Mechanisms of the formation of carboxylic acids and their anhydrides during the liquid-phase oxidation of cyclohexane

Abstract

The channels of formation and transformation of bifunctional (C₅, C₆) and monofunctional acids (C₁, C₅, C₆) and their anhydrides during the liquid-phase oxidation of cyclohexane are reviewed. Adipic acid and adipic anhydride are predominantly formed by the radical-chain oxidation of 2-hydroxycyclohexanone. Destructive transformations of 1-hydroxycyclohexyloxyl and cyclohexyloxyl (at a low conversion of cyclohexane) radicals, which are formed by the homolytic decomposition of 1-hydroperoxy-1-hydroxy- or 1-hydroxy-1-alkylperoxy-, and hydroperoxycyclohexane, respectively, afford 6-hydroxyhexanoic and caproic acids. 6-Oxohexanoic acid can be formed by the decomposition of 2-hydroperoxycyclohexanone via the nonradical and radical routes, as well as by the oxidation of 2-hydroxycyclohexanone with peroxide compounds. Valeric and 5-hydroxypentanoic acids are predominantly resulted from the destructive transformations of the 2-oxocyclohexyloxyl radical. The shortening of carbon chain is explained by decarboxylation, decarbonylation, and elimination of formic acid. The ring closure of 5-hydroxypentanoic acid to 8-valerolactone prevents its further oxidation. The schemes for the formation of 5-oxopentanoic and glutaric acids by the oxidation of 6-oxohexanoic acid at the C(5)-H bonds were proposed. The transacylation of the initially formed anhydrides with carboxylic acids leads to mixed anhydrides, and their alcoholysis by cyclohexanol affords all cyclohexyl esters of mono- and dicarboxylic acids.