Understanding the mechanism of the decomposition reaction of nitroethyl benzoate through the Molecular Electron Density Theory

The molecular mechanism of the decomposition reaction of nitroethyl benzoate (NEB) 1 yielding nitroethylene 2 and benzoic acid 3 has been studied within the Molecular Electron Density Theory (MEDT) using DFT methods at the B3LYP/6-31G(d) computational level. This decomposition reaction takes place through a one-step mechanism. Bonding Evolution Theory (BET) analysis of this reaction provides a complete characterisation of the electron density changes along the reaction. The reaction begins through the synchronous rupture of the O-C and C-H single bonds of NEB 1. Interestingly, while the rupture of the O-C single bond takes place heterolytically, that of the C5-H6 one takes place homolytically, yielding the formation of a pseudoradical hydrogen atom. These changes, which demand a high energy cost of 37.1 kcal mol(-1), are responsible for the high activation energy associated with this decomposition reaction. Formation of the C-C double bond present in nitroethylene 2 takes place at the end of the reaction. The six differentiated phases in which the IRC associated with this reaction is divided clearly point out its non-concerted nature, thus ruling out the proposed pericyclic mechanism. This reaction, whose associated TS presents a more or less distorted six-membered cyclic structure in which all atoms may not necessarily be bound, is categorised as a pseudocyclic reaction.