Oleate hydroperoxides

Homogeneous Oxidation Catalysts. Cobalt(II) carboxylates, such as the oleate, acetate, and naphthenate, are used in the Hquid-phase oxidations of -xylene to terephthaUc acid, cyclohexane to adipic acid, acetaldehyde (qv) to acetic acid, and cumene (qv) to cumene hydroperoxide. These reactions each involve a free-radical mechanism that for the cyclohexane oxidation can be written as... 

The generation of free radicals occurs by the thermal and copper oleate-catalyzed decomposition of formed hydroperoxides with the rate [12]... 

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). 

The products of lipid oxidation in monolayers were also studied. Wu and coworkers (41) concluded that epoxides rather than hydroperoxides might be the major intermediates in the oxidation of unsaturated fatty acids adsorbed on silica, presumably because of the proximity of the substrate chains on the silica surface. In our work with ethyl oleate, linoleate and linolenate which were thermally oxidized on silica, the major decomposition products found were those typical of hydroperoxide decomposition (39). However, the decomposition patterns in monolayers were simpler and quantitatively different from those of bulk samples. For example, bulk samples produced significantly more ethyl octanoate than those of silica, whereas silica samples produced more ethyl 9-oxononanoate than those of bulk. This trend was consistent regardless of temperature, heating period or degree of oxidation. The fact that the same pattern of volatiles was found at both 60°C and 180°C implies that the same mode of decomposition occurs over this temperature range. 

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). 

Hydroperoxides of linolenate decompose more readily than those of oleate and linoleate because active methylene groups are present. The active methylene groups are the ones located between a single double bond and a conjugated diene group. The hydrogen at this methylene group could readily be abstracted to form dihydroperoxides. The possibilities here for decomposition products are obviously more abundant than with oleate oxidation. 

Figure 2-20 Photooxidation. Singlet-oxygen attack on oleate produces two hydroperoxides linoleate yields four hydroperoxides...

Older literature always presents the initial radicals in equivalent resonant positions with equal probability of forming hydroperoxides. The three resonant positions for linoleic acid or its ester and the three hydroperoxides resulting from these are shown in Reaction 41 (224). Comparable resonant structures have been published for oleate, linolenate, and arachidonate (222, 225). 

The latter observations with methyl oleate, together with thermodynamic considerations and EPR evidence for free radical intermediates, suggest an alternative explanation for the dramatic increase in oxidation rates once hydroperoxides accumulate, namely that bimolecular decomposition may be specific to allylic hydroperoxides and proceed via LOO radical-induced decomposition rather than by dissociation of hydrogen-bonded dimers (280). Reaction sequence 63 is analogous to Reactions 49 and 50a, where one slowly reacting radical reacts with a... 

Monohydroperoxides are the primary products of lipid oxidation. A variety of hydroperoxides with positional and geometrical isomers are formed depending on the position and number of double bonds of the unsaturated fatty acids and the oxidation mechanism. A number of reviews have been published on the composition of isomeric hydroperoxides formed from oxidation of oleate, linoleate, and linolenate (286, 287-291). The hydroperoxides formed are odorless, but they are relatively unstable and are the precursors of a variety of volatile and nonvolatile scission products that are important to the oxidized flavor. 

On oxidation with O2, methyl oleate (methyl 9-c/5-octadecenoate) was found to yield a mixture of hydroperoxides of formula CigH3404. In these, the —OOH group was found attached not only to C-8 and C-11 but also to C-9 and C-10. What is the probable structure of these last two hydroperoxides How did they arise Show all steps in a likely mechanism for the reaction. 

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. 

Fatemi, S.H. and Hammond, E.G. (1980) Analysis of oleate, linoleate, and linolenate hydroperoxides in oxidized ester mixtures. Lipids, 15, 379—385. 

