C6H5CH=CHBr + KOH—> C6H5CTBCH + KBr + H2O
Prepared by JOHN C. HESSLER. Checked by J. B. CONANT and E. R. BARRETT.
1. Procedure
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.