Vapour-phase chemistry of arenes. Part 10. Formation of phenols in air oxidation of benzene, chlorobenzene, m-dichlorobenzene, and benzonitrile in the presence of cyclohexa-1,3-diene at ca. 600 K

Abstract

In a flow reactor at 573–623 K, flow time ca. 2 min, vapours of benzene and derivatives, e.g. chlorobenzene and benzonitrile, are not measurably oxidized by air. In the presence of small amounts of cyclohexa-1,3-diene (1), however, substantial conversion of (1) is accompanied by formation of phenols from arenes. The main oxidation product of (1) is benzene, but some phenol, and cyclohex-3-enone (2) is also formed. Conversion of (1) is largely due to hydrogen abstraction by O₂, log(A₂/l mol^–1 s^–1)= 8.9, E₂ca. 104 kJ mol^–1. The resulting cyclohexadienyl radical gives benzene by disproportionation with O₂. Formation of phenol is explained by addition of O₂ to cyclohexadienyl radical, followed by rearrangement and reaction with O₂. Addition of HO₂˙ to (1) is shown to be the likely first step in forming non-aromatic oxygenated products such as (2). This slow combustion of (1) leads to ˙OH radicals which must be responsible for conversion of arene. Product data, especially isomeric composition of substituted phenols, as well as thermochemical–kinetic analysis indicate that, at ca. 600 K, a mechanistic transition occurs, from addition of ˙OH (prevailing at lower temperatures) to hydrogen abstraction to give aryl radicals, which lead to phenol via ArO₂˙ and ArO˙.