Transition state enthalpies of transfer in aqueous dimethyl sulfoxide solutions. The hydroxide catalyzed ionization of chloroform

The accelerating effect of added dimethyl sulfoxide on the rate of hydroxide-catalyzed ionization of chloroform in water has been examined in terms of the solvation of reactants and transition state. Enthalpies of solution of chloroform in water, DMSO, and mixtures of the two have been measured. From these, together with enthalpies of activation for the detritiation of chloroform, and enthalpies of transfer of hydroxide ion from water to aqueous DMSO mixtures, transition state enthalpies of transfer have been calculated. The reaction is accelerated by small concentrations of DMSO because of increased transition state solvation, but by large DMSO concentrations due to reduced solvation of hydroxide ion. Transition state solvation is unlike that of the reactants, but resembles that expected for CCl3− ion.