Electrochemical Reduction of 2,4-Dinitrotoluene in Room Temperature Ionic Liquids: A Mechanistic Investigation

Abstract

The reduction mechanism of 2,4-dinitrotoluene (DNT) has been studied in eight room temperature ionic liquids (RTILs) using cyclic voltammetry (CV), square wave voltammetry (SWV), chronoamperometry, and digital simulation. Two distinctive peaks are observed in the voltammetry, corresponding to the stepwise reduction of the two nitro groups on the aromatic ring. Diffusion coefficients (D) and electron counts (n) were calculated from chronoamperometric transients, revealing an electron count of one in most RTILs, and a linear relationship between D and the inverse of viscosity. Focusing on the first reduction only, the peak appears to be chemically reversible at low concentrations. However, as the concentration increases, the current of the reverse peak diminishes, suggesting that one or more chemical steps occur after the electrochemical step. The results from digital simulation of the CVs in one of the RTILs reveal that the most likely mechanism involves a deprotonation of the methyl group of a parent DNT molecule by the electrogenerated radical anion and/or a dimerisation of two electrogenerated radical anions. Elucidation of the reduction mechanism of DNT (and other explosives) is vital if electrochemical techniques are to be employed to detect these types of compounds in the field.

Electrochemical Behaviour of Organic Explosive Compounds in Ionic Liquids: Towards Discriminate Electrochemical Sensing

“…From the irreversible shapes of the voltammetry in Figures 2 to 5, and the presence of an electrogenerated oxidation peak, it appears that the reaction mechanisms are probably the same in RTILs as they are in traditional solvents. This is in contrast to TNT and DNT, where the RTILs were found to stabilise the electrogenerated radical intermediates, allowing them to display chemically reversible voltammetry at low concentrations [17,18] . As mentioned previously, the detailed mechanisms for the organic explosive in this work could not be determined due to the complexities in fitting chronoamperometric transients to determine the electron count.…”

“…A concentration of 3 m m of explosive was used, because it provides a high enough concentration to ensure that the peaks were clearly distinguishable from the blank CVs, but not such a high concentration that unwanted side reactions occur. It also allows easy comparison to previous studies of TNT and DNT voltammetry, which were performed at the same concentration [17,18] …”

“…In several previous studies of dissolved analytes in RTILs, [17,18,35] microdisk electrodes were employed to simultaneously determine unknown diffusion coefficients and electron counts through fitting to a mathematical expression. The potential is stepped from a position of zero current to a potential after the first reduction process, the current is measured as a function of time, and the experimental transient is fitted to simultaneous equations described by Shoup and Szabo [35] .…”

“…Before a viable sensor can be developed, the fundamental electrochemical behaviour of the analyte needs to be understood. The reaction mechanisms of TNT [17] and 2,4‐dinitrotoluene (DNT) [18] have been previously proposed in RTILs, revealing a single electron reduction of each nitro group, followed by chemical steps that include dimerization reactions. These studies revealed that the RTIL structures, particularly the cation structure, influenced the shape and position of the voltammetry.…”

RACI and Academy of Science Awards 2017–18

“…The first step becomes chemically irreversible at increasing concentration, and simulations suggest the subsequent dimerisation of the radical anion, and/or abstraction of a proton from another DNT molecule. [12] Greg Qiao (The University of Melbourne) won the RACI's 2017 Applied Research Award. He reports a new methodology for the Fenton cancer therapy, whereby MOF nanoparticles were PEGylated via the surface-initiated atom transfer radical polymerisation (SI-ATRP) reaction to produce a PEGylated, reduced MOF (P@rMOF).…”