O-Carboxyphenylmercaptoacetic acid

To a solution of 20 g. of anhydrous sodium carbonate in 150 ml. of water are added 10 g. (0.033 mole) of dithiosalicylic acid [Org. Syntheses Coll. Vol. 2, 580 (1943)] and 15 g. of sodium hydrosulfite. The mixture is refluxed for 30 minutes, and then there is added a solution of 15 g. of chloroacetic acid in 200 ml. of water to which sufficient sodium carbonate has been added to neutralize the chloroacetic acid. After being refluxed for 1 hour, the mixture is cooled and acidified to Congo red. The precipitated product is recrystallized from water to give a 70% yield of o-carboxyphenylmercaptoacetic acid melting at 217-218°. 

A mixture of 40 g. (0.19 mole) of o-carboxyphenylmercaptoacetic acid (p. 68), 90 ml. of acetic anhydride, and 20 g. of sodium acetate is heated slowly in an oil bath until evolution of carbon dioxide begins (about 80°). Heating must be careful or the reaction may go out of control. The temperature of the bath is raised 135-140° and held there for 20 minutes. The mixture is cooled, and suflScient concentrated sodium hydroxide solution is added to give an approximate 10% concentration of sodium hydroxide in the entire mixture. The resulting mixture is refluxed until a clear solution results (about 1 hour) and then is acidified with acetic acid. Steam distillation gives a 75-80% yield of crude 3-thianaphthenol. 

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