A mechanistic study of the liquid phase autoxidation of nonan-5-one

Abstract

The liquid phase autoxidation of nonan-5-one has been studied as a model for the degradation of ketones formed from the oxidation of lubricants in automotive combustion engines. The oxidation, at 120 °C, gives a large number of products (32 identified), including derivatives of the substrate with an intact carbon skeleton and low molecular mass compounds, from radical fragmentations, as well as carbon oxides. The products were identified and quantified by GC-MS and GC, with the characterisation of isomers being aided by measuring the deuterium content of products from the autoxidation of 4,4,6,6-[²H₄]-nonan-5-one. Products arising from hydrogen atom abstraction from all the secondary C–H bonds were observed, with the main site of attack being α to the carbonyl group. The ketoalkyl, ketoperoxyl and ketoalkoxyl radicals formed in the reaction undergo radical dimerisation, disproportionation and hydrogen atom abstraction. With the ketoalkoxyl radicals the dominant pathway is β-scission, indeed ∼58% of the total product yield arises from this fragmentation of the α-ketoalkoxyl radical. Mechanisms for the formation of the observed products are proposed and used to determine the selectivity of the autoxidation for the different C–H bonds in the substrate.