Gas-phase acidity of para-substituted benzoic acids -: a triadic analysis of substituent effects

Deprotonation energies in vacuo of a large number of para-substituted benzoic acids encompassing a wide variety of substituents [F, Cl, Br, CH3, C(CH3)3, CH=CH2, C=CH, CHO, COOH, CF3, BH2, B(CH3)(2), NH2, N(CH3)(2), OH, OCH3, SH, CN and NO2] were studied by a reliable ab initio method. It is shown by using a triadic formula analysis that the enhanced acidity of these compounds is a consequence of the final state as a rule, i.e. it is a result of the greater stability of the conjugate bases. There are just a few exceptions to this rule, involving C(CH3)(3), OCH3, OH and SH substituent groups. In the last two cases molecules deprotonate at the O and S substituent atoms, respectively. The ring-substituted pentacyano derivative of benzoic acid was also examined. It is shown that it exhibits highly pronounced acidity owing to a strong anionic resonance effect in the resulting final state anion. Copyright (c) 2005 John Wiley & Sons, Ltd.