Autoxidation free radical chain reactions

The mechanism by which an oiganic material (RH) undergoes autoxidation involves a free-radical chain reaction (3—5) ... 

Bateman, Gee, Barnard, and others at the British Rubber Producers Research Association [6,7] developed a free radical chain reaction mechanism to explain the autoxidation of rubber which was later extended to other polymers and hydrocarbon compounds of technological importance [8,9]. Scheme 1 gives the main steps of the free radical chain reaction process involved in polymer oxidation and highlights the important role of hydroperoxides in the autoinitiation reaction, reaction lb and Ic. For most polymers, reaction le is rate determining and hence at normal oxygen pressures, the concentration of peroxyl radical (ROO ) is maximum and termination is favoured by reactions of ROO reactions If and Ig. 

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). 

Soluble Co compounds are generally employed in the autoxidation of hydrocarbons, i.e., the oxidation with O2 as the oxidant. In neat hydrocarbons, low concentrations of Co compounds accelerate the autoxidation since the Co2+/Co3+ couple is excellent for decomposing alkyl hydroperoxides and thus initiates free radical chain reactions. However, at high conversions, the Co may be deactivated by formation of insoluble clusters with side products of the hydrocarbon autoxidation. Moreover, high concentrations of a Co compound may actually inhibit the reaction because Co also terminates radical chains by reaction with ROO radicals ... 

Generally speaking, a two-electron transfer (supply or withdrawal) is more appropriate to avoid an undesirable free-radical chain reaction ( autoxidation ), which is apt to be less selective. For this reason, a two-electron supply from the central metal to 02 will be emphasized in the following discussion in which tyrosinase and cytochrome P 450 are used as appropriate examples. 

AutOxidBtion Autoxidation is a free-radical chain reaction, involving a complex series of reactions that initiate, propagate, and terminate the chain. 

The initiation reaction is the hemolytic abstraction of hydrogen to form a carbon-centered alkyl radical in the presence of an initiator. Under normal oxygen pressure, the alkyl radical reacts rapidly with oxygen to form the peroxy radical, which in turn reacts with more unsaturated lipids to form hydroperoxides. The lipid-free radical thus formed can further react with oxygen to form a peroxy radical. Hence, the autoxidation is a free radical chain reaction. Because the rate of reaction between the alkyl radical and oxygen is fast, most of the free radicals are in the form of the peroxy radical. Consequently, the major termination takes place via the interaction between two peroxy radicals. 

The important features of autoxidation are auto-catalytic and free radical chain reactions. The rate of oxidation is initially slow and increases as the reaction progresses. However, once autoxidation is initiated, the reaction continues until the reaction substrate or catalytic factor becomes extinct. In short, unsaturated lipids undergo three reaction phases initiation, propagation, and termination. The participation of reactive oxygen radicals in autoxidation reactions is summarized in the following reaction steps. 

The simplicity of the easily surveyed reaction equation is strongly misleading. The reaction mechanism of the autoxidation of alkyl-substituted aromatic compounds consists of several complex steps - free-radical chain reactions triggered by oxidation catalysts. In general, two initiation steps can be distinguished [5, 6, 10] ... 

In the realm of homogeneous catalysis we often encounter examples of acid- and base-catalyzed hydration-dehydration and hydrolysis, metal-catalyzed hydrolysis and autoxidation, photocatalytic oxidation and reduction, metal-catalyzed electron transfer, acid-catalyzed decarboxylation, photocatalytic decarboxylation, metal-catalyzed free-radical chain reactions, acid-catalyzed nucleophilic substitutions, and enzymatic catalysis. 

We are not aware of any significant study of autoxidation of polypropylene at elevated temperatures, i.e. oxidative pyrolysis. On the other hand, much is known about gas-phase oxidation of hydrocarbons at high temperatures and the cool-flame limit (25,26,27,28). The reactions are recognized as free radical chain reactions propagated by peroxy radicals and hydroperoxides which was essentially a development of Backstrom s scheme for the oxidation of aldehydes (29). These mechanisms may be adapted to the oxidative pyrolysis of polypropylene. 

Organic peroxo compounds are also obtained by autoxidation of ethers, unsaturated hydrocarbons and other organic materials on exposure to air. The autoxidation is a free-radical chain reaction which is initiated almost certainly by radicals generated by interaction of oxygen and traces of metals such as Cu, Co, or Fe.27 The attack on specific reactive C—H bonds by a radical, X, gives first R and then hydroperoxides which can react further ... 

Although autoxidation reactions have been studied for well over a hundred years, clear understanding of the processes had to await development of the concept of free radical chain reactions in the 1920s [1—3]. Credit for first recognizing the radical chain nature of an autoxidation... 

AUTOXIDATION OF SULPHUR-CONTAINING SUBSTRATES IN THE ABSENCE OF OTHER HYDROCARBON FREE RADICAL CHAIN REACTIONS... 