Materials. Styrene-Butadiene Rubber (SBR) Latex. SBR latex was prepared by redox emulsion polymerization using (in parts) butadiene (69) and styrene (31) at 6°-40°C (pinane hydroperoxide/sodium formaldehyde sulfoxylate/Fe++ as initiator) in the presence of potassium oleate (2.7) inorganic electrolytes (0.45) as polymerization aids, and demineralized water (135) until a conversion of 70% was achieved. Residual monomers were then removed. 

Aluminum acetate Aluminum caprylate Aluminum distearate Aluminum myristates/palmitates Aluminum stearate Aluminum tristearate N-2-Aminoethyl-3-aminopropyl trimethoxysilane Aminoethylethanolamine Aminomethyl propanol Aminopropyltriethoxysilane Aminopropyltrimethoxysilane Ammonium benzoate Ammonium borate Ammonium citrate dibasic Ammonium laureth sulfate Ammonium laureth-5 sulfate Ammonium laureth-7 sulfate Ammonium laureth-12 sulfate Ammonium laureth-30 sulfate Ammonium lauryl sulfate Ammonium maleic anhydride/diisobutylene copolymer Ammonium oleate Ammonium persulfate Ammonium polyacrylate Ammonium potassium hydrogen phosphate Ammonium stearate Ammonium sulfamate Ammonium thiocyanate Ammonium thiosulfate Amyl acetate Antimony trioxide Asbestos Asphalt Azelaic acid 2,2 -Azobisisobutyronitrile Barium acetate Barium peroxide Barium sulfatej Bentonite Benzalkonium chloride Benzene Benzethonium chloride Benzothiazyl disulfide Benzoyl peroxide Benzyl alcohol Benzyl benzoate 1,3-Bis (2-benzothiazolylmercaptomethyl) urea 1,2-Bis (3,5-di-t-butyl-4-hydroxyhydrocinnamoyl) hydrazine 4,4 -Bis (a,a-dimethylbenzyl) diphenylamine Bisphenol A Bis (trichloromethyl) sulfone Boric acid 2-Bromo-2-nitropropane-1,3-diol 1,4-Butanediol Butoxydiglycol Butoxyethanol Butoxyethanol acetate n-Butyl acetate Butyl acetyl ricinoleate Butyl alcohol Butyl benzoate Butyl benzyl phthalate Butyidecyl phthalate Butylene glycol t-Butyl hydroperoxide... 

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 classical mechanism for the free radical oxidation of methyl oleate involves hydrogen abstraction at the ally lie carbon-8 and carbon-11 to produce two delocalized three-carbon allylic radicals (Figure 2.1). According to this mechanism, oxygen attack at the end-carbon positions of these intermediates produces a mixture of four allylic hydroperoxides containing OOH groups on carbons 8, 9, 10 and 11, in equal amounts ... 

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

From Porter etal. (1994) oxidation of methyl oleate in hexane solution inititated with di-tert-butyl hyponitrite products analysed as the hydroxyoctadecenoates by normal phase HPLC. Co-oxidation of oleate with tert-butyl hydroperoxide produced only the W-cis, 9-trans,10-transand 8-cis in a ratio of 1 1.2 for ll-cis 9-trans and i-cis 0-trans products. 

Stereochemical studies based on C-nuclear magnetic resonance spectroscopy ( C-NMR) showed the presence of eight cis and trans allylic hydroperoxides (Table 2.1). To determine the isomeric distribution of allylic hydroxyooctadecenoate derivatives, cis and trans fractions were separated by silver nitrate-thin layer chromatography (TLC), a procedure that separates according to the number, position and geometry of double bonds, and they were hydrogenated prior to GC-MS analyses of the TMS ether derivatives. More recently, the six major hydroperoxide isomers of methyl oleate were partially separated by silica HPLC, and identified by chemical-ionization mass spectrometry and IH NMR (Table 2.1). These hydroperoxide isomers were better separated as the hydroxy octadecenoate derivatives by the same silica HPLC method and re-analysed by GC-MS. 

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... 

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