Aqueous and Nonaqueous Polarographic Studies of Substituted 2, 6-Dimethylbenzonitrile N-Oxides

Aqueous and nonaqueous polarographic properties of 4-substituted 2, 6-dimethylbenzonitrile N-oxides (stable nitrile N-oxides) have been studied and compared with those of the substituted pyridine N-oxides and benzylidenemethylamine N-oxides (nitrones) investigated previously by us. The first reduction wave in both the aqueous and N, N-dimethylformamide (DMF) solvent systems is due to the deoxygenation reaction of the nitrile N-oxide group except when certain substituents are present (see text). This conclusion has also been verified by controlled potential electrolysis in aqueous solution and by cyclic voltammetry in DMF solvent. A plot of the Hammett σ constants of the substituents against E1/2 values was linear with a positive slope for both the aqueous and DMF solvent systems. The slope is smaller than in the case of pyridine N-oxides and nitrones, this being reasonably attributable to the triple bond nature of the C〓N〓O group. Half-wave reduction potentials of the nitrile N-oxides are positively shifted compared with those of pyridine N-oxides, particularly in an aqueous solvent. Molecular orbital calculations were applied to interpret the substituent effect on the reduction potentials of the N-oxides.