Electroreduction of isophthalaldehyde: An example of simultaneous reduction of two identical electroactive centers

Abstract

The diprotonated form of isophthalaldehyde is reduced by two electrons to a diradical which, reduced further by two Electrons, reacts with two protons to form a dialcohol. The stability of the diradieal is much lower than that observed for <i>ortho- </i>and terephthalaldehydes. In acidic media isophthalaldehyde thus behaves like a simple benzaldehyde at twice the concentration. This course of the electrode process, based on polarographic reduction, has been confirmed by cyclic voltammetry and controlled potential electrolysis using a dropping mercury electrode. The diprotonated molecules of isophthalaldehyde are oriented during the reduction process in such a way that probability of the electron transfer is the same for both groups. In the single protonated form of isophthalaldehyde, the group protonated at the electrode is reduced in a two-electron process to alcohol. The reduction of the second carbonyl group occurs at more negative potentials. At the time of reduction the molecule is oriented with one CH=OH⁺ group towards the surface. Investigation of the reduction of the unprotonated form is limited by the reaction of the dicarboxaldehyde with hydroxide ions. Additions of hydroxide ions to carbonyl groups result in geminal diol anions which are oxidized in alkaline solutions in two consecuti ve two-electron steps. One of the CH(OH)O^- groups is then oriented preferentially at the electrode surface. In isophthalaldehyde dioxime both protonated oxime groupings are reduced simultaneously in a single, eight -electron step.