Oleate oxidation, autoxidation

There is specificity of the antioxidant action in the presence of heterogeneous catalyst. The kinetics of ionol retarding action on the oxidation of fuel T-6 catalyzed by the copper powder and homogeneous catalyst copper oleate was studied in Ref. [12]. Copper oleate appeared to be very active homogeneous catalyst it was found to catalyze the autoxidation of T-6 in such small concentration as 10 6 mol L-1 (T = 398 K). The kinetics of autoxidation catalyzed by copper salt obeys the parabolic law (see Chapter 4) ... 

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). 

Secondary Decomposition and Polymerization. Reactions which usually occur in the later stages of autoxidation at room temperature may assume increased significance, and their consequences may be encountered much more rapidly, at elevated temperatures. For example, the series of short chain esters, oxo-esters, and dicarboxylic acids - usually formed in only trace amounts in room-temperature oxidation of oleate, linoleate and linolenate - can be found in significant amounts after heating for 1 hr at 180°C. Possible reactions leading to the formation of such compounds are given below. 

The mutual annihilation of free radicals is known as the termination stage (Reactions 12.5 through 12.7), when the free radicals R and ROO" interact to form stable, non-radical products. The rate of oxidation of fatty acids increases with their degree of unsaturation. The relative rate of autoxidation of oleate, linoleate, and linolenate is in the order of 1 40 100 on the basis of oxygen uptake and 1 12 25 on the basis of peroxide formation. ... 

Autoxidation and photo-oxygenation are two aspects of the non-enzymic reaction between oxygen and unsaturated fatty acids. The enzymic reactions are discussed in Section 10.3. Oxidation of lipids during storage and handling, involving complex substrates and ill-defined reaction conditions, proved difficult to understand. This difficulty is enhanced by the fact that the primary oxidation products are labile and readily converted to secondary oxidation products of several kinds. Understanding of these processes has come from studies of simpler substrates such as methyl oleate or methyl linoleate under clearly defined reaction conditions. 

Photo-oxygenation is a quicker reaction than autoxidation and the relative rates for oleate, linoleate and linolenate are shown in Table 10.1. In autoxidation the methylene-interrupted diene system is significantly more reactive than the isolated double bond of a monoene but in photooxygenation the relative rates of oxidation are more closely related to the number of double bonds. On the basis of these values photo-oxygenation of methyl oleate can be 30000 times quicker than autoxidation and for the polyenes photo-oxygenation can be 1000-1500 times quicker. 

Oxidation of methyl oleate has been extensively studied and is considered to be typical of all monoene acids/esters. Photo-oxygenation produces only two products - the 9-hydroperoxide (AlOt) and the 10-hydroperoxide (A8t) - in equal amounts. Reaction is confined to the olefinic carbon atoms and is accompanied by double-bond migration and stereomutation. As shown in Scheme 10.5 autoxidation of methyl oleate forms eight monohydroperoxides of which two (9-OOH A 10c and lO-OOH A8c) are only minor products. This range of compounds arises from initial attack at either allylic carbon atom to give a delocalized radical with oxygen finally attached to any one of four carbon atoms. 

Erankel, E.N., Garwood, R.E., Khambay, B.P.S., Moss, G. and Weedon, B.C.L. Stereochemistry of olefin and fatty acid oxidation. III. The allylic hydroperoxides from the autoxidation of methyl oleate. J. Chem. Soc., Perkin Trans. I. 2233-2240 (1984). 

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. 

The main volatile decomposition products formed from oleate, linoleate and linolenate are those expected from the cleavage of the alkoxyl radicals formed from the hydroperoxides of autoxidized and photosensitized oxidized fatty... 

Figure 4.10. Cleavage produets from methyl oleate hydroperoxides (Frankel et al, 1981). Relative pereent values shown in parentheses are for autoxidation and photosensitized oxidation respeetively.

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... 

Soybean oil containing a mixture of oleate, linoleate and linolenate triacylglycerols can produce 14 positional hydroperoxides by autoxidation and photosensitized oxidation (Chapters 2 and 3). Soybean oil and other vegetable oils, such as safflower oil and com oil, that do not contain linolenate, produce an unexpectedly high concentration of the 12-hydroperoxide isomer at peroxide values below 50. The 12-hydroperoxide isomer appears to be derived from photosensitized oxidation because its concentration is decreased in the presence of singlet oxygen quenchers such as )5-carotene and a-tocopherol (Chapter 3). 

Frankel, E.N., Neff, W.E. and Selke, E. Analysis of autoxidized fats by gas chromatography-mass spectrometry VII. Volatile thermal decomposition products of pure hydroperoxides from autoxidized and photosensitized oxidized methyl oleate, linoleate, and linolenate. Lipids 16, 279-285 (1981). 

Common vegetable oils containing mixtures of oleate, linoleate and linolenate produce 14 positional isomers of hydroperoxides by autoxidation and photosensitized oxidation (Chapters 2 and 3). More secondary products, dimers and... 

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