Mechanism of electrohydrodimerization of acrylonitrile on mercury from aqueous solutions

Abstract

The electrohydrodimerization (EHD) of acrylonitrile (AN) on mercury from aqueous solutions was examined by a polarographic technique with positive feedback for IR compensation as well as by differential-capacity measurements, with the aim of determining optimal experimental conditions under which the formation of the hydrodimer adiponitrile (ADN) is decidedly favoured over that of the saturated monomer propionitrile (PN). Several tetraalkylammonium cations at low concentrations (≈10 −3 M) and several supporting electrolytes were employed for this purpose. The best conditions for EHD were attained in aqueous solutions of 1.3 to 1.5 M Na + or Li + citrate containing 10 −3 M dodecylethyldimethylammonium (DEDMA) bromide. Under these conditions a gradual increase in the AN bulk concentration from 10 −2 M to 0.2 M causes a progressive splitting of the single two-electron wave due to PN formation into two consecutive one-electron waves, the first of which is due to ADN formation. Differential-capacity measurements indicate that such a progressive passage from the formation of the saturated monomer to that of the hydrodimer is accompanied by a gradual coadsorption of AN and DEDMA on the electrode surface, which demonstrates that the coupling reaction yielding ADN takes place in the adsorbed state. Mechanisms for PN formation at AN concentrations <10 −2 M and for ADN formation at the higher AN concentrations at which EHD takes place have been proposed.