Cyclic voltammetric study of the disproportionation reaction of the nitro radical anion from 4-nitroimidazole in protic media

We have chosen 4-nitroimidazole (4-NIm) as a prototype of nitroimidazolic compounds in order to carry out an exhaustive cyclic voltammetric study in protic media, 0.1 M Britton–Robinson buffer+0.3 M KCl with ethanol as a co-solvent. The one electron reduction of 4-NIm in protic media at alkaline pH produces a stable nitro radical anion on the time scale of the cyclic voltammetric technique. The nitro radical anion decays according to a coupled chemical reaction and we have focused the study to follow this reaction using cyclic voltammetric methodology. The one electron reduction of the 4-NIm to form the nitro radical anion and the subsequent reaction of the radical obey an EC2 mechanism that follows very well the cyclic voltammetry theory for the disproportionation reactions previously described by Olmstead and Nicholson (Anal. Chem. 41 (1969) 862). We have obtained the disproportionation rate constant k2 which is strongly dependent on both pH and ethanol content according to the following regression equations: log k2=−0.932pH+12.771 and log(10-3 k2)=−1.998log[% EtOH]+3.873. The results obtained from this study in protic media differ substantially from previous studies in aprotic media wherein the nitro radical anion was not stabilized.