Transition state enthalpies of transfer in aqueous dimethyl sulphoxide solutions. The hydroxide-catalysed racemisation of D-α-methyl-α-phenylacetophenone

Abstract

Enthalpies of solution of D-α-methyl-α-phenylacetophenone in a series of dimethyl sulphoxide–water mixtures have been measured. Taken in conjunction with the enthalpies of transfer of the hydroxide ion from a 27.8 mol% dimethyl sulphoxide–water mixture to solutions of higher dimethyl sulphoxide concentrations, and the previously published enthalpies of activation for the racemisation of the ketone, transition state enthalpies of transfer have been calculated. The results show that the increase in rate accompanying the increase in dimethyl sulphoxide concentration can be mainly accounted for by desolvation of the hydroxide ion but that desolvation of the transition state makes the increase less than it would otherwise be.