Acylation. Part XVII. The aminolysis and hydrolysis of carboxylic and thiolcarboxylic anhydrides

Abstract

The kinetics of the spontaneous and the acid-catalysed hydrolysis of thiolbenzoic anhydride and of the aminolyses of thiolbenzoic, benzoic, thiolacetic, and acetic anhydrides with substituted anilines in dioxan–water mixtures have been examined. Thiolbenzoic anhydride is more easily hydrolysed than benzoic anhydride, but its hydrolysis is less sensitive to acid catalysis. It is also less sensitive to salt effects than that of thiolacetic anhydride. It is concluded generally that bond-breaking plays a more important part in the hydrolyses of the benzoic anhydrides than in those of the acetic derivatives. The aminolyses studied were all first-order in aniline, a result which excludes general base catalysis by the free base. With any given base, and for a given type of anhydride, the sulphur derivative always reacts faster than its oxygen analogue, the rate ratio decreasing as the basic strength of the aniline is increased. The data support the contention that the spontaneous hydrolyses are subject to general base catalysis by other water molecules, and that the attacking nucleophile is incipiently formed hydroxide ion. The results for acetic and thiolacetic anhydrides substantiate the view that in acylation an increase in nucleophilicity of the attacking reagent leads to an increase in the kinetic importance of the bond-forming process. The data as a whole support our previous generalisations about acylation.