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, CH 3 , C(CH 3 ) 3 , CHCH 2 , CCH, CHO, COOH, CF 3 , BH 2 , B(CH 3 ) 2 , NH 2 , N(CH 3 ) 2 , OH, OCH 3 , SH, CN and NO 2 ] 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(CH 3 ) 3 , OCH 3 , 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 © 2005 John Wiley & Sons, Ltd.