CORRELATION-ANALYSIS OF THE PKA VALUES OF MONO-ORTHO-SUBSTITUTED AND DI-ORTHO-SUBSTITUTED BENZOIC-ACIDS

The pK(a) values of 10 benzoic acids occupied simultaneously at the 2- and 6-positions by various substituent combinations were newly measured in water at 25-degrees-C. The effects of the substituents were analyzed quantitatively with regression analysis and free-energy-related parameters of the Hammett-Taft type. The results were compared with those of the analysis of the pK(a) of mono-ortho-substituted acids cited in the literature. The resonance effect of substituents in di-ortho-substituted compounds was insignificant, but the electronic effect of the through-space inductive type in terms of the Charton sigma(I) and the steric effect in terms of the Taft-Kutter-Hansch E(s) were not. The overall effects were additive for two substituents. In mono-ortho-substituted acids, a "regular" electronic effect of ortho substituents similar to that of the corresponding para substituents including the resonance effect component was indicated to work in addition to the steric and the through-space inductive effects. The steric and through-space inductive effects of a substituent were almost unchanged regardless of whether the substituent was involved in mono- or di-ortho substitution. A possible rationale integrating the results for mono- and di-ortho-substituted acids was to assume an intermediate formation for the dissociation process in which the carboxyl group is partly ionized and more easily twisted than the nonionized carboxyl group in the initial state.