Gas phase oxygenation of benzene derivatives at ca 300 K with O(3P) atoms produced by microwave discharge of N2O. Part 1. Rates and mechanism of phenol formation

Abstract

The reactions of O(³P) with benzene derivatives C₆H₅Z (Z = H, Cl, F, CH₃, CF₃, and OCH₃) have been studied in a flow reactor at ambient temperature in a helium atmosphere, using microwave discharge of nitrous oxide as the source of oxygen atoms. The main products are the substituted phenols; substituted cyclohexadienes were also observed. ipso-Substitution, with formation of phenol, is also an important path, except in the reaction with fluorobenzene and trifluorotoluene. The oxygen atom adds to the benzene ring to form a triplet adduct, which can isomerize to phenol or lose an H-atom, producing a phenoxyl radical. This phenoxy radical also leads to phenol via disproportionation with a cyclohexadienyl radical, the latter arising from H-atom addition to the substrate. Relative rates for O-addition were determined by competition experiments with benzene as a reference. A Hammett plot, using σ constants for the meta- and para-positions demonstrated the electrophilic character of O(³P). A good correlation was found for the para-positions, with ρ=–1.1.