Free radicals in the electrochemical reduction of certain mononitro and dinitro derivatives of pyridine

Abstract

In the primary process of electrochemical reduction of substituted 3-nitropyridines in dimethylformamide, their anion radicals are formed. This also takes place in the reduction of 3,5-dinitropyridines and 3-nitropyridines with a nitrophenyl substituent at position 2 or 4. For these dinitro derivatives, however, secondary free radicals are formed as well; in a basic medium, these are the products of reduction of the corresponding Meisenheimer complexes. Serving as the reaction center for electroreduction is the nitro group on the pyridine ring, not the group on the phenyl ring. For the mononitropyridines and dinitropyridines that were studied, free radicals of the nitropyridine type are formed as the primary species. The structure of the primary and secondary free radicals was established by analysis of the hyperfine structure of their ESR spectra.