Hydrogen bonding. Part 10. A scale of solute hydrogen-bond basicity using log K values for complexation in tetrachloromethane

Abstract

A scale of solute hydrogen-bond basicity has been set up using log K values for the complexation of a series of bases (i) against a number of reference acids in dilute solution in tetrachloromethane, equation (i). log Kⁱ=L_A log K_B^H_i+D^A(i)

Thirty-four such linear equations have been solved to yield L_A and D_A values that characterise the acids, and log K_B^H values that characterise the base; all the thirty-four equations intersect at a point where log K=–1.1 with K on the molar scale. This primary set of log K_B^H values involved 215 bases, and through a large number of secondary values we have been able to determine log K_B^H for some 500 bases, that include nearly all the functional groups encountered in organic chemistry. By making use of the ‘magic point,’ we have transformed log K_B^H into an entirely equivalent, but more convenient, scale through equation (ii). β₂^H=(log K_B^H+ 1.1)/4.636 (ii)

Since we can take β₂^H= 0 for all non-basic compounds such as alkanes and cycloalkanes, the new β₂^H hydrogen-bond solute basicity scale covers virtually all classes of base.

We show that β₂^H is not generally related to measures of full proton-transfer basicity such as aqueous pK or gaseous proton affinity (E_pa) values, although family dependence is observed, and we stress that solute hydrogen-bond basicity must not be equated with full proton-transfer basicity. We also briefly investigate the solvent dependence of the β₂^H values in terms of the Maria–Gal θ-value, and we point out a number of exclusions to the ‘reasonably general’β₂^H scale.