Rate enhancement of ester saponification in aqueous dimethyl sulphoxide: a case for the importance of transition state solvation

Abstract

On the basis of kinetic data for the alkaline hydrolysis of a number of acetate esters, glycol di- and mono-esters, and αβ-unsaturated esters and lactones in aqueous dimethyl sulphoxide (DMSO) and aqueous ethanol, an assessment of the relative importance of the two factors generally associated with rate enhancement in aqueous DMSO, the existence of a desolvated and more reactive hydroxide ion and the remarkable ability of DMSO to solvate the transition state, is attempted. A qualitative discussion of the data in the light of an equation connecting the rate increase in a dipolar aprotic solvent with the solvent activity coefficients of the ester molecule, hydroxide ion, and the transition state anion in the two solvent systems is made. It is shown that hydroxide ion desolvation is not the exclusive factor causing rate accelerations in DMSO. Spectral evidence shows that solvation of the ester molecule in the two solvent systems plays a negligible role. The results fit well with the concept of ‘loose’ and ‘tight’ transition states and the influence of the dipolar aprotic and the protic solvent systems on these types of transition states is analysed.