Examination of Mechanisms for Formation of Volatile Aldehydes from Oxidation of Oil-Based Systems

The mechanisms responsible for the production of small volatile aldehydes during low temperature condensed phase oxidation have been the subject of extensive research, and many pathways have been proposed in the literature. However, many of these mechanisms have yet to be explored quantitatively in the context of a kinetic model. A variety of mechanistic postulates for the formation of the volatile species hexanal, such as direct β-scission of alkoxy radicals, Korcek-like decomposition reactions, intramolecular hydrogen shifts, intramolecular reactions of allylic peroxy species, and scission reactions of higher order oligomeric species, were assembled, and quantum chemical calculations were performed where necessary to obtain estimates of kinetic parameters to test each reaction's kinetic relevance in a microkinetic model for the oxidation of a cobalt-catalyzed ethyl linoleate system. Under atmospheric conditions, scission of chain ends from dimeric species was the largest contributor of hexanal, with an induction time evident. A more detailed experimental data set than previously available in the literature with information in the early (<24 h) time window was obtained by gas chromatography/mass spectrometry headspace analysis, and agreement between the model and the experimental data was vastly improved when the mechanism involving decomposition of dimeric species was included.