Alkyl vicinal dibromides are reduced by cob(I)alamin generated at a glassy carbon cathode in weakly acidic aqueous acetonitrile in a two-electron electrocatalytic cycle with a lowering in overpotential of 0.84 V compared to direct reduction. Unlike reactions of primary alkyl halides with cob(I)alamin that yield stable alkylcobalamins, 1,2-dibromobutane and 1,2-dibromoethane are reduced completely to alkenes at potentials somewhat positive of the E°' of Co(II)/Co(I). Spectroelectrochemistry showed

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Alkyl vicinal dibromides were reduced at glassy carbon electrodes In Isooctane/water emulsions stabilized by Aero-sokOT (AOT) and tetraethylammonium perchlorate (TEAP) by using an electrocatalytic cycle Involving vitamin B12. Catalysis lowers the overpotential by about 0.8 V compared to direct reduction In a homogeneous medium. The heterogeneous rate constant (0.0056 cm s-1) for the key Co(II)/Co(I) reduction step of the catalyst was about 3-fold smaller than In pH 2.3 aqueous acetonitrile. Apparent rate constants on the order of 104 L mol"1 s"1 for the rate-limiting chemical reactions of 1,2-dibromoethane and 1,2-dibromobutane with Co(I) were considerably smaller than In aqueous acetonitrile because of the partitioning of substrate Into the hydrocarbon phase. Despite these kinetic limitations, square-wave voltammetry could be used with the catalytic cycle to estimate total ethylene dlhalldes In unleaded and leaded gasoline In AOT/ TEAP emulsions prepared with gasoline.

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