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

Jorge A Amador-Balderas,Michael-Adán Martínez-Sánchez,Ramsés E Ramírez,Francisco Méndez,Francisco J Meléndez

doi:10.3390/molecules25071631

Abstract

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 ΔacidG° values obtained were close to the experimental ones reported in the literature. The good relationship between the ΔacidG° 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.

Highlights

The Hammett equation is a linear Gibbs free energy relationship that uses the properties of substituted benzoic acids as a reference standard to study the effect of substituent on the reactivity and properties of various molecules [1]
Considering that the steric effects are of limited importance for acidity [3] and the usefulness of the electronegativity and polarizability [24,25,26,27,28], we study the electronic effect of the substituent Z in the acidity of the substituted benzoic acids using a two parameter linear model ∆acid G◦ = mx + b, where x is the electronegativity χ or softness S of Z, C6 H4, ZC6 H4, ZC6 H4 COO and ZC6 H4 COOH
The electronegativity and the Fukui function are good descriptors to account for substituent effects in the gas phase acidity of a set of ortho, meta and para-substituted benzoic acids

Summary

Introduction

Chattaraj et al [12] found trends of changes in acidity of meta- and para-substituted benzoic acids with electrophilicity based charge transfer electrons transferred group effects in proton-transfer reactions in gasindex, phase.fractional. Hollingsworth et al density [15] examined the effects substituents on the of a set they found a good correlations with the Löwdin, Mulliken, AIM, and natural population analysis of 16 meta- and para-substituted benzoic acids using density functional theory [B3LYP/6-311G(d, p)]. Naturalthe population substituent on the and of para-substituted phenolsMéndez [16,17,18],etethanol derivatives analysis charges onacidity atoms of thereactivity dissociating carboxylic acid group. Z on the gasthe phase acidityacidity of a set ortho-, meta-metaand and para-substituted benzoic acids.

Results and Discussion

Methodology and Computational Details

Conclusions