Acylation. Part XXII. The mechanism of hydrolysis of acyl cyanides in concentrated aqueous acid

Abstract

The hydrolyses of acetyl and propionyl cyanides, to acetic and propionic acids, respectively, have been examined kinetically in concentrated aqueous perchloric acid (2·5–12M). In neutral or slightly acidic solution the hydrolyses are very rapid but are inhibited by increasing acid concentrations up to ca. 8M, whereafter positive catalysis by acid is found. The results support our previous conclusions about the kinetics and mechanism of the hydrolysis of benzoyl cyanide to benzoic acid.

With acetyl and propionyl cyanides hydrolysis to the α-keto-acid is only observed at both high acidities and high acyl cyanide concentration (>ca. 3M). Thus even in 12M-perchloric acid the hydrolysis of 0·5M-acetyl cyanide leads exclusively to acetic acid. However, with 10^–4M-benzoyl cyanide, although hydrolysis to benzoic acid is quantitative in 1M-perchloric acid, the yield falls to ca. 50% in 12M-acid. It is possible therefore that keto-acid formation is easier in the aromatic series.