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

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 ortho 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 ortho 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 18e–/18H+ reduction of TNT. Last but not least, the
calculations have shown that the thermodynamic stabilities of (isomeric)
neutral radical species (X + e– + H+)presumably the regioselectivity-determining
steps
in the 6e–/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.