Further Examples of para- and ortho-Disubstituted Benzene Derivatives Reduced in a Reversible Two-Electron Step into Products of Limited Stability

Abstract

In the course of the study of benzene derivatives bearing two electronegative groups on the ring, it was possible to demonstrate¹ that the behaviour of p-diacetylbenzene is different from that of most of the other substances studied. The change in the wrave-height of the more negative wave in the region where reduction of the C₆H₅COCH₂-grouping takes place, and the change of the anodic wave of the electrolysis product obtained by rectangular voltage polarization, indicated formation of an intermediate of biradical character with limited stability. In principle, three schemes were considered, involving hydration, protonation of the parent compound, and acid-base catalysed cleavage of the biradical intermediate. A chronopotentiometric investigation² confirmed that Scheme I gives the best explanation for all the experimental results. pH-Dependence of experimentally found rate constants, k _a, k _b, obtained by chronopotentiometry, is analogous to the dependence of the height of the second reduction step of p-diacetylbenzene and of the anodic wave of the primary electrolysis product obtained with the commutator. The reaction limiting the height of the second reduction step and of the anodic wave of the product, is hence the acid-base catalysed cleavage of the biradical. The specific rate constants of this reaction obey a Brønsted relation².