene is being dropped in (Note
3), the temperature of the oil bath is raised very gradually to 215–220° and is kept there until all the
bromostyrene has been added. Finally the temperature is raised to 230° 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°, is
37 g. (
67 per cent of the theoretical amount) (Note
4).
2. Notes
1. A single Pyrex flask can be used for only three or four runs. The flask should be emptied while still very hot. The use of
steel or copper vessels in place of a glass flask seems to diminish the yield slightly.
2. The
potassium hydroxide usually available contains sufficient moisture so that it will liquefy at 200°. If pure dry
potassium hydroxide is used, it is necessary to add a little water so that the fusion point will be lowered to the temperature indicated.
It is suggested that equally good yields can be obtained if the eutectic of
sodium hydroxide and
potassium hydroxide is used, under which conditions the alkali is molten at 200° and the reaction takes place very readily.
1
3. Toward the end of the reaction, a crust of
potassium bromide may tend to cover the melted
potassium hydroxide. The crust can be broken 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.
4. The yield of material can be somewhat increased by working with small lots (
25 g. of β-bromostyrene).
3. Discussion
This preparation is referenced from:
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