Quantum chemical study on the mechanisms of nucleophilic substitution of a nitro group in 2,4,6-trinitrotoluene and 1,3,5-trinitrobenzene

Abstract

The semiempirical MNDO-PM3 method is employed for calculations of the enthalpies of formation of Meisenheimerortho- andipso-σ-complexes of 1,3,5-trinitrobenzene (TNB) and 2,4,6-trinitrotoluene (TNT) with the methoxide ion in the gas phase and in water (using the pseudocontinual point-dipole solvation model). The calculated reaction enthalpies for the replacement of a TNB and TNT nitro group with a methoxy group reveal in principle the possibility for TNB to react with the methoxide ion in water by theSNAr mechanism through an intermediateipso-σ-complex. In the gas phase this reaction can not follow theSNAr mechanism due to its strong endothermicity (20 kcal/mol). The analogous nucleophilic substitution of a TNT nitro group by a methoxy group may be suppressed by the faster formation of the stable trinitrobenzene anion.