Peroxidation chain reaction

Diarylamiaes fuactioa as mbber antioxidants by breaking the peroxidative chain reactions leading to mbber deterioration. Nearly all commercial synthetic mbbers (see Elastomers, synthetic), including neoprene, butyl, styrene—butadiene, and the acrylonitrile—butadiene mbbers, can be protected with about 1—2% of an alkylated diphenylamine. DPA itself is not used as a mbber antioxidant. An objectionable feature of these antioxidants is that they cause discoloration and staining which limits their use to applications where this is not important. 

Reactive alkyl radicals and nonradical products generated by lipid peroxidation chain reactions are potential alkylating agents. Reactive methyl radicals can also arise by the irradiation or oxidation of methyl compounds such as methylhydrazine (33). 

In the presence of an antioxidant such as vitamin E (a-tocopherol), the peroxyl radical abstracts the hydrogen atom from the antioxidant, and the radical chain reaction is interrupted [7], Hydrogen abstraction from the antioxidant itself generates a radical. The antioxidant radical, however, has greatly increased stability compared to the fatty acid peroxyl such that it fails to initiate another peroxidation chain reaction. The more efficient at transferring a hydrogen atom to the peroxyl radical, the better the antioxidant. Importantly, antioxidants do not intercept the carbon-centered fatty acyl radical but only the peroxyl radical. 

Nitric oxide can both promote and inhibit lipid peroxidation (Hogg and Kalyanaraman (1999). By itself, NO acts as a potent inhibitor of the lipid peroxidation chain reaction by scavenging propagatory lipid peroxyl radicals (formula [81]). It can also inhibit many potential initiators of lipid peroxidation, such as peroxidase enzymes. In the presence of 02 , NO forms peroxynitrite (see equation [46]), a powerful oxidant capable of initiating lipid peroxidation and oxidising lipid soluble antioxidants. 

As noted above, the primary products of the oxidative degradation (the peroxidation chain reaction) of polyolefins are hydroperoxides, which are unstable and undergo thermolysis or photolysis with chain scission. The products are lower molar mass materials including carboxylic acids, alcohols, aldehydes, and ketones (14,15). Depending on the amoimts of antioxidant and other stabilizers that are present, and on the nature of the environment in which they are discarded, it may take a few years or even decades before conventional polyolefins undergo sufficient oxidative degradation to become brittle and disintegrate. 

Reaction (4) is rate controlling. The kinetics of the peroxidation chain reaction has been discussed in many reviews and standard texts and the reader is directed to these for further information . 

Unlike the peroxidation of the hydrocarbon polymers, the oxidation of lignin occurs by a stoichiometric process and not a chain reaction. Because phenols are antioxidants, the phenoxyl radicals formed are too stable to participate in a peroxidation chain reaction and the aromatic system is converted to quinoid compounds and ultimately humus. Both abiotic transition metal ion catalysed peroxidation and biological oxidation ire involved in the conversion of lignin to hiunus. 

Hydroperoxides are of fundamental importance to polymer degradation. Not only are the free radicals produced by their dissociation (reaction 2) the main initiators of the peroxidation chain reaction, [12,13], but PO is also the source of the ultimate low molar mass degradation products that are readily bioassimilated by microorganisms. 

Oxidation begins with the breakdown of hydroperoxides and the formation of free radicals. These reactive peroxy radicals initiate a chain reaction that propagates the breakdown of hydroperoxides into aldehydes (qv), ketones (qv), alcohols, and hydrocarbons (qv). These breakdown products make an oxidized product organoleptically unacceptable. Antioxidants work by donating a hydrogen atom to the reactive peroxide radical, ending the chain reaction (17). 

Above about 250°C, the vapor-phase oxidation (VPO) of many organic substances becomes self-sustaining. Such oxidations are characterized by a lengthy induction period. During this period, peroxides accumulate until they can provide a source of new radicals to sustain a chain reaction. Once a critical threshold peroxide concentration is reached, the reaction accelerates very rapidly. 

Autoca.ta.Iysis. The oxidation rate at the start of aging is usually low and increases with time. Radicals, produced by the homolytic decomposition of hydroperoxides and peroxides (eqs. 2—4) accumulated during the propagation and termination steps, initiate new oxidative chain reactions, thereby increasing the oxidation rate. 

In this work the development of mathematical model is done assuming simplifications of physico-chemical model of peroxide oxidation of the model system with the chemiluminesce intensity as the analytical signal. The mathematical model allows to describe basic stages of chemiluminescence process in vitro, namely spontaneous luminescence, slow and fast flashes due to initiating by chemical substances e.g. Fe +ions, chemiluminescent reaction at different stages of chain reactions evolution. 

A widely used synthetic procedure is radical polymerization, polymerization by a radical chain reaction (Section 13.9). In a typical procedure, a monomer (such as ethene) is compressed to about 1000 atm and heated to 100°C in the presence of a small amount of an organic peroxide (a compound of formula R—O—O -R,... 

Radical-mediated silyldesulfonylation of various vinyl and (a-fluoro)vinyl sulfones 21 with (TMSlsSiH (Reaction 25) provide access to vinyl and (a-fluoro)vinyl silanes 22. These reactions presumably occur via a radical addition of (TMSlsSi radical followed by /)-scission with the ejection of PhS02 radical. Hydrogen abstraction from (TMSlsSiH by PhS02 radical completes the cycle of these chain reactions. Such silyldesulfonylation provides a flexible alternative to the hydrosilylation of alkynes with (TMSlsSiH (see below). On oxidative treatment with hydrogen peroxide in basic aqueous solution, compound 22 undergoes Pd-catalyzed cross-couplings with aryl halides. 

The easy homolysis of C-Br bond in CBr4 allowed us to conduct the radical chain reaction of CBr4 with 3,3,3-trifluoropropene under common conditions (benzoyl peroxide), although in this case the strong electrophiles are used as reagents (an addend and a monomer), i.e. a very unfavorable combination of polar factors for proceeding the process takes place (ref. 6). 

As a side product of this reaction, we regenerate another Br, which can go and react with another alkene. We call this a chain reaction, and the reaction occurs very rapidly. In fact, when peroxides are present (to jump-start this chain process), the reaction occurs much more rapidly than the competing ionic addition of HBr that we saw... 

The main function of vitamin E is as a chain-breaking, free radical trapping antioxidant in cell membranes and plasma lipoproteins. It reacts with the lipid peroxide radicals formed by peroxidation of polyunsaturated fatty acids before they can establish a chain reaction. The tocopheroxyl free radical product is relatively unreactive and ultimately forms nonradical compounds. Commonly, the tocopheroxyl radical is... 

Therefore, a chain reaction initiated by step (91), occurs in which a considerable amount of hydrogen peroxide and peroxydisulphate will be converted according to a 1 1 stoichiometry. 

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