Acylation. Part XXXVIII. Kinetics and mechanism of hydrogen ion catalysed hydrolysis of thiobenzamide

Abstract

Catalysed hydrolysis of thiobenzamide in moderately concentrated aqueous mineral acid leads initially to both benzamide and thiobenzoic acid, the former constituting ca. 25% of the product at 8·5M-HCl and 60°. The subsequent hydrolysis of thiobenzoic acid is relatively ver rapid, but that of benzamide is ca. 3-fold slower than that of thiobenzamide under the same conditions. The general kinetic features of the thiobenzamide hydrolysis are otherwise similar to those found for benzamide and other O-amides, the rate passing through a maximum value as the catalyst concentration is increased. For thiobenzamide we find E_a 19·4 kcal mol^–1 and ΔS‡–21 cal mol ^–1 K^–1. The pK_a is –2·38 at 25°. The mechanism of hydrolysis is discussed in the light of mechanisms possible for O-amides. A carbonyl addition mechanism is required to produce benzamide and can alone explain all the facts. A contribution from a synchronous mechanism is not, however, excluded.