Kinetics of SN1 Reactions in Binary Liquid Mixtures near the Critical Point of Solution

Abstract

We have measured the rates of the SN1 hydrolysis reactions of 2-chloro-2-methylbutane in isobutyric acid + water and 2-bromo-2-methylpropane in trietylamine + water near their respective consolute points. In the former mixture, two phases coexist below the critical solution temperature, while in the latter, two phases coexist above the critical solution temperature. Because each hydrolysis reaction produced a strong electrolyte among its products, we could follow the progress of the reaction by making measurements of the conductance. To interpret these measurements properly, however, we had to take into account the following phenomena: (1) the effect of the composition on the critical temperature, (2) the effect of the liquid−liquid-phase transition when it occurred during the course of the run, and (3) the competition between the effects of temperature and the effects of concentration on the response of the conductance probe. With these phenomena accounted for, however, we found in the case of both reactions that the specific rate of hydrolysis slowed at temperatures above the critical solution temperature and accelerated at temperatures below it. From the point of view of thermodynamics, the causative factor in slowing down is the derivative of the reaction Gibbs free energy with respect to the extent of reaction, which goes to zero at the critical point. In contrast, speeding up has no ready theoretical explanation. The experimental results imply, nevertheless, that speeding up is not confined to the two-phase side of the phase diagram.