Analysis of the Gas Phase Acidity of Substituted Benzoic Acids Using Density Functional Concepts

A theoretical study of the effect of the substituent Z on the gas phase acidity of substituted benzoic acids ZC₆H₄COOH in terms of density functional theory descriptors (chemical potential, softness and Fukui function) is presented. The calculated gas phase Δ_acidG° values obtained were close to the experimental ones reported in the literature. The good relationship between the Δ_acidG° values and the electronegativity of ZC₆H₄COOH and its fragments, suggested a better importance of the inductive than polarizability contributions. The balance of inductive and resonance contributions of the substituent in the acidity of substituted benzoic acids showed that the highest inductive and resonance effects were for the -SO₂CF₃ and -NH₂ substituents in the para- and ortho-position, respectively. The Fukui function confirmed that the electron-releasing substituent attached to the phenyl ring of benzoic acid decreased the acidity in the trend ortho > meta > para, and the electron-withdrawing substituent increased the acidity in the trend ortho < meta < para.