Linoleate oxidation, autoxidation mechanism

The basic mechanism of autoxidation at elevated temperatures is similar to that of room-temperature oxidation, i.e., a free radical chain reaction involving the formation and decomposition of hydroperoxide intermediates. Although relative proportions of the isomeric hydroperoxides, specific for oleate, linoleate and linolenate, vary with oxidation temperatures in the range 25°C -80°C, their qualitative pattern is the same (. Likewise, the major decomposition products isolated from fats oxidized over wide temperature ranges are those reflecting autoxidation of their constituent fatty acids (2 -6). The mechanisms and products of lipid oxidation have been extensively studied. The reader is referred to the numerous monographs, reviews and research articles available in the literature (1,A,7,8,9,10,11). 

Linoleic acid and its esters were frequently used in the study of autoxidation Being a 1,4-diene, linoleic acid has one particularly weak a-methylenic C... H bond, so that, at least in the initial stages, the hydroperoxide which becomes conjugated is practically the only oxidation product. The generally accepted reaction mechanism is the chain reaction... 

Chlorophyll, methylene blue, protoporphyrins and erythrosine react with unsaturated fatty esters by type II photosensitized oxidation in which singlet oxygen produces hydroperoxides by an entirely different mechanism from free radical autoxidation. In marked contrast to autoxidation, the distribution of hydroperoxides produced from oleate, linoleate and linolenate in the presence of singlet oxygen is very different, and is discussed in Section B below. 

As with oleate, some volatile decomposition compounds are formed from linoleate hydroperoxides that cannot be explained by the classical A and B cleavage mechanism, including acetaldehyde, 2-pentylfuran, methyl heptanoate, 2-octenal, 2,4-nonadienal, methyl 8-oxooctanoate, and methyl 10-oxodecanoate. Although hydroperoxides derived from both autoxidation and from photosensitized oxidation of linoleate form the same volatile decomposition products, significant quantitative differences are noted. The autoxidized linoleate hydroperoxides produced more pentane, 2-pentyl furan, 2,4-decadienal and... 

---