Initiation of autoxidation

Table 7.4 Reactions responsible for initiation of autoxidation. (After Scott )...

The fimction of an antioxidant is to divert the peroxy radicals and thus prevent a chain process. Other antioxidants fimction by reacting with potential initiators and thus retard oxidative degradation by preventing the initiation of autoxidation chains. The hydroperoxides generated by autoxidation are themselves potential chain initiators, and autoxidations therefore have the potential of being autocatalytic. Certain antioxidants fimction by reducing such hydroperoxides and thereby preventing their accumulation. 

Also autooxidation or auto-oxidation. A slow, easily initiated, self-catalyzed reaction, generally by a free-radical mechanism, between a substance and atmospheric oxygen. Initiators of autoxidation include heat, light, catalysts such as metals, and free-radical generators. Davies (1961) defines autoxidation as interaction of a substance with molecular oxygen below 120°C without flame. Possible consequences of autoxidation include pressure buildup by gas evolution, autoignition by heat generation with inadequate heat dissipation, and the formation of peroxides. 

Some TMSP systems that preclude i-oxo dimer formation and exhibit the right proton-linked redox potentials should facilitate selective catalytic aerobic oxidation such as that defined in equations 3-6. Selective catalytic aerobic oxidation has yet to be achieved, however. The few TMSP systems examined thus far in conjunction with C>2-based oxidations function as redox initiators of autoxidation. 

A number of transition metals are now known147-156 to form stable dioxygen complexes, and many of these reactions are reversible. In the case of cobalt, numerous complexes have been shown to combine oxygen reversibly.157 158 Since cobalt compounds are also the most common catalysts for autoxidations, cobalt-oxygen complexes have often been implicated in chain initiation of liquid phase autoxidations. However, there is no unequivocal evidence for chain initiation of autoxidations via an oxygen activation mechanism. Theories are based on kinetic evidence alone, and many authors have failed to appreciate that conventional procedures for purifying substrate do not remove the last traces of alkyl hydroperoxides from many hydrocarbons. It is usually these trace amounts of alkyl hydroperoxide that are responsible for chain initiation during catalytic reaction with metal complexes. 

It may be concluded from the preceding discussion that at this juncture there is no bona fide evidence for the initiation of autoxidations by direct hydrogen transfer between metal-dioxygen complexes and hydrocarbon substrates. Although such a process may eventually prove feasible, in catalytic systems it will often be readily masked by the facile reaction of the metal complex with hydroperoxide. The choice of cumene as substrate by many investigators is somewhat unfortunate for several reasons. Cumene readily undergoes free radical chain autoxidation under mild conditions and its hydroperoxide readily decomposes by both homolytic and heterolytic processes. 

This reaction scheme is presented in Scheme 22.1, where hv is the source of the formation of radicals (other ways of initiation of autoxidation are also mentioned in the Scheme high temperature, shear or the presence of residues of polymerisation catalysts). The cyclical chain length v of the oxidation chain is given by... 

Reduced transition metal complexes are traditionally implicated as the initiators for the autoxidation of fats, lipids, and foodstuffs. However, whether this involves direct activation of O2 or of reduced dioxygen (02 -, H00% and HOOH) is unclear. Reduced metal plus HOOH yields HO via Fenton chemistry and probably is the pathway for initiation of autoxidation in many systems. Although there has been an expectation that one or more of the intermediates from the autoxidation of reduced transition metals (equation 112) can act as the initiator for the autoxidation of organic substrates, direct experimental evidence has not been presented. 

It has been contended that the direct attack of oxygen on the double bond has low thermodynamic probability (21, 22), and it has been considered that trace metal contaminants catalyze the initiation of autoxidation by producing free radicals through electron transfer. Alternative pathways are as follows, using cobalt as an... 

The evidence for the hypothesis that initiation of autoxidation involves trace metal impurities and that autoxidation would in fact not occur in the absence of trace metals can be summarized as follows ... 

Many oxygenated complexes of transition metals have now been isolated and used as catalysts for the oxidation of olefins and recent evidence supports the initiation of autoxidation through the formation of a metal hydroperoxide catalyst complex. 

The reaction of the superoxide radical anion with hydrogen peroxide should be emphasized in relation to initiation of autoxidation. This is the so-called Fenton reaction in particular of an Fe-complex ... 

Gugumus F. Novel role for tropospheric ozone in initiation of autoxidation. Polym Degrad Stab 1998 62 403-6. 

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