THE MNDO-PM3 STUDY OF THE MECHANISM OF NUCLEOPHILIC-SUBSTITUTION OF THE PHENOXIDE ANION FOR THE NITRO-GROUP IN 1,3,5-TRINITROBENZENE AND 2,4,6-TRINITROTOLUENE IN THE GAS-PHASE AND IN POLAR-SOLVENTS

The semi-empirical quantum chemical MNDO-PMS calculations of the entha lpies of formation of Meisenheimer ortho- and ipso-sigma-complexes of 1,3,5-trinitrobenzene (TNB) and 2,4,6-trinitrotoluene (TNT) with the p henoxide anion in the gas phase and in water are performed within the framework of the point dipole model. Based on the calculated heats and activation barriers to substitution of the nitro group by the phenoxy l group in TNB and TNT, the possibility of the reactions of TNB and TN T with the phenoxide anion in water is shown. These reactions in water occur via the SNAr mechanism involving the corresponding ipso-sigma-c omplex as an intermediate. In the gas phase, the SNAr mechanism is imp ossible, because the reaction is strongly endothermic. In the case of TNT, the exothermic reaction of elimination of a proton from the methy l group by the phenoxide anion competes with nucleophilic substitution in a polar solvent. The activation energy calculated for this exother mic reaction is 8 kcal mol(-1).