The possibilities of polarography in the chemistry of nitroazoles

Marjanović³

et al. 1993

Reduction of the nitro group in l-alkyl-4-cyano-5-nitropyrazoles follows a pattern similar to that of 1-alkyl-5-nitropyrazoles, involving the sequence H+, e, H+, e, 2e, 2H+. On the other hand, the sequence of proton and electron transfers in the reduction of the nitro group in l-alkyl-3-nitro-4-cyanopyrazoles differs from that in 1-alkyl-3-nitropyrazoles. Similarly, the reduction of 4-cyano-3( 5)-nitropyrazole differs from that of 3(5)-nitropyrazole, even when both of these reductions are affected by the dissociation of the NH group with pK, = 6.26 and 9.81, respectively. Similarly, the behavior of I-alkyl-4-carboxamido-5-nitropyrazoles resembles that of l-alkyl-5-nitropyrazoles, but that of l-alkyl-3-nitro-4-carboxamidopyrazoles is different from that of 1-alkyl-3-nitropyrazoles. In l-alkyl-4-carboxamido-5-hydroxylaminopyrazoles, the carboxamido group is reduced in a two-and four-electron process, but the low absolute value of the limiting current of wave i2 indicates the presence of competitive side reactions.

Section: Introductionmentioning

confidence: 87%

Polarographic and electrochemical studies of some aromatic and heterocyclic nitro compounds: Part V. Polarographic reduction of cyano‐and carboxamido‐nitropyrazoles

Marjanović³

et al. 1993

“…ArNOzHo + e + ArN02H- (11) ArN02Hp + H+ * ArNO + H,O (12) Parallel E,,,-pH plots ( Figure 4) make it possible to compare structural effects at any pH. Most striking difference is caused by the position of the reduced nitro group in pyrazolium salts II and 111.…”

Section: I-nitropyrazole (I)mentioning

confidence: 99%

Polarographic and electrochemical studies of some aromatic and heterocyclic nitro compounds, part II: Polarographic and voltammetric reduction of some nitramines, pyrazolium, and imidazolium nitro derivatives

Sužnjević

et al. 1992

Reduction of 1-nitropyrazole (I) results in acidic media in a cleavage of the N-N bond and formation of nitrous acid which is further reduced at more negative potentials. At pH > 4 a competitive reduction of the nitramine to nitrosamine occurs. Four-electron reduction of nitro derivatives of pyrazolium (11) and (111) and imidazolium (IV) and (V) salts results in a formation of a hydroxylamine derivative. The reduction follows the scheme: H+, e, e, H + , 2e, 2H+. In the reduction product the azolium ring is reduced at more negative potentials. Whereas the effect of pyrazolium and irnidazolium rings on the reducibility of the nitro group is comparable, the interaction between the nitro group and the azolium ring markedly depends on the position of the nitro group on the ring.

Polarographic and electrochemical studies of some aromatic and heterocyclic nitro compounds, part III: Electroreduction of mono‐ and dinitropyrazoles and ‐imidazoles

Sužnjević

et al. 1992

Institute of General arzd Physicul C b m i s q , Studentski wg 12-16, ABSTRlCTThe reduction of mono-and dinitropyrazoles and of nitroimidazoles f o l l o~~ 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, Ht, respectively. For nitropyrazoles and nitroimidazoles without an alk~7l substituent on the ring nitrogen, the reduction process is further complicated by the dissociation of the NH-group in the heterocyclic ring. For l-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 sixelectron 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.