New Insights into Electrocatalysis and Dissociative Electron Transfer Mechanisms: The Case of Aromatic Bromides

The electrochemical reduction of a series of aromatic bromides including substituted bromobenzenes and polycyclic bromoarenes has been investigated in acetonitrile at silver and glassy carbon (GC) electrodes. Whereas GC behaves as a noncatalytic electrode, Ag exhibits remarkable electrocatalytic activities for the reduction of all compounds. The electrocatalytic effects are strongly influenced by the molecular structure of the aromatic bromide, decreasing with increasing electron-withdrawing ability of the substituents as well as with extension of the polycyclic aromatic system. Dissociative electron transfer (ET) to an organic halide RX may occur either in a single step (concerted mechanism) or in two distinct steps with the formation of an intermediate radical anion (stepwise mechanism). The mechanism of the reduction process was analyzed at both catalytic and noncatalytic electrodes. Electroreduction of all compounds at GC occurs according to a stepwise mechanism, whereas at Ag both mechanisms were observed depending on the structure of the molecule. This study reports unprecedented examples of a passage from one dissociative ET mechanism to the other upon a change of the nature of the electrode material. The process at Ag involves adsorption of reagents, intermediates and products and is catalytic regardless of the mechanism of the dissociative ET. Analysis of the data herein reported together with literature data on the electroreduction of different types of organic halides sheds some light on the relation between catalysis and dissociative ET mechanism.