Radical chain reactions autooxidation

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

Each aromatic amine molecule, InH, terminates many free radical chains in autooxidation of alcohols and amines due to the ability of oxyperoxy and aminoperoxy radicals to oxidize InH as well as to reduce In to InH (JO. However, the coefficient of inhibition, f > 2, can be very often observed in oxidizing hydrocarbons too (2 ). Therefore, some reduction of aminyl radicals to InH proceeds in oxidizing hydrocarbons. To ellucidate the ways of such reduction we have studied the products and kinetics of the reactions of diphenylaminyl radical In. ... 

In commercial cumene oxidation processes, the radicals from the thermal decomposition of CHP are the initiators for the free-radical chain reaction. Therefore, the reaction towards CHP in the cumene oxidation is called autooxidation. 

Literature precedent suggests that SC-CO2 is inert to stabilized carbon-centered radicals (e.g., benzyl). For example, McHugh reported the autooxidation of cumene in SC-CO2 via the free radical chain process outlined in Scheme 3 19). This report is significant because it demonstrates for the first time that it is possible to conduct free radical chain reactions in SC-CO2. 

Furthermore, these hydroperoxides ean start a radical chain reaction, leading to additional aroma-active fat degradation products and enhanced autooxidation. The hydroperoxide degradation can happen spontaneously and may also be catalyzed by enzymes. Finally it should be emphasized that XO contributes to lipid oxidation in milk fat only if an appropriate substrate is present. 

Autooxidation. Liquid-phase oxidation of hydrocarbons, alcohols, and aldehydes by oxygen produces chemiluminescence in quantum yields of 10 to 10 ° ein/mol (128—130). Although the efficiency is low, the chemiluminescent reaction is important because it provides an easy tool for study of the kinetics and properties of autooxidation reactions including industrially important processes (128,131). The light is derived from combination of peroxyl radicals (132), which are primarily responsible for the propagation and termination of the autooxidation chain reaction. The chemiluminescent termination step for secondary peroxy radicals is as follows ... 

We call this type of reaction autooxidation because it is a an autocatalytic process (the reaction generates radical intermediates that propagate chain reactions) and it is an oxidation that converts alkanes into alkyl peroxides. 

Reactions that take place via radical intermediates are occasionally also begun by radical initiators, which are present unintentionally. Examples are the autooxidation of ethers (see later Figure 1.28) or one of the ways in which ozone is decomposed in the upper stratosphere. This decomposition is initiated by, among other things, the fluorochlorohydrocarbons (FCHCs), which have risen up there and form chlorine radicals under the influence of the short-wave UV light from the sun (Figure 1.10). They function as initiating radicals for the decomposition of ozone, which takes place via a radical chain. However, this does not involve a radical substitution reaction. 

The proposed free radical chain mechanism for this reaction is given in Scheme 3. The striking catalytic effect of the metal ions such as Cu2+ and Fe3+ is attributed to their ability to accept an electron from the enamine in the chain initiation step. The autooxidation of the SchifFs bases of a,/ -unsaturated ketones is thought to proceed similarly via the enamine form of the SchifFs bases. 

The reactions responsible for the deviation are, in Gautron s opinion, of the autooxidation type, i.e., oxidative chain reactions of free-radical intermediates with the initiators being, most likely, peroxides from the solvent or solute. This... 

Moreover, formation of radical transients with S.-.O bonds is kinetically preferred, but on longer time scale they convert into transients with S.-.N bonds in a pH dependent manner. Ultimately transients with S.-.N bonds transform intramolecularly into C-centred radicals located on the C moiety of the peptide backbone. Another type of C-centred radicals located in the side chain of Met-residue, a-(aikylthio)alkyl radicals, are formed via deprotonation of MetS +. C-centred radicals are precursors for peroxyl radicals (ROO ) that might be involved in chain reactions of peptide and/or protein oxidation. Stabilization of MetS +through formation of S.-.O- and S.-.N-bonded radicals might potentially accelerate oxidation and autooxidation processes of Met in peptides and proteins. Considering that methionine sulfoxide, which is the final product coming from all radicals centred on sulphur, is restored by the enzyme methionine sulfoxide reductase into MetS, stabilization of MetS +appears as a protection against an eventual peroxidation chain that would develop from a carbon centred radical. 

Another method for photodegrading polyethylene is to include metal salts, which catalyze photooxidation reactions, in the solid polymer. The compounds most generally used for that purpose are divalent transition-metal salts of higher aliphatic acids, such as stearic acid or dithiocarbonates or acetoacetic acid. The photochemical reaction is an oxidation-reduction reaction that forms free radicals capable of reacting with polyethylene, RH, to initiate an autooxidation chain reaction, as follows ... 

In this usually slow reaction, RH may represent another molecule of AB, the solvent, or some other constituent with a relatively active hydrogen atom, such as a compound containing allylic hydrogen. The formation of new radicals in this step provides other species able to continue to react with O2 and thus to continue the autooxidation by a so-called chain reaction. 

