The electrochemical reduction of a series of chloro-, bromo-, and iodoacetophenones in N, N-dimethylformamide has been shown to be a one-electron process with the formation of an unstable anion radical. The anion radical decomposes to form an acetylphenyl radical and halide ion. The acetylphenyl radical abstracts a hydrogen atom from the solvent system to form acetophenone, which is reduced at more negative potentials to complete the reaction sequence. Since the reduction of p-chloroacetophenone in the presence of cyanide ion results in the formation of the p-cyanoacetophenone anion radical, the intermediacy of an acetylphenyl radical is suggested. Although the decomposition rates of the ortho- and para-halogenated acetophenone anion radicals are quite rapid (>104 s-1), the kinetics for the decomposition of the m-chloroacetophenone anion radical to the acetylphenyl radical (k = 10 s-1) has been shown to be consistent with a simulated ECE model mechanism, with the second electron transfer being the reduction of acetophenone to its anion radical. The acetylphenyl radical is apparently not reduced further to the anion under anhydrous, aprotic conditions at the platinum cathode. The results obtained here are contrasted with previously reported decomposition pathways of halogenated aromatic anion radicals.
ASJC Scopus subject areas
- Organic Chemistry