Electrochemical behaviour of the nitrobenzene family in aprotic media

Abstract

The electrochemical reduction of nitrobenzene, nitrosobenzene, azoxybenzene and azobenzene in aprotic media has been studied by conventional voltammetric and controlled­potential electrolysis techniques. The influence of the following experimental parameters has been investigated: electrode material, solvant, supporting electrolyte, proton donor, temperature, and the concentration of the electroactive substance. The voltammetric reduction of the four compounds in DMF using a quaternary ammonium salt as supporting electrolyte consists of two steps. The first reduction step is a reversible one-electron transfer process ta yield the radical anion and the second reduction step is usually irreversible due oa the fast protonation of the dianion. The total number of electrons involved in the second step depends on the nature of the compound. For nitrobenzene, the second peak (wave) involves a three­electron reduction to yield the phenylhydroxylamine anion. For nitrosobenzene and azobenzene, the second peak (wave) corresponds to an one-electron reduction to the dianion. The reversibility of this step depends on the supporting electrolyte and the temperature. When Me? N+ and Bu?N+ salts are used as supporting electrolyte, the anodic peak corresponding to the oxidation of the dianion to the radical anion increases in intensity upon lowering the temperature. However, with Et?N+ salt as supporting electrolyte, protonation of the dianion is much faster and the oxidation peak of the dianion is not observed even at -30 °C. For azoxybenzene, the second peak (wave) corresponds to an irreversible two-electron reduction of the azoxybenzene radical anion to the azobenzene radical anion. Further reduction of azobenzene radical anion can be observed at a more negative potential. As a result, three reduction peaks (waves) can be observed in the cyclic voltammogram (polarogram) of azoxybenzene. Partial electrolysis of nitrobenzene at -2.35 V (1.2 F/mole) gave phenylhydroxylamine as the predominant product (= 70% yield). The exhausitive controlled-potential electrolyses of nitrobenzene, nitrosobenzene, and azobenzene at -2.35 V gave similar results: hydrazobenzene was the predominant product. The first electron transfer on nitrobenzene and on azoxybenzene was found to be less reversible on copper and nickel electrodes than on platinum and mercury electrodes due to surface oxides on copper and nickel formed during the conventional polishing process. The surface oxide was removed either by reduction by molecular hydrogen at 280 °c or by cathodic polarization at -2.20 V in TEAP - DMF. On these pretreated copper and nickel electrodes, the anodic to cathodic peak separation was nearly the same as that on platinum and mercury electrodes. With nitrosobenzene and azobenzene, the presence of oxides had no influence on the reversibility of the first electron transfer. ln the presence of lithium perchlorate, only one reduction peak for nitrobenzene, azoxybenzene, and azobenzene, and only two reduction peaks for nitrosobenzene can be observed in their cyclic voltammograms. These peaks decrease rapidly in intensity on the second and subsequent cycles due to the formation of a non-conducting grey film on the electrode surface. At HMDE, a new redox systems which consist of two anodic peaks and two cathodic peaks can be observed at potentials more positive than those due to the parent substances; they could be attributed to the formation (anodic peak) and reduction of organomercury compounds between the azobenzene dianion strongly associated with lithium cations and mercury. A new example of a chain reaction induced electrochemically was found for the reduction of nitrosobenzene in the presence of fluorene or indene. The products of this chain process are N-fluorenyl-N-phenyl nitrone (1) and fluorenone anil (2) for fluorene and N-indenyl-N-phenyl nitrone (3) and indenone anil (4) for indene. At -30 ° C , fluorenone anil was the sole product formed and was isolated in a 97% yield. The mechanism for the chain reactions was proposed and the rate constants of the chain reaction of nitrosobenzene at -30 °c are 3.1 x 10? (k?) and 4.5 x 10? (k?) in the presence of five-fold excess of fluorene, and 6.9 x 10? (k? ) and 7.4 x 10? (k?) in the presence of five-fold excess of indene