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Org. Synth. 1932, 12, 60
DOI: 10.15227/orgsyn.012.0060
PHENYLPROPIOLIC ACID
[Propiolic acid, phenyl-]
Submitted by T. W. Abbott
Checked by Henry Gilman and G. F. Wright.
1. Procedure
A solution of potassium hydroxide is prepared by dissolving 252.5 g. (4.5 moles) of potassium hydroxide (Note 1) in 1.2 l. of 95 per cent alcohol contained in a 3-l. round-bottomed flask provided with a reflux condenser and heated on a steam bath. To the alkaline solution, cooled to 40–50°, is added 336 g. (1 mole) of crude ethyl α,β-dibromo-β-phenylpropionate (p. 270). When the initial reaction has subsided, the contents of the flask are refluxed for five hours on the steam bath.
The reaction mixture is cooled, and the salts which separate are filtered by suction. The filtrate is treated with concentrated hydrochloric acid until neutral to litmus (Note 2), and the salts which precipitate are separated by filtration. The filtrate is then distilled until the vapor reaches 95°. The residue and the precipitated salts, previously separated by filtration, are combined, dissolved in 800 cc. of water, and chilled by the addition of cracked ice to make a volume of 1.8 l. (Note 3). The cooled solution is immersed in an ice-water bath and stirred mechanically while a 20 per cent sulfuric acid solution is added until the solution is strongly acid to litmus. After stirring for twenty minutes the phenylpropiolic acid is filtered by suction and washed with four 30-cc. portions of a 2 per cent sulfuric acid solution.
The acid thus obtained as a light brown, granular product is dissolved in 1 l. of 5 per cent sodium carbonate solution, treated with 20 g. of Norite, and heated on a steam bath for thirty minutes with occasional stirring. The mixture is then filtered and cooled externally, and about 200 g. of cracked ice is added. The solution is stirred mechanically while a 20 per cent solution of sulfuric acid is added slowly. The precipitated acid is filtered by suction, washed first with 50 cc. of a 2 per cent sulfuric acid solution and then with a little water, and air-dried. The yield of acid melting between 115° and 125° is 112–118 g. (77–81 per cent of the theoretical amount).
One hundred grams of the crude acid can be purified by crystallization from 200–300 cc. of carbon tetrachloride, yielding 70 g. of phenylpropiolic acid melting at 135–136°.
2. Notes
1. The best yields are obtained when a 50 per cent excess of potassium hydroxide is used. The concentration of alkali has little or no effect on the yield.
2. The alcohol is best distilled from neutral rather than from alkaline solution.
3. In order to prevent decarboxylation, the temperature should be kept as low as possible. If this precaution is not observed, the yield is lowered and the product is less pure.
3. Discussion
The procedure described is essentially that of Perkin.1 Phenylpropiolic acid can also be prepared from ether solutions of β-bromostyrene2 and β-chlorostyrene3 with sodium and carbon dioxide; from β-bromostyrene and butyllithium in ether;4 by the action of alcoholic alkali on α-bromocinnamic acid,2 β-bromocinnamic acid,5 or ethyl α-bromocinnamate;6 and by the action of carbon dioxide on sodium phenylacetylide.2, 7
The preparation of phenylpropiolic acid by the action of alkali on α,β-dibromocinnamic acid, a more direct synthesis than that involving the ester, has not been much used because of the difficulty of preparing the dibromo acid. It has been reported, however, that α,β-dibromocinnamic acid can be prepared easily and in a 95 per cent yield by the addition of bromine to cinnamic acid in boiling carbon tetrachloride, and that the crude product can be used for the preparation of phenylpropiolic acid.8 A simplified procedure for the preparation of small amounts of phenylpropiolic acid from α,β-dibromocinnamic acid is described in the same article.

References and Notes
  1. Perkin, J. Chem. Soc. 45, 172 (1884); Liebermann and Sachse, Ber. 24, 4113 (1891).
  2. Glaser, Ann. 154, 140, 162 (1870).
  3. Erlenmeyer, Ber. 16, 152 (1883).
  4. Gilman, Langham, and Moore, J. Am. Chem. Soc. 62, 2328 (1940).
  5. Barisch, J. prakt. Chem. (2) 20, 180 (1879).
  6. Michael, Ber. 34, 3647 (1901).
  7. E. I. du Pont de Nemours and Company, U. S. pat. 2,194,363 [C. A. 34, 4745 (1940)].
  8. Reimer, J. Am. Chem. Soc. 64, 2510 (1942).

Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)

alcohol (64-17-5)

sulfuric acid (7664-93-9)

hydrochloric acid (7647-01-0)

ether (60-29-7)

sodium carbonate (497-19-8)

bromine (7726-95-6)

carbon tetrachloride (56-23-5)

carbon dioxide (124-38-9)

Norite (7782-42-5)

potassium hydroxide (1310-58-3)

sodium (13966-32-0)

cinnamic acid (621-82-9)

β-bromostyrene (103-64-0)

Phenylpropiolic acid,
Propiolic acid, phenyl- (637-44-5)

β-Chlorostyrene (622-25-3)

α-bromocinnamic acid

Ethyl α,β-dibromo-β-phenylpropionate (5464-70-0)

butyllithium (109-72-8)

β-bromocinnamic acid

ethyl α-bromocinnamate

sodium phenylacetylide

α,β-dibromocinnamic acid