Medium effects on activation parameters of alkaline hydrolysis of acetylmandelate ion

Abstract

The kinetic parameters for the reaction between hydroxide and acetylmandelate ions have been determined in water, 20% ethanol–water, and two sets of isodielectric ethanol–water mixtures having dielectric constants (D) 63·77 and 40·98 respectively. One set of isodielectric media (D= 63·77) comprising ethylene glycol–water mixtures has also been used. The components due to the electrostatic and nonelectrostatic factors have been computed by dissecting each parameter. An analysis of the activation energy and the entropy of activation indicates that the presence of water as well as ethanol in optimum proportions in the medium leads to enhanced solvation and hence stabilisation of the activated complex. Solvation of the transition state is greater in ethanol–water than in glycol–water of the same dielectric constant (63·77). The free energy of activation is in general little affected by a change in the medium.