THE REDUCTION-MECHANISM OF AROMATIC NITRO-COMPOUNDS IN AQUEOUS-MEDIUM .5. THE REDUCTION OF NITROSOBENZENE BETWEEN PH-0.4 AND PH-13

The electrochemical reduction of nitrosobenzene to phenylhydroxylamine has been examined in aqueous medium between pH 0.4 and 13, by polarography and by cyclic voltammetry. The results are analyzed using the theory of the 9-member square scheme with protonations at equilibrium (E. Laviron, J. Electroanal. Chem., 146 (1983) 15; R. Meunier-Prest and E. Laviron, J. Electroanal. Chem., 328 (1992) 33). A study of the variations of the apparent heterogeneous and surface rate constants shows that the sequences of addition of the electrons and protons are successively H(+)e(-)H(+)e(-), e(-)H(+)H(+)e(-) and e(-)H(+)e(-)H(+). The values of the elementary surface electrochemical rate constants deduced from our results are of the order of 10(9) s(-1), i.e. of the order of magnitude predicted by Brown and Anson (J. Electroanal. Chem., 92 (1978) 133). The elementary heterogeneous rate constants are much higher than predicted by the theory of the square scheme, which can be attributed to an increase in the apparent reversibility, owing to the occurrence of the 'surface' path, parallel to the heterogeneous path (cf. E. Laviron, J. Electroanal. Chem., 124 (1981) 19). The global 4e(-) reduction of an aromatic nitro compound to the corresponding hydroxylamine takes place via two successive 9-member square schemes linked by the dehydration of the intermediate dihydroxylamine; the reaction paths are determined.