Reduction of diprotonated form of aryl hydrazones

Abstract

Hydrazones derived from aromatic aldehydes and ketones are reduced at pH 2 to about 8 in a four-electron step. The species reduced in this step bears two positive charges on adjacent nitrogen atoms. This has been proved by the pH-dependence of half-wave potentials of N,N,N-trialkylhydrazonium ions, which indicates a protonation of the azomethine nitrogen prior to the first electron uptake. Similar species with two adjacent positive charges is generated by diprotonation of hydrazones, adsorbed at the electrode surface. The existence of such species as reactive intermediates at electrode surface has been in some instances confirmed, based on steep plots of imax/id = f(pH). The steep shape of these plots has been confirmed for acetophenone (II) and fluorenone (III) hydrazones using conventional buffers and for benzophenone hydrazone (IV) after extrapolation of buffer concentration to zero. The shape of the imax/id = f(pH) can be namely achieved by protonation not only by H+ ions, but also by acid buffer components, as in general acid catalysis.