POLAROGRAPHIC AND ELECTROCHEMICAL STUDIES OF SOME AROMATIC AND HETEROCYCLIC NITRO-COMPOUNDS .3. ELECTROREDUCTION OF MONOPYRAZOLES AND DINITROPYRAZOLES AND NITROIMIDAZOLES

The reduction of mono- and dinitropyrazoles and of nitroimidazoles follows the general pattern of reduction of aromatic nitro compounds: The nitro group is reduced in a four-electron step to a hydroxylamino group and the protonated form of the hydroxylamino group is-in the lower pH range-further reduced to an amine. This reduction differs from that of nitrobenzenes in participation of a second hydrogen ion probably involved in protonation of the heterocyclic ring. This second proton is for nitroimidazoles transferred before the uptake of the first electron, for nitropyrazoles probably after this uptake. The transfer of the second electron is indicated to be the potential determining step. The two sequences are H+, H-, e, H-, e, 2e, H+ and H+, e, H+, H+, e, 2e, H+, respectively. For nitropyrazoles and nitroimidazoles without an alkyl substituent on the ring nitrogen, the reduction process is further complicated by the dissociation of the NH-group in the heterocyclic ring. For 1-alkyl-5-nitroimidazoles, for 4(5)-nitroimidazole and for N-unsubstituted-4- and 3(5)-nitropyrazoles (but not for 2-nitroimidazoles, 1-alkyl-4-nitroimidazoles and 1-alkylnitropyrazoles) the hydroxylamino derivative formed in the first four-electron step undergoes a base catalyzed dehydration yielding a quinone-like ketimine. Easy reduction of this species results in alkaline solutions in a limiting current which is significantly higher than corresponds to a four-electron and limits to a six-electron reduction. Such dehydration reactions occur considerably faster for dihydroxylamino derivatives formed in the reduction of dinitropyrazoles resulting in two waves with total transfer of up to 12 electrons.