Characteristics of cationic phase-transfer catalysts in the oxidation of hydrocarbons by O2

Abstract

The oxidation of tetralin and cyclohexene by O₂ was investigated in the presence of cationic phase-transfer catalysts (PTCs). It was found that the oxidation takes place analogously to the recently investigated catalysed decompositions of hydroperoxide initiator molecules. The natures of both the onium cation and the counteranion are determining factors. The catalytic activity of the onium salt is determined by the effective charge on the onium ion and by the size and polarizability of the anion. For both hydrocarbons, the primary product of oxidation is the corresponding hydroperoxide, which may undergo further oxidation. For tetralin, tetralyl hydroperoxide underwent disproportionation into O₂ and tetralol, and reuse of the O₂ thus produced resulted in a considerable “overoxidation” in the closed reactor. Tetralone was formed in a smaller amount. The main products in the oxidation of cyclohexene were its hydroperoxide, cyclohexene oxide and 2-cyclohexen-1-ol. In contrast to the oxidation of tetralin, “overoxidation” did not occur here, and the formation of 2-cyclohexen-1-one was never observed. The oxidations of these hydrocarbons in the presence of cationic PTCs proved to be strongly influenced by the concentration of homogeneously dissolved water. The oxidation products of these hydrocarbons also exerted considerable influence on the progress of the oxidation.