THE REDUCTION-MECHANISM OF AROMATIC NITRO-COMPOUNDS IN AQUEOUS-MEDIUM .3. THE REDUCTION OF 4-NITROPYRIDINE-N-OXIDE BETWEEN H-0=-6 AND PH-9

The electrochemical reduction of 4-nitropyridine-N-oxide has been examined in an aqueous medium between H-o = -6 and pH 9 by polarography and cyclic voltammetry. The results, analysed on the basis of the theory of the bicubic scheme with protonations at equilibrium, parallel and confirm those obtained for 4-nitropyridine. In the first two-electron stage, leading to dihydroxylamine, the addition of the second electron is rate determining; the sequence of addition of electrons and protons is complex because of the protonation on the greater than or equal to N --> O group, but it is e(-)H(+)e(-)H(+) for all acidity values on the electroactive group proper. The pH calculated for the minimum on the curve of the logarithm of the apparent rate constant as a function of pH, whose value is imposed, and by considering the Hammett correlations for the pK(a) values of nitropyridines and nitropyridine-N-oxides coincides exactly with the experimental Value (pH 2.95); it was equal to 8.1 for 4-nitropyridine. The results also confirm that the elementary surface electrochemical constants have values of the order of 10(9) s(-1), and that the large increase observed for the apparent heterogeneous constant should be due to the occurrence of the ''surface'' path, parallel to the heterogeneous path.