Hydrogen-bonded and ion-pair complexes in aprotic solvents

Abstract

The proton-transfer reactions between acids and bases in aprotic solvents of low relative permitivity lead to the production of ion-pair complexes via hydrogen-bonded reaction intermediates. For most systems the rate-limiting step is the formation of the hydrogen-bonded intermediate. The rate of these reactions, as measured by the temperature-jump technique, agree well with those calculated by a refined theory of diffusion-controlled rates which takes into account rotation within the encounter complex. The reactions of Bromophenol Blue with pyridine basis are anomalously slow, the rate-limiting step being proton-transfer within the hydrogen-bonded complex to form the ion-pair. Values of the enthalpy, entropy and volume of activation, and the primary kinetic isotope effect, enable the structure of the transition state to be deduced.