DFT study of the mechanism of nitration of toluene with nitronium

Abstract

AbstractReaction profiles of the isomeric nitration of toluene with nitronium ion were studied. Stationary points including three isomeric transition states were successfully located and characterized for the first time by employing hybrid DFT procedures at the B3LYP/6–311G** level without any restriction on the internal coordinates. Further correlation energy corrections by MP2/6–311G**//B3LYP/6–311G** were performed to evaluate the activation barrier heights. Mechanistic studies on the geometry, charge, energy and IR spectrum of the stationary points were carried out to illustrate the microscopic nitration process. A comparison of the nitrations of toluene and benzene shows that the methyl group plays dual roles in the nitration of toluene for its inductive and superconjugative effects. That is, the introduction of a methyl group creates favorable sites for the attacker and stabilizes the isomeric toluene–NO complexes. Both roles are critical to the positional selectivity. The order of the activation barrier height is para < ortho < benzene < meta. This relative reactivity order is not changed with either B3LYP/6–311G** or MP2/6–311G**//B3LYP/6–311G** calculations, although the absolute magnitude of the latter is larger. The order of the stability of complexes is p‐toluene–NO > o‐toluene–NO > m‐toluene–NO > benzene–NO. The relationship between the energy and the electron migration of NO2 group was established to elucidate the stabilization caused by the interaction between the two moieties of the complex. The vibrational shifts of tetrahedral C—N and C—H bonds suggest that the nitration of toluene may undergo the classical electrophilic substitution pathway, namely an SE2 mechanism. Copyright © 2004 John Wiley & Sons, Ltd.