The Reduction Mechanism of the >C=O Group. Part 4. The Electrochemical Reduction of Isonicotinamide in an Aqueous Medium.
作者:Dorothée Dion、Rita Meunier-Prest、Etienne Laviron、Erik Jonas Pedersen、Elemér Fogassy、Charles A. McAuliffe、Stenbjörn Styring、Cecilia Tommos、Kurt Warncke、Bryan R. Wood
DOI:10.3891/acta.chem.scand.51-0411
日期:——
The electrochemical reduction of isonicotinamide NRCONH2 (INA), in which NR represents the pyridine ring, has been examined in an aqueous medium between H-0 = -3 and pH 13.75. For pH<6, as shown by cyclic voltammetry and polarography, a global 2e(-), 2H(+) reversible transfer is followed by two first-order chemical reactions and a 2e(-), 2H(+) reduction (EC1C2E process), as in the case of methyl isonicotinate NRCOOMe. The first chemical reaction C-1 is an internal proton transfer (rate constant K-1) The second chemical reaction C-2 (rate constant K-d), which is much slower than C-1, involves the loss of NH3 to give 4-pyridinecarbaldehyde, which is more easily reduced than INA. Reaction C-2 is base-catalyzed but no acid catalysis was found even in very acidic media (H-0 = -3). At pH > 6, another first-order reaction C-1' (rate constant K-1') competes with C-1 to give 1,4-dihydropyridine-4-carboxamide which is not further reducible. The rate constant K-1 is independent of pH between pH 1.8 and 5. Above pH 6, K-1' is independent of pH, whereas log K-1 decreases linearly with a slope of -2; this result can be interpreted as a consequence of adsorption, the actual rate constant K-s1 of the reaction, which occurs on the electrode surface, being inversely proportional to the concentration of the H+ ions.