Density functional theory study on the reaction of triazol-3-one with nitronium: direct nitration versus acidic group-induced nitration

Abstract

The nitration mechanism as well as the kinetics of triazol-3-one (TO) with nitronium (NO₂⁺) in both a concentrated nitric acid and a nitric–sulfuric acid system was theoretically studied. Firstly, the density functional theory (DFT) with a B3LYP functional was employed to investigate the mechanism of the mentioned reactants towards the targeted product, 5-nitro-2,4-dihydro-1,2,4-triazol-3-one (NTO). An unexpected induction effect, which derived from the coexisting acid group (NO₃⁻ and/or HSO₄⁻), was proclaimed. The impact of the induction effect on the nitration of TO was systematically demonstrated. It is found that unlike the nitration of most aromatics, the nitration of TO with NO₂⁺ to form NTO does not follow the typical electrophilic substitution mechanism. Based on the results calculated in each acid system, the nitration mechanisms, including the NO₂⁺ direct nitration (path A), NO₃⁻-induced nitration (paths Bn–Dn) and HSO₄⁻-induced nitration (paths Bs–Ds), were proposed. It is indicated that path A is unlikely or unfavorable due to the high activation barrier in the rate-determining step, whereas paths Bn–Dn and Bs–Ds are favorable, mainly attributed to the significant decrease of the activation energy induced by NO₃⁻ and HSO₄⁻ during the nitration process, especially for the NTO-oriented path Bn and Bs. Secondly, the canonical variational transition (CVT) state theory with small curvature tunneling (SCT) correction was used and the rate constants of the rate-determining steps for all paths at different temperatures were calculated. It is shown that the nitration rate in either path Bn or path Bs outdistances that in path A, indicating that NO₃⁻ and HSO₄⁻ accelerate the nitration of TO with NO₂⁺, and ultimately favour the formation of NTO due to the proposed induction effect of each acid group. An enhanced catalytic effect of the nitric acid or/and sulfuric acid is thought to be embodied in not only the acceleration to the formation of NO₂⁺, but also the induction effects of NO₃⁻ and HSO₄⁻ during the nitration processes. Meanwhile, it is suggested that the concentration of nitric acid and sulfuric acid in each nitration system should be well controlled since the favourable condition to produce NO₂⁺ and NO₃⁻/HSO₄⁻ differs in the concentrations of the corresponding acids.