Decomposition reactions of bifenox anion radical involving intramolecular electron transfer

The nitropesticide bifenox (methyl 5-(2,4-dichlorophenoxy)-2-nitrobenzoate) is reduced in aprotic media in two reduction steps. The first heterogeneous electron transfer involves the acceptance of one electron leading to the formation of a relatively stable nitro anion radical that was characterized by EPR spectroscopy. The electrochemically generated radical anion eventually decomposes to two stable products. Intramolecular electron transfer coupled with chemical steps leads in the first case to the cleavage of the carboxymethyl group and the formation of the nitropesticide nitrofen ((2,4-dichlorophenyl)(4-nitrophenyl) ether) and in the second case to a trans-esterification process, i.e., an exchange of the methyl group for an alkyl of the supporting electrolyte. Both these final products retain pesticidal activity, which explains the high efficiency of bifenox as a pesticide. The electrochemical characteristics show that the radical anion undergoes a further two-electron reduction at more negative potentials. The second reduction step is strongly influenced by the nature and concentration of the tetraalkylammonium cation of the supporting electrolyte. The influence of the electrolyte concentration was accounted for by the effect of the electrode double-layer structure on the electron transfer kinetics (Frumkin effect) due to repulsion of the radical anion from a negatively charged electrode I electrolyte interface. (C) 2005 Elsevier B.V. All rights reserved.