Electroreduction of Organic Compounds with Two Reactive Centers. II. Isophthal Aldehyde
Reduction of all three isomeric diformylbenzenes in aqueous solutions depends on protonation of the aldehydic groups. The reductions of diprotonated forms in acidic media show considerable differences of the behavior for individual isomers. Diprotonated forms of terephthaladehyde and orthophthaladehyde are reduced in a single reversible two-electron step, yielding a stable diradical or a quinomethide. Isophthaladehyde is reduced in acidic media up to pH 5 (where the rate of protonation of both formyl groups is sufficiently fast) in a single two-electron process. The irreversibility is caused by subsequent reactions. Each CHOH+ group is converted into a CHOH· group that can undergo dimerization. The simultaneous transfer of two-electrons in aqueous media is facilitated by protonation which occurs as a surface reaction. This allows an orientation of both CHOH+ groups with equal probability of an electron transfer. This makes the process in a protic medium different from processes in aprotic media, where after the transfer of the first electron the second group acts as a substituent, affecting transfer of the second and subsequent electrons. In the monoprotonated form the protonated — CHOH+ group is reduced first. This reduction occurs in the medium pH — range (pH 5–8) where the uptake of the second electron occurs on radical — CHOH· at more positive potentials than the first electron uptake. First reduction occurs in a single two-electron step, the potentiâl of which is affected by the second CHO group as substituents. The reduction of the unprotonated form at pH > 9 occurs similarly as in aprotic media: First two electrons are transferred in two separated one-electron steps.