Reduction Pathways of 2,4,6-Trinitrotoluene: An Electrochemical and Theoretical Study

Abstract

The reduction pathways of trinitrotoluene are studied using electrochemical and computational methods. The electrochemical reduction of three nitro groups in 2,4,6-trinitrotoluene (TNT) is characterized by three major reduction peaks in cyclic voltammograms at the peak potentials of −0.310, −0.463, and −0.629 V vs a normal hydrogen electrode (NHE). The second and third peaks coincide with the two peaks observed for the 2-amino-4,6-dinitrotoluene (at the potentials of −0.475 and −0.627 V vs NHE), whereas the two peaks in the 4-amino-2,6-dinitrotoluene voltammograms appear at −0.537 and −0.623 V and deviate more significantly from the corresponding two peaks in 2,4,6-trinitrotoluene. It suggests that the first NO₂ group reduced in the overall process is the one in <i>ortho</i> position with respect to the CH₃ group. Analogously, the 2,6-diamino-4-nitrotoluene exhibits a reduction peak at −0.629 V, almost identical to the third and second reduction peaks of 2,4,6-trinitrotoluene and 2-amino-4,6-dinitrotoluene, respectively. Since the other isomer, 2,4-diamino-6-nitrotoluene, exhibits a reduction peak at −0.712 V, we conclude that the second reduction occurs also in the <i>ortho</i> position with respect to the methyl group. Most of these observations are corroborated by quantum chemical calculations, which yielded reduction potentials in a good agreement with the experimental values (in relative scale). Thus, studying in detail all of the possible protonation and redox states in the reduction of the first nitro group and the key steps in the reduction of the second and third nitro groups, we have obtained a comprehensive and detailed picture of the mechanism of the full 18<i>e</i>^–/18H⁺ reduction of TNT. Last but not least, the calculations have shown that the thermodynamic stabilities of (isomeric) neutral radical species (<b>X </b>+<b> </b><i><b>e</b></i>^<b>–</b><b> </b>+<b> H</b>^<b>+</b>)presumably the regioselectivity-determining steps in the 6<i>e</i>^–/6H⁺ reductions of the individual NO₂ groupsare within 2 kJ·mol^–1 (i.e., comparable to RT). Therefore, the course of the reduction can be governed by the effect of the surroundings, such as the enzymatic environment, and a different regioselectivity can be observed under biological conditions