Mechanism of Auto-oxidation

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. These are shown by reactions 8.3.1.1-8.3.I.2. 

Note that for these reactions an organic radical initiator (In ) and no metal complexes are used. As will be seen later, product formations take place both in the propagation and in the termination steps. 

In metal-catalyzed auto-oxidation the role of the metal ion is to initiate the radical chain. Reactions 8.3.1.6 and 8.3.1.7 show the initiation steps when metal ions are present. The initial hydroperoxide required for metal-catalyzed decomposition, reactions 8.3.1.6 and 8.3.1.7, is normally present in trace quantities in most hydrocarbons. 

Note that only a metal ion with easily accessible oxidation states, which differ by one unit, can efficiently catalyze both reduction and oxidation of organic hydroperoxides. These reactions initiate a radical chain, and oxidation products are formed by the usual propagation and termination steps. 

With purified hydrocarbons where trace hydroperoxide impurities have been scrupulously removed, a long induction time precedes metal-catalyzed auto-oxidation. During the induction period, trace quantities of hydroperoxide is formed. 

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Chan, H.W.S. 1987. The mechanism of auto-oxidation. In Auto-oxidation of Unsaturated Lipids (H.W.C. Chan, ed.), pp. 1-16, Academic Press, London. 

Farmer, E.H., Bloomfield, G.F., Sundralingam, A., Sutton, D.A. 1942. The course and mechanism of auto-oxidation reactions in olefinic and polyolefinic substances, including rubber. 

Electron transfer is considered to take place to the oxygen molecule to give the Oa radical with subsequent hydrogen-atom abstraction from the hydroxylamine yielding HOa and HNO. The mechanism of auto-oxid-ation of triethylborane in the presence of iodine has been described. Induction periods observed are dependent on the iodine oxygen concentration ratios, and a chain-reaction mechanism is proposed involving both ethyl and ethylperoxy-radicals. 

Table 2.2 Mechanism of Auto-oxidation and Role of Antioxidants...

It is believed that one mechanism for auto-oxidation of such Fe(n) complexes involves the initial binding of 02 followed by a rapid redox process involving dimerization via an oxygen-bridged species to yield a Fe(m) fi-oxo dimer as the final product ... 

Porter, N.A. 1986. Mechanisms for auto-oxidation of polyunsaturated lipids. Acta Chem. Res. 19, 262-268. 

Scheme 2 Inhibition of auto-oxidation by different antioxidant mechanisms...

The discussion of the mechanism of catalytic oxidation falls into two extreme cases. (1) A pure auto-oxidation can be postulated wherein the oxygen is activated by the platinum and a peroxide intermediate is formed which decomposes to yield an aldehyde and hydrogen peroxide. 

NaCl reduces the rate of auto-oxidation in sweet-cream butter but increases it in ripened cream butter (c. pH 5) the mechanism in unknown. 

Kinetics of the complex reaction process of auto-oxidation can be described by the mechanism of the so-called kinetic chain reaction. In a kinetic chain reaction, reactive reaction products (radicals) ate first formed in one or more initiation reactions, in the so-called chain start or chain initiation, and these products can then enter so-called propagation reactions. The propagation reactions are characterised by the fact that the radicals ate... 

Reaction with ambient oxygen termed auto-oxidation also causes changes in the chemical structure and properties of polymers. A generic mechanism for auto-oxidation as a thermal oxidation is provided in Scheme 6.1. Extensive discussions of variations in mechanisms of thermal oxidation are discussed in the sections to follow. 

Harvey (1952) demonstrated the luciferin-luciferase reaction with O. phosphorea collected at Nanaimo, British Columbia, Canada, and with O. enopla from Bermuda. McElroy (1960) partially purified the luciferin, and found that the luminescence spectrum of the luciferin-luciferase reaction of O. enopla is identical to the fluorescence spectrum of the luciferin (A.max 510 nm), and also that the luciferin is auto-oxidized by molecular oxygen without light emission. Further investigation on the bioluminescence of Odontosyllis has been made by Shimomura etal. (1963d, 1964) and Trainor (1979). Although the phenomenon is well known, the chemical structure of the luciferin and the mechanism of the luminescence reaction have not been elucidated. 

Recently there has been an increasing interest in self-oscillatory phenomena and also in formation of spatio-temporal structure, accompanied by the rapid development of theory concerning dynamics of such systems under nonlinear, nonequilibrium conditions. The discovery of model chemical reactions to produce self-oscillations and spatio-temporal structures has accelerated the studies on nonlinear dynamics in chemistry. The Belousov-Zhabotinskii(B-Z) reaction is the most famous among such types of oscillatory chemical reactions, and has been studied most frequently during the past couple of decades [1,2]. The B-Z reaction has attracted much interest from scientists with various discipline, because in this reaction, the rhythmic change between oxidation and reduction states can be easily observed in a test tube. As the reproducibility of the amplitude, period and some other experimental measures is rather high under a found condition, the mechanism of the B-Z reaction has been almost fully understood until now. The most important step in the induction of oscillations is the existence of auto-catalytic process in the reaction network. 

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. 

The detailed mechanism for these Co AlPO-18- and Mn ALPO-18-cata-lyzed oxidations are unknown, but as previously pointed out vide supra) and by analogy to other metal-mediated oxidations a free-radical chain auto-oxidation (a type IIaRH reaction) is anticipated [63], This speculation is supported by several experimental observations that include (1) an induction period for product formation in the oxidation of n-hexane in CoAlPO-36, (2) the reduction of the induction period by the addition of free-radical initiators, (3) the ability to inhibit the reaction with addition of free-radical scavengers, and (4) the direct observation of cyclohexyl hydroperoxide in the oxidation of cyclohexane [62],... 

Vanoppen et al. [88] have reported the gas-phase oxidation of zeolite-ad-sorbed cyclohexane to form cyclohexanone. The reaction rate was observed to increase in the order NaY < BaY < SrY < CaY. This was attributed to a Frei-type thermal oxidation process. The possibility that a free-radical chain process initiated by the intrazeolite formation of a peroxy radical, however, could not be completely excluded. On the other hand, liquid-phase auto-oxidation of cyclohexane, although still exhibiting the same rate effect (i.e., NaY < BaY < SrY < CaY), has been attributed to a homolytic peroxide decomposition mechanism [89]. Evidence for the homolytic peroxide decomposition mechanism was provided in part by the observation that the addition of cyclohexyl hydroperoxide dramatically enhanced the intrazeolite oxidation. In addition, decomposition of cyclohexyl hydroperoxide followed the same reactivity pattern (i.e., NaY < BaY... 

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