Linear solvation energy relationships. Part 38. An analysis of the use of solvent parameters in the correlation of rate constants, with special reference to the solvolysis of t-butyl chloride

Abstract

Arguments are advanced to show that at least four solvent parameters are needed in any general equation for solvent effects on rate constants and on Gibbs energies of individual solutes: the parameters we use in our general equation are π*, the solvent dipolarity, α, the solvent hydrogen-bond acidity, β, the solvent hydrogen-bond basicity, and δ_H², the solvent cohesive energy density. Parameters such as Y and N cannot represent unique solvent properties, because there exists more than one set of Y values and more than one set of N values. It is shown that the various Y values are ‘blends’ of π* and α in varying proportions, that the N values are ‘blends’ of π* and β, and that Y⁺ values are nearly pure β. Conditions for the collapse of the several Y scales into one scale, and for the collapse of the (mY+IN) equation into just mY, are outlined, based on the collinearity of π*, α, and β in certain sets of solvents.