Oleates, autoxidation

Porter, N.A., Mills, K.A. and Carter, R.L. A mechanistic study of oleate autoxidation Competing peroxyl H-atom abstraction and rearrangement. J. Am. Chem. Soc. 116, 6690-6696 (1994). 

The photobleaching of P-carotene by fluorescent light in fatty acid ester solutions showed an autoxidation kinetic profile with the rate of degradation of P-carotene in the order laurate > oleate > linoleate (Carnevale et al. 1979). The presence of a radical scavenger retarded the autoxidation, thus leading to the view that protection against autoxidation is built into the system by the unsaturation in the fatty acid. 

Recent detailed studies on autoxidation reactions have been published for tetralin 44-46) cumene and ethylbenzene 46,47) methyl oleate 48,49) and benzaldehyde 50h... 

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) ... 

Farmer, E, H. and Sutton, D. A. 1943. The course of autoxidation reactions in polyiso-prenes and allied compounds. Part IV. The isolation and constitution of photochem-ically-formed methyl oleate peroxide. J. Chem. Soc. 1943, 119-122. 

Privett, O. S. and Nickell, E. C. 1959. Determination of structure and analysis of the hydroperoxide isomers of autoxidized methyl oleate. Fette, Seifen. Anstrichm. 61, 842-845. 

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. 

Esters of the fatty acid oleic acid are components of membranes that are subject to autoxidation. Explain which hydrogens of an oleate ester you expect to be abstracted most readily by a radical. Show the structures of the major autoxidation products that would be formed from an oleate ester. 

Figure 2.3 Regiomeric hydroperoxides from autoxidation of oleates.

As a further example the four hydroperoxides obtained in the autoxidation of oleate would be expected to give either the aldehydes and radical esters shown in the following equation or, alternatively, the oo-oxoesters and alkane and alkene radicals if the fi scission takes place on the other C-C bond. The free radicals can then react with neutral molecules or inactivate one another (Figure 2.12). 

Photooxidation is much faster than autoxidation the reaction of linoleate with singlet oxygen is approximately 1500 times faster than that with triplet oxygen (47). There is less difference in the rate of photooxidation between monoenes and polyenes than is seen in autoxidation. The relative rates for oleate, linoleate, linolenate, and arachidonate are 1.0, 1.7, 2.6, and 3.1 (48, 49). This contrasts with the 40-fold increase in rate of autoxidation between oleate and linoleate. 

The rate of autoxidation increases with the degree of unsaturation. In neat systems without adding initiators, linoleate having two double bonds was 40 times more reactive than oleate, which has only one double bond, hnolenate having three double bonds was 2.4 times more reactive than linoleate, and arachidonate having four double bonds was 2 times more reactive than linolenate (2, 3). 

Alemany P, Del Pozo A. Autoxidation of ethyl oleate protection with antioxidants [in Spanish], Galenica Acta 1963 16 335-338. 

Linstead and Whalley showed that urea complexes can be used for effective separation of straight- and branched-chain carboxylic esters. Swem s group used the method to advantage in working up autoxidized methyl oleate for isolation of long-chain hydroperoxides. These peroxides behave like branched-chain compounds because of the bulky peroxide group and remain in solution when nonperoxidic components of the mixture are precipitated as urea complexes. 

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. 

Table 10.1 Relative rates of autoxidation and photooxygenation of oleate, linoleate and linolenate...

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. 

The four hydroperoxides from autoxidation of oleate give four aldehydes (8 0, 9 0, 10 1 2t, 11 1 2t) and the two hydroperoxides from autoxidized linoleate give hexanal and deca-2,4-dienal. The aldehydes produced from other hydroperoxides (Tables 10.2 and 10.3) can be derived from the equation given above. Hept-cw-4-enal is reported to give a creamy flavour to butter but a... 

The relative rates of autoxidation of different unsaturated fatty acids and esters were compared on the basis of oxygen absorption measurements (Table 1.1). In neat systems without added initiator, linoleate was 40 times more reactive than oleate, linolenate was 2.4 times more reactive than linoleate, and arachidonate was 2 times more reactive than linolenate. The oxidizability of polyunsaturated fatty acid (PUFA) esters was also compared on the basis of oxygen uptake measured kinetically by the induction period method described above, in solution in the presence of azo initiators. The oxidizability of 18 2,18 3,20 4, and 22 6 was linearly related to the number of bis-aUyUc positions present in the fatty esters. From this relationship, the oxidizability of each PUFA was increased approximately two fold for each active bis-allylic methylene group. Thus, the oxidizability of 22 6 was 5 times greater than that of 18 2. 

Table 2.1. Hydroperoxides from autoxidation of methyl oleate (as % of total) ...

A number of mechanisms have been advanced to explain the formation of all eight cis and trans isomers of 8-, 9-, 10- and 11-hydroperoxides in autoxidized methyl oleate. In one mechanism, the delocalized radicals formed by abstraction of the hydrogens allylic to the double bond of oleate lose their... 

Frankel, E.N., Neff, W.E., Rohwedder, W.K., Khambay, B.P.S., Garwood, R.F. and Weedon, B.C.L. Analysis of autoxidized fats by gas chromatography-mass spectrometry I. Methyl oleate. Lipids 12,901-907 (1977a). 

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. 

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