Acylation. Part XXIV. The cyanide catalysed hydrolysis of esters

Abstract

The hydrolysis of ethyl thiolacetate is catalysed by cyanide ion and by hydrogen cyanide, whereas the hydrolyses of ethyl benzoate and ethyl p-nitrobenzoate are unaffected. With ethyl thiolacetate the use of cyanide buffers in both water and deuterium oxide establishes the rate equation –d[Ester]/dt=(k_OH^–[OH^–]+k_CN^–[CN^–]+k_HCN[HCN])[Ester], with (k_CN^–)_H2O/(k_CN^–)_D2O= 1·25 and (k_HCN)_H2O/(k_DCN)_D2O∼ 1·1. The evidence identifies the cyanide catalysis as nucleophilic in type. The acetyl cyanide intermediate is not formed in a rapid pre-equilibrium and the cyanolysis step could be either synchronous or involve a carbonyl addition intermediate. The hydrogen cyanide catalysis represents an unusual acid catalysed nucleophilic catalysis of ester hydrolysis.