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

Abstract

Three different attacks of 3Σg O2 on the hydroxycyclohexadienyl radical intermediate I (generated from the 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 hydroxycyclohexadienyl 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/oxepin, 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.