Autoxidation of Unsaturated Fatty Acid Methyl Esters. II.

Nonconjugated Compounds

Abstract

Methyl linoleate and its trans, trans isomer, methyl linolenate, and a mixture of methyl eicosapentaenoate and methyl docosahexaenoate (50 : 50%) were autoxidized in the conditions free from natural antioxidants and metals. Their autoxidations gave the induction period of 49, 52, 18 and 11h, and showed the oxidation rates of 3.61, 3.08, 5.24 and 9.41×10^-5mol/h, respectively, at the autoxidation level after the induction period (specified as the time required to gain 10mg by weight). Relative autoxidation rates derived from the induction periods were almost similar to the ratio of the number of the active methylene group, but those from the oxidation rates were equal to the ratio of unsaturation degree. In comparison with the oxidation rate of methyl oleate, the abstraction of hydrogen from the active methylene groups by peroxyl radicals was superior by 1.54×10^-5mol/h in the rate to that from α-methylene groups. An Increase in the unsaturation degree of autoxidation substrates permitted the rapid and large formation of hydroperoxides, radicals, polymers, α, β, γ, δ-unsaturated carbonyls and etc. But in comparison at the same autoxidation level (weight gains) free from oxidative polymerization and decomposition, the amounts of these autoxidation products were similar each other. These data showed that the rate-determing step during induction period of autoxidation was the abstraction of hydrogen from the active methylene groups by oxygen molecules, and after the induction period was the diffusion of peroxyl radicals. The geometric isomeric effect that methyl linoleate was autoxidized easier than the trans, trans isomer was observed. This effect suggested the interaction of the oxygen molecule with the double boud of substrates in initiating the chain reaction.