Radicals autooxidation

To date, the observed reactivity of Fe =0 species toward olefins does not lend unequivocal support for this idea. Although Fe =0 species derived from 2 or 3 were found to react with cyclooctene generating epoxide in about 50% yield, oxidation of olefins with FI202 catalyzed by 2 or 3 was not very efficient under argon (20% conversion of oxidant to product or less) and showed evidence for a significant contribution from radical autooxidation when carried out in air [28]. Furthermore, the Fe =0 complex supported by a 15-membered macrocyclic ligand was inactive in olefin epoxidation, but became quite active only in the presence of excess oxidant (PhIO). A PhIO Fe =0 adduct was proposed as the oxidant instead [64]. 

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. 

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. 

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 ... 

Because the chemiluminescence intensity can be used to monitor the concentration of peroxyl radicals, factors that influence the rate of autooxidation can easily be measured. Included are the rate and activation energy of initiation, rates of chain transfer in cooxidations, the activities of catalysts such as cobalt salts, and the activities of inhibitors (128). 

Tertiary peroxyl radicals also produce chemiluminescence although with lower efficiencies. For example, the intensity from cumene autooxidation, where the peroxyl radical is tertiary, is a factor of 10 less than that from ethylbenzene (132). The chemiluminescent mechanism for cumene may be the same as for secondary hydrocarbons because methylperoxy radical combination is involved in the termination step. The primary methylperoxyl radical terminates according to the chemiluminescent reaction just shown for (36), ie, R = H. 

Improving giycaemic control may not only reduce the rate of non-enzymatic glycosyiation and monosaccharide autooxidation, but lower polyol pathway activity. In addition, it should have a beneficial effect on other haemodynamic and hormonal factors involved in the development of diabetic vascular disease. However, in studies of diabetic retinopathy, rapid control of glucose levels by intensive insulin therapy has been shown to worsen vascular disease initially and it could be postulated that a sudden improvement in retinal blood flow promotes further free-radical damage as part of a reperfusion-ischaemic injury. 

Hunt, J.V., Smith, C.T. and WolflF, S.P. (1990). Autooxidative glycosylation and possible involvement of peroxides and free radicals in LDL modification by glucose. Diabetes 39, 1420-1424. 

Zhang, H, E Kotake-Nara, H Ono, and A Nagao. 2003. A novel cleavage product formed by autooxidation of lycopene induces apoptosis in HL-60 cells. Free Radic Biol Med 35(12) 1653-1663. 

There are still some non-explained observations. For example, syndiotactic PP was reported [45,46] as being more stable than isotactic polymer. At 140°C, the maximum chemiluminescence intensity was achieved after 2,835 min for syndiotactic PP, while isotactic polymer attained the maximum after only 45 min. Atactic PP was reported to be more stable than the isotactic polymer [46]. An explanation has been offered that the structure of isotactic PP is much more favourable for autooxidation, which proceeds easier via a back-biting mechanism where peroxyl radicals abstract adjacent tertiary hydrogens on the same polymer chain. 

Most ethers react slowly with oxygen by a radical process called autooxidation to form hydroperoxides and peroxides. 

Fig. 36 A Wagnerova Type II hydroperoxy radical chain initiated autooxidation.

Radical hydroxylation of hydrocarbons by autooxidation yields alcohols (major products), ketones, and acids (minor products). Cyclohexanol, for example, is formed in 90% yield from cyclohexane and peroxyacetic acid (275). The high -ol/-one ratio at low conversions can sometimes be used as a partial diagnostic tool to distinguish between the radical and electrophilic pathways. The predominant reaction of electrophilic radicals, such as HO, ROO, and CH 3 is H-atom abstraction from reactants (S-H) or peracids, as exemplified by the following ... 

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. 

An interesting process of C-C bond formation is represented by the autooxidation of Mercurialis perennis L. plant alkaloid hermidin. The reaction proceeds through the formation of a transient blue anion-radical, which dimerizes with the transfer of the reaction center to give, eventually, chryso-hermidin as a dimeric hexaketone (Wasserman et al. 1993 Scheme 7.59). 

Photolysis Abiotic oxidation occurring in surface water is often light mediated. Both direct oxidative photolysis and indirect light-induced oxidation via a photolytic mechanism may introduce reactive species able to enhance the redox process in the system. These species include singlet molecular O, hydroxyl-free radicals, super oxide radical anions, and hydrogen peroxide. In addition to the photolytic pathway, induced oxidation may include direct oxidation by ozone (Spencer et al. 1980) autooxidation enhanced by metals (Stone and Morgan 1987) and peroxides (Mill et al. 1980). 

The slow spontaneous oxidation of compounds in the presence of oxygen is termed autoxidation (autooxidation). This radical process is responsible for a variety of transformations, such as the drying of paints and varnishes, the development of rancidity in foodstuff fats and oils, the perishing of rabber, air oxidation of aldehydes to acids, and the formation of peroxides in ethers. 

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. 

Procarbazine (Matulane) may autooxidize spontaneously, and during this reaction hydrogen peroxide and hydroxyl free radicals are generated. These highly reactive products may degrade DNA and serve as one mechanism of procarbazine-induced cytotoxicity. Cell toxicity also may be the result of a transmethylation reaction that can occur between the A-methyl group of procarbazine and the N7 position of guanine. 

The main features of this hydroperoxidation reaction are that in any case a shift of the double bond is connected with this reaction, and that no free radicals are involved i.e., no hydrogen abstraction from the carbon atom a to the double bond prior to the C—O bond formation occurs as is the case in the well-studied autooxidation reactions. In the latter reactions two different hydroperoxides are formed as... 

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... 

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