Abstract
The electroreductions and electrooxidations of 2-, 3-, and 4-pyridinecarboxaldehyde have been investigated in solutions of sodium hydroxide of concentration up to 10 M ( J = 16.7) using dc and differential pulse polarography, linear sweep voltammetry, and controlled potential electrolysis. Both reductions and oxidations of these compounds were influenced by the formation of geminal diol anions (PyCH(OH)O −) in the bulk of the solution and in the vicinity of the electrode. The electroreductions of the 3- and 4-aldehydes proceed by a one-electron uptake, followed by a radical dimerization yielding pinacols. The two-electron reduction of the 2-aldehyde is complicated by competing dimerization and polymerization. In strongly alkaline solutions, the Cannizzaro reaction completes with bulk electrolysis. To undergo a two-electron oxidation in alkaline media to pyridinecarboxylic acids, pyridinecarboxaldehydes must be converted into PyCH(OH)O −. At J − < 14 this anion is present in equilibrium with both the free aldehydes (PyCHO) and their geminal diol forms (PyCH(OH) 2). To be electrooxidized the anion Py(OH)O − is either transported to the electrode from the bulk of the solution or formed in the vicinity of the electrode either by addition of OH − ions to PyCHO or by dissociation of PyCH(OH) 2. Measurements of kinetic currents of oxidation of 3-pyridinecarboxaldehyde at pH 10–12 made it possible to estimate rate constants for the formation of PyCH(OH)O −. At J −> 14 the geminal diol anion reacts with another hydroxide ion. This reaction results in a shift of half-wave potentials of the anodic wave to more negative values with incrasing a OH−.
Published Version
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