Electroreduction and oxidation of terephthalaldehyde and the role of hydration

Electroreduction of terephthalaldehyde (benzene-1,4-dicarboxaldehyde) (I) in protic media is complicated by acid-base and hydration-dehydration equilibria. At pH 0-5.5 the diprotonated form of I is reduced in a reversible two-electron step to a diradical-quinonemethide. This species is at pH 1-3 converted into an aldol (4-HOCH2C6H4CHO), which is further reduced at a more negative potential. At pH 5.5-8.5 the monoprotonated form is reduced. The protonated formyl group is reduced first and then, at a more negative potential, the second formyl group in the aldol, formed in the first step, is further reduced. As terephthalaldehyde is in aqueous solutions present in about 15% as a monohydrate, the limiting currents in this pH range are controlled by the hydration-dehydration equilibrium, the establishment of which is acid and base-catalyzed. The reduction of the unprotonated form occurs at pH > 8.5. At pH > 10 addition of OH− ions takes place. The formation of a geminaldiol anion results in a decrease of reduction waves of the aldehyde, but an increase in the oxidation anodic wave of the diol. Dependence of anodic waves on activity of OH− ions indicates a consecutive addition of OH− ions to the two formyl groups. The understanding of the sequence of chemical (protonation, hydration) and electrochemical reaction steps enabled application of changes of polarographic i-E curves with time as an analytical method for following (in real time) concentration changes of the parent compound, its monohydrazones and monoximes as well as dihydrazones and dioximes.