While, for example, the thermal autoxidation reaction of ( + )-limo-nene (1) proceeds as a free radical chain reaction (61), the photosensitized oxygenation of 1 occurs according to the scheme shown at the top of the next page (51, 52). 

It is known that manganese salts cause oxidation of hydrocarbons, like cumene, by initiating free radical chain reactions. However, this is normally done by catalytic decomposition of trace amounts of hydroperoxides found in the hydrocarbons. In our case, the catalyst does not seem to decompose CHP, as demonstrated in an independent experiment (see above). If it did, the rate of decomposition should increase in time as the reaction progresses leading to an increase in the autoxidation rate. While we do observe for cumene an initiation period up to the accumulation of 3-5% hydroperoxide, from that point on up to greater than 50% CHP accumulation, the oxidation rate is constant. This initiation period may be due to surface activation of the catalyst. 

Prooxidant activity of phenolic compounds Phenolic antioxidants can initiate an autoxidation process and act like prooxidants under conditions that favor their autoxidation. Instead of terminating a free-radical chain reaction by reacting with a second radical, the phenoxy radical may also interact with oxygen and produce quinones (P = 0) and superoxide anion (02 ). PO -I-O2—>P = 0- -02 . Small phenolic compounds which are easily oxidized, such as quercetin, gallic acid, possess prooxidant activity while high-molecular weight phenolic compounds, such as condensed and hydrolyzable tannins, have little or no prooxidant activity. 

Lipid oxidation in foods is a complex chain of reactions that first consist of the introduction of a functional group containing two concatenated oxygen atoms (peroxides) into unsaturated fatty acids, in a free-radical chain reaction, that afterward gives rise to secondary oxidation products. Different pathways for lipid oxidation have been described radical mechanism or autoxidation, singlet oxygen-mediated mechanism or photooxidation, and enzymatic oxidation. 

Thermal Oxidative Stability. ABS undergoes autoxidation and the kinetic features of the oxygen consumption reaction are consistent with an autocatalytic free-radical chain mechanism. Comparisons of the rate of oxidation of ABS with that of polybutadiene and styrene—acrylonitrile copolymer indicate that the polybutadiene component is significantly more sensitive to oxidation than the thermoplastic component (31—33). Oxidation of polybutadiene under these conditions results in embrittlement of the mbber because of cross-linking such embrittlement of the elastomer in ABS results in the loss of impact resistance. Studies have also indicated that oxidation causes detachment of the grafted styrene—acrylonitrile copolymer from the elastomer which contributes to impact deterioration (34). 

Many hydroperoxides have been prepared by autoxidation of suitable substrates with molecular oxygen (45,52,55). These reactions can be free-radical chain or nonchain processes, depending on whether triplet or singlet oxygen is involved. The free-radical process consists of three stages ... 

Another method for producing petoxycatboxyhc acids is by autoxidation of aldehydes (168). The reaction is a free-radical chain process, initiated by organic peroxides, uv irradiation, o2one, and various metal salts. It is terrninated by free-radical inhibitors (181,183). In certain cases, the petoxycatboxyhc acid forms an adduct with the aldehyde from which the petoxycatboxyhc acid can be hberated by heating or by acid hydrolysis. If the petoxycatboxyhc acid remains in contact with excess aldehyde, a redox disproportionation reaction occurs that forms a catboxyhc acid ... 

While it is generally accepted that autoxidation is a chain reaction involving R and R02 radicals, the mechanism of initial free radical formation is both controversial and incompletely understood. To overcome the difficulties inherent in postulating the initiation step ... 

Iron(III) weso-tetraphenylporphyrin chloride [Fe(TPP)Cl] will induce the autoxidation of cyclohexene at atmospheric pressure and room temperature via a free radical chain process.210 The iron-bridged dimer [Fe(TPP)]2 0 is apparently the catalytic species since it is formed rapidly from Fe(TPP)Cl after the 2-3 hr induction period. In a separate study, cyclohexene hydroperoxide was found to be catalytically decomposed by Fe(TPP)Cl to cyclohexanol, cyclohexanone, and cyclohexene oxide in yields comparable to those obtained in the direct autoxidation of cyclohexene. However, [Fe(TPP)] 20 is not formed in the hydroperoxide reaction. Furthermore, the catalytic decomposition of the hydroperoxide by Fe(TPP)Cl did not initiate the autoxidation of cyclohexene since the autoxidation still had a 2-3 hr induction period. Inhibitors such as 4-tert-butylcatechol quenched the autoxidation but had no effect on the decom-... 

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. 

We have mentioned in Section II.B.2 studies of the oxidation of olefins by molecular oxygen in the presence of low-valent Group VIII metal complexes, with the expectation of effecting homogeneous, nonradical oxidation processes. However, these reactions were shown to involve the usual free radical chain autoxidation, and no direct transfer of oxygen from a metal-dioxygen complex to an olefin was demonstrated. 

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