Electrochemical Oxidation of 4-Hydroxyimino-2-pyrazolin-5-ones in Acetonitrile

Cyclic voltammetry and electrolysis in combination with ESR were used to study the electro- chemical oxidation of 1-R-4-hydroxyimino-3-methyl-2-pyrazolin-5-ones (R = H, C6H5, 4-BrC6H4, and 2-NO2-4-CF3C6H3) in acetonitrile on the background of 0.1 M Et4NClO4on glassy carbon and platinum electrodes in the absence and in the presence of bases (pyridine, piperidine, tetraethylammonium hydroxide). With all the compounds, one-electron transfer gives rise to iminoxyl radicals (g2.0045, aN630.78-31.33, aN11.87- 2.12). In alkaline medium the iminoxyl radical derived from 4-hydroxyimino-3-methyl-2-pyrazolin-5-one is unstable and oxidizes to 3-methyl-4-nitroso-1-pyrazolin-5-one. The oxidation potentials are much reduced with decreasing electron-acceptor power of the substituents and increasing basicity of the bases.