Electroreduction of 2,4,6-Trinitrotoluene in Room Temperature Ionic Liquids: Evidence of an EC2 Mechanism

Abstract

The reduction of 2,4,6-trinitrotoluene (TNT) has been studied in eight room temperature ionic liquids (RTILs) on a gold (Au) microdisk electrode and a Au thin film electrode (TFE). Three reduction peaks were observed in all RTILs, corresponding to the reductions of each of the three nitro groups in the TNT structure. TNT was the easiest to reduce in imidazolium RTILs, followed by pyrrolidinium and then tetraalkylphosphonium. Diffusion coefficients (D) and electron counts (n) were calculated from potential-step chronoamperometry on the first reduction peak. D's ranged from 0.7 × 10-11 to 4.1 × 10-11 m2 s-1, and a plot of D against the inverse of viscosity was linear, indicating that the Stokes-Einstein relation holds well for TNT in RTILs. The electron count was one in most RTILs-in stark contrast to the widely accepted six-electron reduction in protic solvents. An electrogenerated red solid was formed after the first reduction peak, believed to be an azo (or azoxy) compound formed by dimerization of two TNT radicals, although characterization of the product(s) proved difficult. The behavior at different concentrations revealed different degrees of chemical reversibility of reduction peak. This evidence points toward the possibility of an EC2 mechanism, which was supported by digital simulation of the experimental voltammograms. Understanding the reduction mechanism of TNT is essential if RTILs are to be used for TNT sensing applications, particularly at high concentrations.