Electrochemical and thermodynamic behaviour of oxygenated nitrogen compounds and aromatic hydrocarbons in nitromethane, and the nitration process with NO +2 (solvation phenomena)

Abstract

The electrochemical properties of oxygenated nitrogen compounds in anhydrous deoxygenated nitromethane have been studied by means of linear and cyclic voltammetry at a platinum electrode. Taking into account previous works in sulpholane, the various electrochemical couples involving nitrogen oxides at oxidation states from II to V have been identified in nitromethane, and their standard potentials have been determined at 298 K. Furthermore we have induced electrochemically the molecular association: NO + 1 2 N 2O 4 → N 2O 3 (I) on a Pt electrode surface in order to reach the equilibrium constant of this reaction (I). All these findings have also allowed us to calculate the ionic dissociation constants of N 2O 3 and N 2O 4 according to N 2O 3 ⇆ NO + + NO − 2 (II) and N 2O 4 ⇆ NO + 2 + NO − 2 (III), respectively. It has been shown that thermodynamic data are comparable with those obtained in non-polar inert solvents and the standard potential of a couple hardly varies at all in various aprotic media if the nitrogen species involved in these chemical and electrochemical processes are solvated weakly (the solvent molecules barely intervene in these processes). On the other hand, we have undertaken the electrochemical study of aromatic hydrocarbons (ArH) in nitromethane in order to assess solvation effects on the aromatic radical cation (ArH + ) and hence to evaluate the standard potentials of ArH/ArH + couples. Finally, the survey of the redox process NO + 2 + ArH ⇆ NO 2 + ArH + (IV) in nitromethane has allowed us to obtain energetic and mechanistic information about the aromatic nitration with NO + 2 and to compare it with that found in other aprotic media by reference to solvation phenomena.