The role of onium salts in the oxidation of hydrocarbons by O2 catalysed by cationic phase-transfer reagents

Abstract

Experimental and theoretical evidence is presented that cationic phase-transfer catalysts promote the homolytic decomposition of hydroperoxide initiators into radicals, this being a fundamental step in the catalysis of the oxidation of hydrocarbons by O₂. Such decomposition of the model substance tert-butyl hydroperoxide (t-BHP) results in O₂, tert-butanol (90–95%) di-tert-butyl peroxide (5–10%) and traces of CO₂. The stoichiometric ratio Δ[t-BHP]/ΔO₂ was found to have a value of 2, independently of the nature of the counteranion present. It is assumed that the interaction between hydroperoxide and onium cation is mainly electrostatic in nature and that its effectivity depends on the positive charge density on the onium cation, which is controlled by the nature and dimensions of the counteranion. The role of water in the decomposition of t-BHP is also elucidated.