Electrochemical behaviour of 4-nitroimidazole and 2-methyl-5-nitroimidazole

• Published in: Journal of electroanalytical chemistry and interfacial electrochemistry 1982-12, Vol.142 (1), p.263-275
• Main Authors: Roffia, S., Gottardi, C., Vianello, E.
• Format: Article
• Language: English
• ISSN: 0022-0728
• DOI: 10.1016/S0022-0728(82)80020-X

The electrochemical behaviour of the title imidazoles (HRNO 2) has been investigated in dimethylformamide and acetonitrile by polarography, cyclic voltammetry and controlled-potential electrolysis and coulometry. The above nitroimidazoles present similar behaviour, displaying two reduction waves paralleled by two reduction peaks in linear sweep voltammetry. The first peak is irreversible up to a sweep rate of 250 V s −1, while the second one appears to be reversible even at low sweep rates. The experimental data in the first process are consistent with the hypothesis that the irreversibility of the first wave (peak) is due to a rapid decay of the primary anion radical HRNO 2 because of a fast protonation reaction by the starting nitroimidazole (a father-son type of reaction), with the formation of the conjugate base RNO 2 − and of the neutral radical HRNO 2H. This last radical should give rise, as final reduction product, to the hydroxylamino derivative, the necessary protons being supplied by the starting HRNO 2. The second process is attributable to the reduction of RNO 2 − to RNO 2 2. As long as HRNO 2 has not been consumed, during controlled-potential electrolysis at second wave potentials, it is conceivable that the radical dianion RNO 2 2 is oxidized by the starting nitroimidazole to give its conjugate base together with the radical HRNO 2, which follows the reaction path already described for the first wave. Once all the HRNO 2 has been consumed, the homogeneous reduction of the supporting electrolyte cation (C 2H 5) 4N +, catalysed by the redox system RNO 2 −/RNO 2 2, takes place. Electrochemical and spectroscopic investigations on 1-methyl-4-nitro and 1-methyl-5-nitro derivatives of the above nitroimidazoles have provided both an indirect support to the above mechanism and information on the type of tautomer predominantly present in solution at equilibrium.