Liquid-phase oxidation of cyclohexane. Cyclohexyl hydroperoxide, cyclohexanol, and cyclohexanone, mechanisms of formation and transformation

Abstract

The mechanisms of formation and transformation of cyclohexyl hydroperoxide, cyclohexanol, and cyclohexanone in the liquid-phase oxidation of cyclohexane are reviewed. Cyclohexyl hydroperoxide is formed in chain propagation reactions and decomposes via the degenerate branching reactions under the action of alkoxyl, alkyl, and peroxyl radicals, as well as via nonradical channels to form alcohol and ketone. The radicals of two types, \({\rm{H}}{{\rm{O}}_2}^ \cdot \) radical and α-hydroxycyclohexylperoxyl radical, are involved in chain propagation in the course of cyclohexanol oxidation. In the chain propagation reactions, \({\rm{H}}{{\rm{O}}_2}^ \cdot \) radicals are more reactive than α-hydroxycyclohexylperoxyl radicals. The dynamic equilibrium was established in the dissociation of 1-hydroxycyclohexylperoxyl radical to cyclohexanone and radical \({\rm{H}}{{\rm{O}}_2}^ \cdot \). During oxidation of cyclohexane, cyclohexanone is formed from peroxyl radicals, as well as by the decomposition of cyclohexyl hydroperoxide and chain-radical oxidation of cyclohexanol. The oxidation of cyclohexanone in positions 2 and 6 affords 2-hydroperoxycyclohexanone, which predominantly decomposes to form 2-hydroxycyclohexanone. The oxidation of 2-hydroxy-cyclohexanone proceeds via the main channels of destruction of the carbon chain of cyclohexane leading to adipic acid and adipic anhydride. The oxidation products of C—H bonds of all types in alcohol and ketone are accumulated simultaneously with the oxidation products of C(1)—H bonds in cyclohexanol and of C(2)—H and C(6)—H bonds in cyclohexanone.