Mechanism of Autoxidation

Auto-oxidation processes consist of a very large number of simultaneous and consecutive radical reactions. Most of these reactions can be categorized into three basic types, and these are shown by 8.11 to 8.15. Note that for these reactions an organic radical initiator In2 and no metal complex is used. The role of a metal ion in autoxidation processes is that of a catalyst for the initiation steps. 

The facility with which metal complexes bring about reactions 8.16 and 8.17 depends on several factors, one of the important ones being the half-cell potential (E°) of the M +/M(n+1)+ couple. It should be remembered, however, that most E° values for metal ions have been measured in an aqueous environment. On complexation and in an organic liquid these values are expected to change substantially. The initial hydroperoxide required for metal-catalyzed decomposition, reactions 8.16 and 8.17, is normally present in trace quantities in most hydrocarbons. 

With purified hydrocarbons where such trace impurities have been scrupulously removed, a long induction time precedes metal-catalyzed autoxidation. The mechanism of formation of initial R02H in the absence of a radical chain is not known in any detail, but metal ions do not seem to be involved. The most important point to note in this context is that various types of metal-dioxygen complexes have been isolated and fully characterized. However, such complexes do not seem to play any role in metal ion initiated autoxidation reactions. 

With selected organic substrates direct oxidation by metal could be an additional initiation step. This is shown by reaction 8.18. The thermodynamic feasibility of such a reaction depends on the stability of the radical R as well as the E° value of the M(n+1)+/Mn+ couple. The oxidation potential of Co3+/ Co2+ is as high as 1.82. As we will see, direct electron transfer from p-xylene to Co3+ is an important initiation step for the oxidation of p-xylene. 

In a real system many reactions that fit into the general categories represented by 8.13 to 8.15 are possible. This is because the organic intermediates and products themselves may undergo further rearrangement, oxidation, and other reactions. Mechanistic studies for these reactions are therefore invariably based on kinetic models. In these models a set of reactions and associated rate constants are assumed. Through simulation and optimization methods the model is then refined so that best fit between observed and predicted data points are obtained. 

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Shikama K. 1998. The molecular mechanism of autoxidation for myoglobin and hemoglobin A venerable puzzle. Chem Rev 98 1357. 

Hydroperoxide is the first product of hydrocarbon oxidation and plays a key role in the chain mechanism of autoxidation. Hydroperoxide possesses a weak O—O bond and decomposes... 

Mechanism of Autoxidation Reactions in Olefine and Polyolcfinic Substances, including Rubber. Trans. Faraday Soc. 38, 348 (1942). 

Wells-Knecht KJ, Zyzak DV, Litchield JE, Thorpe SR and Baynes JW (1995a) Mechanism of autoxidative glycosylation identification of glyoxal and arabi-nose as intermediates in the autoxidative modification of proteins by glucose. Biochemistry 34, 3702-3709. 

The mechanism of autoxidation is a radical-chain process that is initiated by formation of a hydrocarbon radical, R. ... 

For review articles on mechanisms of autoxidation of polyunsaturated lipids see Ref. 158. 

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 complete mechanism of autoxidation is, however, very complex (see Reich and Stivala, 1969). 

The hydroperoxides formed in the propagation part of the reaction are the primary oxidation products. The hydroperoxide mechanism of autoxidation was first proposed by Farmer (1946). These oxidation products are generally unstable and decompose into the secondary oxidation products, which include a variety of compounds, including... 

For detailed description of mechanism of autoxidation see Refs l,3,58t6... 

Yoshida, T., Mori, K., Hatano, T., Okumura, T., Uehara, I., Komogoe, K., Fujita, Y., and Okuda, T. 1989. Studies on inhibition mechanism of autoxidation by tannins and flavonoids. V. Radical-scavenging effects of tannins and related polyphenols on l,l-diphenyl-2-picrylhydrazyl radical. Chem. Pharmacol. Bull. 37(7) 1919—1921. 

J Gierer, N-O Nilvebrant. Studies on the degradation of residual hgnin structures by oxygen. Part I. Mechanism of autoxidation of 4,4 -dihydroxy-3,3 dimethoxystilbene in alkahne media. Holzforschmg 40(suppl) 107-113, 1986. 

In 1931 Haber and Willstatter (1) suggested that the radicals OH and HO2 play an important role in the reactions of hydrogen peroxide and that these radicals could operate in a simple chain mechanism. Previously, Franck and Haber (2) had introduced these radicals into the mechanism of autoxidation processes in solution. 

Alben et al. (1968) have studied the autoxidation of iron porphyrins, with emphasis on Fe(II) compounds and have discussed mechanisms of autoxidations, which they based on electronic, infrared, and NMR spectra. 

Self-coloured materials such as plastics or anodized aluminium, and adhesive plastics film also had their limitations. There is therefore every reason to assume that oil-based paints will be here in the year 2000. Since the general natures of the mechanisms of autoxidation and polymerization were realized about 30 years ago there have been few significant scientific papers and these subjects are not current research items. Sufficient is now known to ensure the most effective use of the fatty oils in commercial paint products. 

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