Mechanistic Aspects on the Electrografting of Carbon Surfaces by Oxidation of Carboxylates Bearing Unsaturated Groups

The anodic oxidation of tetrabutylammonium carboxylates bearing aromatic or unsaturated terminal groups is a reaction that allows the covalent grafting of organic moieties on carbon electrodes in acetonitrile as solvent. The electron transfer step is followed by a fast decarboxylation reaction, which releases the respective unsaturated free radicals, whose transient formation at the interface explain the surface grafting phenomenon, however; the lack of grafting during the oxidation of saturated aliphatic carboxylates is unexpected. Therefore, this work shows that the covalent grafting by oxidation of unsaturated carboxylates is favored for a parallel orientation of the radicals on the electrode surface, which is originated by the positive polarization of the electrode and the partially negative electronic density on the terminal unsaturated groups in the radical structure. This orientation is furthermore the result of a previous similar orientation of the unsaturated carboxylates, which is transitorily retained because of the very high rate of the decarboxylation step. Cyclic voltammetry experiments performed using different saturated and unsaturated alkyl-carboxylates, their respective carboxylic acids, as well as the use of ferrocene derivatives as redox probes were used to indirectly support the main conclusions given here.