Electrochemical Reduction of 2,4-Dinitrotoluene in Aprotic and pH-Buffered Media

Olson, E. J.; Isley, W. C.; Brennan, J. E.; Cramer, C. J.; Bühlmann, P.
J. Phys. Chem. C 2015, 119, 13088 (doi:10.1021/acs.jpcc.5b02840).

As an intermediate in the industrial 2,4,6-trinitrotoluene (TNT) production, 2,4-dinitrotoluene (DNT) is one of the most prevalent impurities in TNT. Its high volatility as compared to TNT makes DNT a telltale sign of TNT explosives, and the electroactivity of its nitro groups lends itself to electrochemical detection as a particularly promising route for DNT sensing. In this work, the electrochemical reduction mechanism of DNT was investigated by cyclic voltammetry. In anhydrous solvents and in the absence of another source of protons, DNT exhibits two well resolved one-electron transfers. The radical anion formed by reduction of DNT is shown to be sufficiently basic to deprotonate the weakly acidic methyl group of still unreacted DNT, giving rise to an intense blue color. In contrast, in the presence of a pH buffer, DNT is readily reduced in acetonitrile to 2,4-bis(N-hydroxylamino)toluene by an irreversible transfer of eight protons and eight electrons. Density functional calculations suggest that the pathway to 2,4-bis(N-hydroxylamino)toluene is thermodynamically more favorable than the formation of aminonitrotoluene. This occurs at a substantially less negative potential than in the absence of protons, which shows that the reduction of DNT in the presence of protons is directly coupled to the protonation of DNT. The enhancement in signal observed for DNT in the presence of protons indicates that the incorporation of a proton source to electrochemical sensing setups will greatly enhance the sensitivity of electroreductive DNT analysis.