Commentary on the Kamlet-Taft Scale of Solvent Polarity for Aprotic Lewis Bases

Abstract

The generalized scale of solvent polarity (r*) developed by Kamlet and Taft was justified originally from a modified form of reaction field theory. However, results from critical statistical tests demonstrate that such a rationalization of the -* scale is inadequate for both nonpolar and polar aprotic solvents. By contrast, the McRae treatment of dispersive and dipolar interactions between the solute and solvent provides a more suitable model for interpreting the seemingly general characteristics of the ir* numbers when these are used in linear free energy correlations. Several derived functions correlating medium effects to spectral changes and reaction kinetics for inorganic species have utilized the bulk dielectric constant and index of refraction as the variables representing the essential properties of the solvent. In an extensive critical review of literature sources Fowler, Katritzky and Rutherford (1971) noted the limitations on such single parameters when used as independent probes of solvent-solute interactions. As an alternative approach, a number of investigators have proposed various empirical parameters which are seemingly related to solvent polarity; and, among the more recent of these, the multiparameter system of Kamlet, Abboud, and Taft (1977) appears to be the most successful in interpreting a broad range of observed solvent effects upon reactions. The general linear free energy function in equation I relates the three Kamlet-Taft numbers for a given solvent to the observable property P and its specified reference state P).