Electrochemical characterization and redox reaction scheme of the diphenyl ether herbicide nitrofen

Abstract

Redox behavior of nitrofen (2,6-dichlorophenyl- p-nitrophenyl ether) at a glassy carbon working electrode in aprotic and protolytic solvents was examined using cyclic voltammetry. The electrode reaction of 2 m M nitrofen in dimethylformamide (0.1 M tetra- n-butylammonium hexafluorophosphate as supporting electrolyte) was quasireversible. Measured half-wave potential was −1.11 V vs saturated calomel reference electrode (SCE) and peak potential separation was 0.110 V. The reduction product was chemically stable on the time scale of the experiment; cathodic peak current equalled anodic peak current. Linear sweep voltammetry employing a rotating, glassy carbon disk electrode indicated that electron stoichiometry for reduction of nitrofen in dimethylformamide and acetonitrile was one. Reduction of nitrofen under aprotic conditions was most likely a one-electron reduction to the nitro radical anion. Reduction of 1 m M nitrofen in 50% ( v v ) ethanol/water buffered at pH 5.0, 7.0, and 10.2 with 0.25 M sodium acetate, 0.1 M potassium phosphate, and 0.1 M sodium carbonate was chemically irreversible and characteristic of an EC … -type reaction mechanism; heterogeneous electron transfer followed by homogeneous chemical reactions. Cathodic peak potentials at pH 5.0 and 7.0 were −0.760 and −0.820 V vs SCE. At pH 10.2 two cathodic peaks were observed at −0.840 and −1.17 V vs SCE. In protolytic solvent at pH 5.0 cathodic peak current varied linearly as a function of the square root of the scan rate. Thus, the electrode reaction was limited by diffusion. Electron stoichiometry for reduction of nitrofen in protolytic solvent was four. The fate of new redox species generated in protolytic solvent at pH 7.0 was examined upon subsequent multiple scans at 200 mV/s. Initial reduction at −0.820 V vs SCE was most likely centered at the nitro group, yielding the four-electron hydroxylamine product. Oxidation at 0.020 V vs SCE was thought to correspond to the two-electron oxidation of hydroxylamine to the nitroso derivative.