The electrochemical reduction of 5-nitroamino-and 2-methyl-5-n#roaminotetrazoles at the first wave potential consumes six electrons, while the reduction at the second wave potential consumes eight or nine electrons. The preparative reduction of 2-methyl-5-nitroaminotetrazole at the second wave potential leads to 2-methyl-5-aminotetrazole and ammonia. A mechanism is proposed for these reactions.
A six-electron reduction wave is observed in the electrochemical reduction of aromatic and aliphatic N-nitroamines leading to substituted hydrazines [1, 2]:
In contrast, two reduction waves, which alternate upon change in the acidity of the medium, are observed in the reduction of primary N-nitroaminotetrazoles [3]. However, it has not been clear whether the limiting currents of these waves are diffusionat or kinetic. Also, the number of electrons participating in the reduction of N-nitroaminotetrazoles at the first and second wave potentials as well as the reaction products have remained unknown.
The limiting currents of both waves proved diffusional in the pH ranges, at which they have their maxima. Thus, the activation energy of the limiting currents are 7.37 kJ at pH 3.06 for 5-nitroaminotetrazole (I) and 8.08 kJ at pH 0.20 and 9.36 kJ at pH 3.94 for 2-methyl-5-nitroaminotetrazole (II). The slopes of the i--t curves in the first drop in logarithmic coordinates (c~) are 0.20-0.22. Kinetic limitations to the current appear in the segments characterized by a decrease in the limiting currents (e~ 0.25-0.33). The kinetic nature of the limiting current in the region characterized by a decrease in the first wave and increase in the second wave was also indicated by voltamperometry. The voltamperometric curves show two peaks, corresponding to the first and second waves on the polarographic curves. The ratio of the height of the first peak to the height of the second peak changes with increasing rate of potential application. The first peak disappears completely with a further increase in this rate. Such an effect may occur only if there are two forms of the nitroaminotetrazoles in equilibrium with each other. In the case of a stepwise reduction mechanism, the magnitude of the first peak cannot decrease below some level such as the one-electron level. In previous work [3], we proposed that the protonated and unprotonated forms are the equilibrium nitroaminotetrazole forms. The protonated forms are reduced in more acidic media (first wave), while the unprotonated forms are reduced in less acidic media (second wave).
Coulometric studies have shown that six electrons are consumed in the reduction of I and II at the first wave potentials. By analogy with previous results [1, 2], we may assume that 5-hydrazino-and 2-methyl-5-hydrazinotetrazoles are formed in this reduction.
Comparison of the limiting currents of the first and second reduction waves and the coulometric results suggest that eight or nine electrons are consumed in the reduction of I and II at the second wave potentials (the experimental precision does not permit an unequivocal determination of these values). There are no data in the literature on the reduction of N-nitroamines involving eight or nine electrons. In order to identify the reaction products, we carried out the preparative reduction of IIat