Probing the photoreactivity of aryl chlorides with oxygen

Abstract

Molecular oxygen was used to probe the mechanism of the phototransformation of chlorobenzene and 4-chloroanisole in organic solvents. Laser flash photolysis, electron paramagnetic resonance and product distribution studies clarified the reaction mechanisms of these compounds under a wide range of conditions. The main primary photochemical reaction step is the homolytic cleavage of the C–Cl bond to produce a triplet radical pair in the solvent cage. In non-polar solvents hydrogen abstraction, after radical diffusion, leads to reduction. In polar solvents, in addition to H-abstraction, electron transfer within the caged radical pair occurs and leads to an ion pair (phenyl cation and Cl⁻). In the presence of oxygen, phenyl radicals can form phenylperoxyl radicals which have a bathochromically shifted absorption, thus making the homolytic cleavage visible by flash photolysis. The peroxyl radicals can couple, leading to more polar compounds, or undergo back reaction to the phenyl radical. For concentrations of the aryl chlorides of higher than 10⁻³ M, dimerization becomes an important transformation process and occurs after reaction of the transients with ground state molecules. In addition, excimer formation is postulated to be involved in the dimerization process.