Solvation and steric hindrance in methyl-substituted benzoic acids

Abstract

Dissociation constants of all isomers of methyl-substituted benzoic acids have been measured in methanol and in dimethyl sulfoxide. From the pK values, the substituent effects of the methyl groups are calculated and tentatively divided into polar and steric effects. Also, in the case of polymethyl derivatives, the buttressing effect is calculated with reference to monomethyl derivatives. These quantities are processed within the framework of correlation analysis using as reference the corresponding quantities in the gas phase. The steric effects may be classified as steric hindrance to resonance—observed only in derivatives with two ortho methyl groups, and electrostatic induction in the deprotonated molecules—observed in all derivatives. Both effects make the acids stronger and both are attenuated in solution, in methanol more than in DMSO. The electrostatic effect is attenuated in a specific way: the effect of more distant methyl groups is practically cancelled and only that of the ortho groups remains. As a result, the overall substituent effects are rather different in the two solvents and in the gas phase.