REDUCTIVE BOND CLEAVAGE PROCESSES - II

Attempts to detect radical-anions from alkyl halides by γ-irradiation in a low temperature glass, using esr spectroscopy, have generally been unsuccessful. Irradiation of the glass generates solvated electrons that attach to the alkyl halide to generate an alkyl radical and a halide ion. There is an evidence for a weak interaction, but not bond formation, between alkyl radicals and bromide ion formed in acetonitrile at 77 K. In the more solvating methanol, there is no interaction between radicals and bromide ions. Trifluoromethyl halide molecules are the exceptions to this otherwise general behavior. These give radical-anions on irradiation in methyltetrahydrofuran at 77 K with free electron density largely in the carbon-halogen σ *-orbital. The trifluoromethyl radical is pyramidal so the tendency to complete separation from the halide ion is arrested in the glassy matrix. Alkyl radicals are planar, precluding any, strong association with the halide ion. Intramolecular dissociative electron transfer in aryl halide radical-anions involves an interaction between the π -aromatic orbital and the σ-type carbon-halogen bond. These orbitals are orthogonal, but bending of the carbon-halogen bond allows the necessary interaction. Qualitatively, the influence of several factors on the bond cleavage rate can be discerned.