Basicity of carboxylic acids: resonance in the cation and substituent effects

Basic properties of acetic, formic and substituted benzoic acids were investigated by the density functional theory at the levels B3LYP/6-311+G(d,p) and/or B3LYP/6-311++G(2d,2p). The protonated form prefers unsymmetrical conformation E, Z on the two partially double C-O bonds; only in the case of formic acid the E, E form is still observable. Lower basicity of carboxylic acids as compared to ketones is due to the higher energy of the cation while the effect in the uncharged acid molecule is negligible. Contribution of resonance and of the inductive effect was approximately estimated by means of relatively sophisticated model compounds. The inductive effect of the hydroxy group in the cation is the deciding factor for the lowered basicity. Resonance in the cation is strong, stronger than in the acid molecule or in the carboxylate anion, but cannot overcome the inductive effect. Relative basicities of meta- and para-substituted benzoic acids are controlled by the Hammett equation with marked deviations of the para donor substituents that have been not observable with the basicities in solution.