THE REDUCTION-MECHANISM OF AROMATIC NITRO-COMPOUNDS IN AQUEOUS-MEDIUM .2. THE REDUCTION OF 4-NITROPYRIDINE BETWEEN H0=-6 AND PH96

The electrochemical reduction of 4-nitropyridine has been examined in aqueous medium between H-0 = -6 and pH 9.6, by polarography and by cyclic voltammetry. It occurs in three main steps: (i) the nitro compound is reduced (2e-) to the dihydroxylamine; (ii) the dihydroxylamine dehydrates to give the nitroso compound; (iii) the nitroso compound is reduced (2e-) to the hydroxylamine. Analysis of the first 2e- stage, using the theory of the bicubic scheme with fast protonations as discussed by Meunier-Prest and Laviron (J. Electroanal. Chem. 328 (1992) 33) leads to the conclusion that the addition of the second electron is rate determining. A study of the variations in the apparent heterogeneous rate constant, from H-0 = -6 to pH 2, and of those of the surface rate constant, from pH 4 to 9.6, allows the sequence of addition of the electrons and protons to be determined; this sequence is complex, because of the protonation of the pyridine nitrogen, but it can be established that, on the electroactive site proper, the sequence is e- H+ e- H+ for all acidity values. The global reaction is of the electrochemical-chemical-electrochemical (ECE) type, and as such, has been analysed in the framework of the theory of Nadjo and Saveant (J. Electroanal. Chem., 48 (1973) 113). The case of 4-nitropyridine is exemplary, because the figurative point can be situated in each of the four main zones of the kinetic diagram, according to the acidity. The mechanism established here for 4-nitropyridine should be that of all aromatic nitro compounds. This excludes in particular a pre-protonation of the nitro group. 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 bicubic 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. the work of Laviron J. Electroanal. Chem., 124 (1981) 19).