The solvent effect in the electrocatalytic reduction of organic bromides on silver

The electrochemical reduction of two model organic bromides, acetobromoglucose and benzyl bromide, on glassy carbon (assumed as non-catalytic reference material) and silver (the best electrocatalytic material for organic halide reduction) is investigated in six popular organic solvents of widely different properties (acetonitrile, dimethylformamide, dimethylsulfoxide, propylene carbonate, acetone and methanol). Analysis of the solvent effect on the electrocatalysis at Ag required the use of an appropriate reference system for a reliable intersolvental comparison of the electrode potentials. The redox behaviour of two ferrocene derivatives to be used as internal reference systems was therefore investigated in the selected solvents. Also the role of specific bromide anion adsorption on the silver surface was taken into consideration in the assessment of solvent effects on the electrocatalytic process. In all solvents tested, Ag exhibits a remarkable electrocatalytic effect for the reduction of both compounds. In particular, its catalytic activity is outstanding in the protic solvent methanol. For both compounds the electrocatalytic activities linearly increase with decreasing primary medium effect on the bromide anion (DG°Br-,WtoS), i.e. with increasing solvent ability in bromide anion coordination.