Linear solvation energy relationships. Part 4. Correlations with and limitations of the α scale of solvent hydrogen bond donor acidities

Abstract

The solvatochromic comparison method is employed to amend and expand the data base for the α scale of solvent HBD (hydrogen bond donor) acidities. In combination with the π* scale of solvent polarities, the α scale serves to rationalize solvent effects on a number of chemical properties and reaction parameters (XYZs) through equations of the form, XYZ=XYZ₀+sπ*+aα. Properties considered include electronic spectral data, free energies of transfer between solvents, ¹⁹F and ³¹P n.m.r. shifts, and log reaction rate constants. It is shown that Dimroth's E_T(30) and Kosower's Z scales, intended as measures of solvent polarity (i.e.π*-equivalent), and Gutmann's ‘Acceptor Number (AN)’ scale, intended as a measure of solvent electrophilicity (i.e.α-equivalent), correspond in fact to linear combinations of π* and α.