Fig. 4. Chain reaction of lipid peroxidation. An oxidant removes an electron from a PUFA (step 1) to form a lipid radical. Molecular rearrangement causes formation of a reactive conjugated diene (step 2). This can react with active singlet molecular oxygen ( 02), which is in an excited state rather than the ground state to form a peroxyl radical (step 3). Also, transition metals can react with oxygen to produce potent metal-containing oxidants that may allow simultaneous binding or bridging of a biomolecule and oxygen (B6, K4, W5). The peroxyl radical can be detoxified by an antioxidant to a lipid peroxide (step 4) or the peroxyl radical can act as an oxidant to remove an electron from another PUFA (step 5), effecting a chain reaction of autooxidation. PUFA, polyunsaturated fatty acid (R5). Dot indicates unpaired electron in radical forms.

Also potentially hazardous are compounds that undergo autooxidation to form organic hydroperoxides and/or peroxides when exposed to the oxygen in air (see Table 3.12). Especially dangerous are ether bottles that have evaporated to dryness. A peroxide present as a contaminant in a reagent or solvent can be very hazardous and change the course of a planned reaction. Autoxidation of organic materials (solvents and otho" liquids are most frequently of primary concern) proceeds by a free-radical chain mechanism. For the substrate R—H, the chain is initiated by ultraviolet... 

A radical scavenger acts by capturing and eliminating free radicals to stop the autooxidation chain reaction, and mainly includes hindered amine derivatives. 

The initiation step provides a radical source by thermal or photochemical dissociation of initiators, which then provides bromine radicals by reaction with Br2. Initiators are sometimes present in the alkene as allyl hydroperoxides which may be present due to inadvertent, prior autooxidation. Bromine or HBr may be present in trace amounts in NBS. Reaction of the bromine radical 20 with the substrate 1 proves selective for allylic or benzylic hydrogens due to the near thermoneutral nature of the reaction which breaks the C-H bond and forms the H-Br bond. Reaction of the formed carbon-centered radical 21 with Br2 provides the desired bromide 3 and Br 20. Hydrogen bromide 17 reacts with NBS to form succinimide 4 and resupplies the required low concentration of Br2. Alternatively, reaction of substrate radical 21 with NBS 2 provides product 3 and succinimidyl radical 22 (S ). Due to energy and kinetics considerations, abstraction of the allylic hydrogen by the S should be slower than abstraction of bromine from NBS by an allyl radical. In using solvents in which NBS, succinimide 4 or it s radical 22 are not very soluble, S is not the key chain-carrier. Byproducts and side-reactions can occur with S. ... 

Alkyl and peroxyl radicals alternating lead the chain process. Therefore, oxidation can be retarded by acceptors of both alkyl and peroxyl radicals. Autooxidation develops as a self-initiated ROOH forming chain reaction. Hence, autooxidation can be retarded by the decomposition of hydroperoxide or decreasing the rate of its decomposition to radicals. According to the corrqrlicated oxidation mechanism, inhibitors in mechanism of their action can be divided into the following six groups. 

The core of the crystalline region of irradiated PE contains residual free radicals. These diffuse slowly to the interface with the amorphous region, where, in the presence of dissolved oxygen, whose equilibrium concentration is maintained by diffusion, they initiate an autooxidative chain of degradation.89 Postirradiation annealing in an inert atmosphere at a temperature above the alpha-transition temperature (85°C) leads to a rapid mutual reactions of the free radicals and eliminates the problem.90... 

Inhaled ozone is known to initiate free-radical autooxidation of unsaturated fatty acids in animal pulmonary lipids (Pryor et al., 1981). These reactions lead to the formation of such typical autooxidation products as conjugated dienes and short-chain alkanes like ethane and pentane. Whether these reactions also occur in water treatment is uncertain. Glaze et al. (1988) showed that 9-hexadecenoic acid (83) reacted readily in aqueous solution to form the expected C, and C, aldehydes and acids. Linoleic acid (84) was converted to a mixture of aldehydes and acids (Carlson and Caple, 1977) notably, 3-nonenal (85) was among the products. Isolation of an unsaturated aldehyde is significant because of the high reported toxicity of these compounds. Carlson and Caple (1977) also implied that the epoxide of stearic acid was formed when an aqueous solution of oleic acid was ozonized the product probably derives from an indirect attack on the double bond by peracids or peroxy radicals (Equation 5.39). Nevertheless, it is conceivable that similar reactions could occur in natural waters. 

Few reactions of CIO2 with hydrocarbons have been reported except under conditions very far removed from water treatment practice. Saturated alkanes and alkyl side chains appear almost inert in its presence except where unusually stable radicals may result (benzylic hydrocarbons, e. g.). In these cases, typical products of autooxidation (see Chapter 4) have been isolated, probably by attack of O2 on intermediate free radicals formed by electron transfer (Ozawa and Kwan, 1984 Rav-Acha and Choshen, 1987 Merenyi et al., 1988) or (less likely) hydrogen atom abstraction (Chen et al., 1982). A few polycyclic hydrocarbons have been shown to be partially converted to chlorinated derivatives and quinones by CIO2 (Thielemann, 1972a Taymaz et al., 1979). 

Fig. 5.2. Functional groups in the polyolefin polymer chain relevant for autooxidation Alkyl-radical (above), which reacts very fast with molecular oxygen, forming the peroxy radical (middle). Further abstraction of hydrogen leads to the hydroperoxide (below). Decomposition of hydroperoxides initiates new reaction chains...

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