Hydrogen bonding. Part 10. A scale of solute hydrogen-bond basicity using log K values for complexation in tetrachloromethane
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=LA log KBHi+DA(i)
Thirty-four such linear equations have been solved to yield LA and DA values that characterise the acids, and log KBH 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 KBH values involved 215 bases, and through a large number of secondary values we have been able to determine log KBH 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 KBH into an entirely equivalent, but more convenient, scale through equation (ii). β2H=(log KBH+ 1.1)/4.636 (ii)
Since we can take β2H= 0 for all non-basic compounds such as alkanes and cycloalkanes, the new β2H hydrogen-bond solute basicity scale covers virtually all classes of base.
We show that β2H is not generally related to measures of full proton-transfer basicity such as aqueous pK or gaseous proton affinity (Epa) 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 β2H values in terms of the Maria–Gal θ-value, and we point out a number of exclusions to the ‘reasonably general’β2H scale.