Portal de Periódicos da CAPES

The reduction mechanism of aromatic nitro compounds in aqueous medium Part 3. The reduction of 4-nitropyridine-N-oxide between H0 = −6 and pH 9

1993; Elsevier BV; Volume: 359; Issue: 1-2 Linguagem: Inglês

10.1016/0022-0728(93)80411-a

1873-2569

Robert Lacasse, Rita Meunier‐Prest, E. Laviron, A. Vallat,

Analytical Chemistry and Sensors

The electrochemical reduction of 4-nitropyridine-N-oxide has been examined in an aqueous medium between H0 = −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 ≥ 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 pKa 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 109 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.