Formation of Carbonium Ions from Electrooxidation of Alkyl Bromides

Primary, secondary, and tertiary bromoalkanes were potentiostatically oxidized at platinum gauze. The anolyte was acetonitrile–lithium perchlorate or tetraethylammonium fluoborate and the reference electrode Ag/0.1 M AgNO₃. Carbon–bromine bond cleavage, leading to the formation of N-alkylacetamides, was observed to be the exclusive route of these oxidations. Each of the oxidations of 2-bromopropane, 2-bromobutane, tert-butyl bromide, and neopentyl bromide yielded a sole amide, whereas 1-bromobutane, 1-bromopentane, 1-bromohexane, 1-bromo- 2-methylpropane, 1-bromo-3-methylbutane, 2-bromopentane and 3-bromohexane gave mixtures of amides. A mechanism involving an initial electron transfer from the nonbonding orbital of the bromine is proposed. This intermediate is thought to undergo attack by the nucleophilic solvent and/or undergo carbon–bromine bond breaking to generate highly energetic carbonium ions, which react with the acetonitrile directly or after rearrangement.