QUANTUM-CHEMICAL STUDY ON THE MECHANISMS OF NUCLEOPHILIC-SUBSTITUTIONOF A NITRO-GROUP IN 2,4,6-TRINITROTOLUENE AND 1,3,5-TRINITROBENZENE

The semiempirical MNDO-PM3 method is employed for calculations of the enthalpies of formation of Meisenheimer ortho- and ipso-sigma-complexe s 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 pseudoconti nual 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 methoxi de ion in water by the S(N)Ar mechanism through an intermediate ipso-s igma-complex. In the gas phase this reaction can not follow the S(N)Ar mechanism due to its strong endothermicity (20 kcal/mol). The analogo us nucleophilic substitution of a TNT nitro group by a methoxy group m ay be suppressed by the faster formation of the stable trinitrobenzene anion.