Pairwise solute–solute interaction parameters and their application to solvent and salt effects on the kinetics of chemical reactions in aqueous solutions

Abstract

Dependences on the nature and molalities of added solutes of rate constants for reactions in aqueous solutions have been examined, using the formalism of transition-state theory, in terms of the corresponding dependences of activity coefficients for solutes. Activity coefficients for neutral reactants and transition states are discussed in terms of pairwise solute–solute interactions and then in terms of group interaction parameters using the Savage–Wood additivity of groups (SWAG) procedures. The treatment is extended to a consideration of trends in enthalpies of activation in terms of enthalpic interaction parameters. Systems are grouped into four classes on the basis of changes in hydrophobic and hydrophilic character of substrates on proceeding from initial to transition states and on the basis of the hydrophobic and hydrophylic character of added solutes. In the case of reactions between hydroxide and bromophenol blue anions the dependence of kinetic parameters on added salt is understood, using Pitzer's equations, in terms of charge-charge and pairwise ion-ion interactions, the latter accounting for specific salt effects.