Benzene Oxidation in the Troposphere. A Theoretical Investigation on the Possible Competition of Three Postulated Reaction Channels

Three different attacks of 3Σg O2 on the hydroxy-cyclohexadienyl radical intermediate I (generated from reaction of OH with benzene) have been studied by Density Functional Theory. Both abstraction by O2 of the hydrogen gem to OH in I (affording phenol) and O2 addition to the Π-delocalized system of I (producing a hydroxy-cyclohexadienyl peroxyl radical intermediate) appear to be very viable, with ΔH‡ = 3-4 kcal mol-1. The former reaction is exothermic by 27 kcal mol-1, the latter only by 1 kcal mol-1. In contrast, a recently repropounded pathway, which would lead to benzene oxide/oxepine, via hydrogen abstraction from the hydroxyl in I operated by O2, appears not to be competitive, showing a significantly higher barrier (ΔH‡ = 32 kcal mol-1). Benzene oxide and oxepin are estimated to lie 21 and 19 kcal mol-1 above I respectively.