Acylation. Part XXI. Kinetics and mechanism of the hydrolysis of acyl cyanides

Abstract

The hydrolyses of benzoyl and p-chlorobenzoyl cyanides to the corresponding benzoic acids have been studied in water. In the absence of general acids or bases (other than those derivable from water) the rate of hydrolysis is a function of pH. At high pH (>5·0) the reaction is first order in [OH^–]. As the pH is reduced there exists a region where the rate is largely independent of pH, followed by a region where (unexpectedly) the rate falls as [H₃O⁺] increases. p-Chlorobenzoyl cyanide hydrolyses about two-fold faster than benzoyl cyanide over the entire pH range. The kinetic data at various temperatures both in pure water and in buffer solutions, the solvent isotope effect, and the effect of the chloro-substituent are all compatible with an interpretation of the hydrolysis in terms of two mechanisms: a dominant bimolecular reaction between hydroxide ion and acyl cyanide at high pH (>5·0) and a dominant water reaction at lower pH, this latter reaction being susceptible to general base catalysis and to inhibition by acids. It is difficult to rationalise the data at pH <5·0 without the assumption of some variety of carbonyl addition intermediate, and the observed effects are compatible with a two-step mechanism for carbonyl hydration, but not with a one-step cyclic hydration.

Benzoyl cyanide proves an unexpectedly powerful acylating agent (towards water) coming below benzoyl chloride but above benzoyl fluoride and benzoic anhydride. There is no evidence, however, for the operation of an ionisation mechanism like that thought by some to apply for benzoyl chloride in water-rich solvents.