ELECTROCHEMICAL REDUCTION OF 1-HALO-2-BUTENES IN DIMETHYLFORMAMIDE

The electrochemical reduction of 1-bromo-2-butene and 1-chloro-2-buten e in DMF at a Hg electrode has been studied by polarography, cyclic vo ltammetry (CV), a rotating ring-disk electrode and controlled-potentia l coulometry. A CV study using a gold electrode has also been carried out for these compounds to identify the detected intermediates. Two co nsecutive one-electron reduction processes are found for 1-bromo-2-but ene in polarography and in CV using a Hg electrode. The first process is initiated by the irreversible one-electron cleavage of the carbon-b romo bond to give the allyl radical and Br-, which is the rate-determi ning step. The second one follows a first-order EC mechanism, being in itiated by generation of the allylmercury anion via a one-electron red uction of the allyl-mercury radical, previously formed by reaction of the allyl radical with Hg. A single irreversible two-electron process is found for 1-chloro-2-butene under all voltammetric conditions and f or both compounds in CV using a gold electrode. Additional anodic peak s detected in CV, as well as anodic waves found at the rotating Hg rin g electrode, are ascribed to oxidation of the allylmercury anion and t he allyl anion.