Are one-step aromatic nucleophilic substitutions of non-activated benzenes concerted processes?

Abstract

Aromatic nucleophilic substitution (S_NAr) reactions of non-electrophilically activated benzenes have been studied within the Molecular Electron Density Theory (MEDT) at the B3LYP/6-311+G(d) computational level. These reactions, taking place through a one-step mechanism, present a high activation Gibbs free energy, ΔG^≠ = 31.0 kcal mol⁻¹, which decreases to 22.1 kcal mol⁻¹ in the intramolecular process. A topological analysis of the electron localisation function along the reaction paths permits establishing the non-concerted nature of these S_NAr reactions. A series of unstable structures, with similar electronic structures to those of Meisenheimer intermediates, are characterised. The present MEDT study makes it possible to establish that even these one-step S_NAr reactions involving only two single bonds are non-concerted processes.