Add time:07/16/2019         Source:sciencedirect.com

The electrochemical reduction of 1-methyl-4-methoxycarbonylpyridinium iodide (A) and 1-methyl-4-carboxypyridinium iodide (B) has been examined in an aqueous medium between H0 = −3 and − 4.5 and pH 9 and 14, respectively. As in the case of isonicotinic acid and its methyl ester, up to pH 7, a global 2e−, 2H+ reversible transfer is followed by two successive first order reactions. The first is a fast internal proton transfer; the second a demethanolation for A (rate constant kd < 5 × 10 −3 s−1) and a dehydration for B (rate constant kd = 0.45 s−1). Comparison with earlier results obtained for isonicotinic acid and its methyl ester shows that the dehydration rate constant is at least two orders of magnitude larger when the pyridine nitrogen bears a plus charge, and that the rate constant is at least 100 times smaller for the demethanolation than for the dehydration. Above pH 7, two le− waves, of which the first is reversible, are observed; reduction of the pyridine ring probably occurs.

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