Organic Syntheses Part 11

When only small quant.i.ties of the acid are required, the following modified procedure is of value. One hundred grams of benzyl cyanide are added to a mixture containing 100 cc. of water, 100 cc.

of concentrated sulfuric acid, and 100 cc. of glacial acetic acid.

After this has been heated for forty-five minutes under a reflux condenser, the hydrolysis is practically complete.

The reaction mixture is then poured into water, and the phenylacetic acid isolated in the usual manner.

The odor of phenylacetic acid is disagreeable and persistent.

3. Other Methods of Preparation

The standard method of preparation of phenylacetic acid is by the hydrolysis of benzyl cyanide with either alkali[1a] or acid.[2a] The acid hydrolysis runs by far the more smoothly and so was the only one studied.

There are numerous other ways in which phenylacetic acid has been formed, but none of them is of practical importance for its preparation.

These methods include the following: the action of water on phenyl ketene;[3a] the hydrolysis and subsequent oxidation of the product between benzaldehyde and hippuric acid;[1] the reduction of mandelic acid;[2] the reduction of benzoylformic acid with hydriodic acid and phosphorus;[3] the hydrolysis of benzyl glyoxalidone;[4]

the fusion of atropic acid with pota.s.sium hydroxide;[5] the action of alcoholic potash upon chlorophenylacetylene;[6] the action of benzoyl peroxide upon phenylacetylene;[7] the alkaline hydrolysis of triphenylphloroglucinol;[8] the action of ammonium sulfide upon acetophenone;[9] the heating of phenylmalonic acid;[10]

the hydrolysis of phenylacetoacetic ester;[11] the action of hydriodic acid upon mandelonitrile.[12]

[1a] Ann. 96, 247 (1855); Ber. 14, 1645 (1881); Compt.

rend. 151, 236 (1910).

[2a] Ber. 19, 1950 (1886).

[3a] Ber. 44, 537 (1911).

[1] Ann. 370, 371 (1909)a

[2] Chem. (2) 1, 443 (1865); Ber. 14, 239 (1881).

[3] Ber. 10, 847 (1877)

[4] J. prakt. Chem. (2) 82, 52, 58 (1910).

[5] Ann. 148, 242 (1868).

[6] Ann. 308, 318 (1899).

[7] J. Russ. Phys. Chem. Soc. 42, 1387 (1910); Chem. Zentr.

1911 (I) 1279.

[8] Ann. 378, 263 (1911).

[9 Ber. 21, 534 (1888); J. prakt. Chem. (2) 81, 384 (1910).

[10] Ber. 27, (1894).

[11] Ber. 31, 3163 (1898)

[12] Inaugural Dissertation of A. Kohler (1909), Univ. of Bern.

XVIII

PHENYLACETYLENE

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 pota.s.sium 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 pota.s.sium 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 pota.s.sium 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 pota.s.sium bromide may tend to cover the melted pota.s.sium hydroxide. One can break the crust by shaking the distilling flask gently, or by using a gla.s.s rod inserted through a second hole in the stopper holding the dropping funnel.

It is convenient to have such a rod or stirrer pa.s.sing 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 pota.s.sium 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 gla.s.s flask seems to diminish the yield slightly.

3. Other Methods of Preparation

Phenylacetylene has been prepared by the elimination of carbon dioxide from phenylpropiolic acid by means of phenol[1] or aniline[2]

or by heating with barium hydroxide;[3] from styrene dibromide, by heating with pota.s.sium hydroxide in alcohol;[4] by heating b-bromo or chloro styrene with sodium ethylate or pota.s.sium hydroxide in alcohol;[5] by pa.s.sing the vapors of a-dichloroethylbenzene over hot soda lime;[6] by the action of alcoholic pota.s.sium hydroxide on dibenzal-acetone tetra-bromide;[1b] by the action of aqueous pota.s.sium hydroxide on phenyl propargylaldehyde;[2b] by the action of molten pota.s.sium hydroxide on b-bromo-styrene.[3b]

[1] Ber. 20, 3081 (1887).

[2] Rec. trav. chim. 16, 157 (1896).

[3] Arm. 221, 70 (1883).

[4] Ann. 154, 155 (1870); 235, 13 (1886); Bull. soc. chim. 35, 55 (1881); (3) 25, 309 (1901).

[5] Ann. 308, 265 (1899); 342, 220 (1905).