Organic Syntheses, CV 2, 515
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).
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
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