Saturated Hydrocarbons from Autoxidizing Methyl Linoleate

Abstract

EXAMINATION of the vapours of autoxidizing methyl linoleate by gas-liquid chromatography has revealed the presence of several compounds with retention times corresponding to methane, ethane, propane, butane and pentane. Identification of these compounds from autoxidation of a purified unsaturated ester has not, to our knowledge, been previously reported. This finding confirms a prediction made by C. D. Evans et al. that low-molecular-weight hydrocarbons would be produced by autoxidation of unsaturated fatty acids^1,2. They also reported saturated and unsaturated short chain hydrocarbons had been found in products from autoxidation of soybean oil³. Buttery and associates identified C₁ to C₅ normal saturated hydrocarbons in dehydrated potatoes, in which linoleic acid is the principal unsaturated acid in the lipids⁴. They speculated that the hydrocarbons might have been formed in autoxidizing dehydrated potatoes by hexanal decomposition that had been catalysed by fat peroxides. However, our findings with methyl linoleate show that the saturated hydrocarbons arise early in the autoxidation process, when aldehydes are either absent or present in undetectable amounts. Therefore, the hydrocarbons are being formed either by an extremely rapid decarboxylation of aldehydes present at a low steady-state concentration or by a totally different mechanism. Japanese workers identified ethane and higher molecular weight alkanes and alkenes from air-oxidation of soybean oil at 240° C (refs. 5 and 6). Their results cannot be compared with ours, however, because our ester was relatively pure and our experiments were performed at room temperature.