| Summary: | A theoretical study of the effect of the substituent Z on the gas phase acidity of substituted benzoic acids ZC<sub>6</sub>H<sub>4</sub>COOH in terms of density functional theory descriptors (chemical potential, softness and Fukui function) is presented. The calculated gas phase Δ<sub>acid</sub><i>G</i>° values obtained were close to the experimental ones reported in the literature. The good relationship between the Δ<sub>acid</sub><i>G</i>° values and the electronegativity of ZC<sub>6</sub>H<sub>4</sub>COOH 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 -SO<sub>2</sub>CF<sub>3</sub> and -NH<sub>2</sub> substituents in the <i>para</i>- and <i>ortho</i>-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 <i>ortho</i> > <i>meta</i> > <i>para</i><b>,</b> and the electron-withdrawing substituent increased the acidity in the trend <i>ortho</i> < <i>meta</i> < <i>para</i>.
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