Effect of solvent composition on the acidity of phenols in dimethyl sulfoxide-water mixtures

Abstract

The effect of solvent composition on the acidity of substituted phenols in DMSO–H2O systems was examined. It is shown that in DMSO–H2O mixtures with up to 35 mol% DMSO there is a linear relationship between the pKa values of monosubstituted phenols in the binary solvent mixtures of different compositions and the corresponding pKa values in water. This leads to an equation relating the slope and intercept terms, pKa(DMSO–H2O) = slope[pKa(H2O)] + intercept. The basis of this relationship in the Kirkwood–Westheimer equation is examined. The significance of a possible ‘supersolvent’ is discussed. The applicability of the relationships derived herein to other rate–equilibria relationships and solvent mixtures is considered. Copyright © 1998 John Wiley & Sons, Ltd.