Auto-oxidization, peroxide formation

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. 

Wasserman and Keehn ") have also carried out the photosensitized auto-oxidation of anti-[2.2](l,4)naphthalenophane (34A). Irradiation of anti-34 in methanol and simultaneous reaction with singlet oxygen affords the oxidation product 127 in 20% yield. The primary step in the reaction is assumed to be formation of a peroxide (128) whose geometry permits an intra-annular Diels—Alder reaction as second step methanol-ysis then leads to 127 which was isolated. 

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... 

The alkylhydroperoxides are also interesting because of their formation in natural products. Unsaturated fatty acids and their esters (plant oils) are oxidized in air to peroxides (auto-oxidation). This leads to a stepwise breaking of the double bonds leading to the formation of aldehydes, ketones and fatty acids, all of which make their presence known through their strong odor intensities (e.g. rancid oil). 

Perhaps the most significant aspect of the superoxide-, hydroxide-, or electron-induced auto-oxidation of donor molecules (1,2-diphenylhydrazine, dihydrophenazine, dihydro-lumiflavin, and reduced flavoproteins) is the activation of dioxygen to hydrogen peroxide in biological matrices. Thus, within the normal cytochrome P-450 metabolic cycle, either hydroxide ion or an electron-transfer co-factor acts as an initiator (probably to produce Fl -) and reduced flavoproteins act as the H-atom donor. In contrast, the introduction of 02 - or hydrated electrons (from ionizing radiation or a disease state) into a biological matrix that contains donor molecules leads to the uncontrolled formation of hydrogen peroxide. If reduced metal ions are present, Fenton... 

The polymer radicals thus produced interact with oxygen to form alkyl peroxy radicals (P i—O2) that can abstract hydrogen of the neighboring molecules in various ways, as shown in the mechanism of the auto-oxidation process of Table 2.2. The formation of hydroperoxide in step C of the sequence of reactions is the most important source of initiating radicals. In practice, the following three kinds of antioxidant and stabilizer are used. Peroxide decomposers are materials that form stable products with radicals formed in the auto-oxidation of Table 2.2 ... 

Benzo[a]pyrene-3,6-quinol and other quinols are involved in toxic quinone/quinol redox cycles (Lo-RENTZEN and Ts o 1977, Loeentzen etal. 1979). Quinols are formed from the corresponding qui-nones by several reductases. They are rapidly auto-xidizes while superoxide anions are formed. Benzo[fl]pyrene-3,6-quinone has been shown to be mutagenic in the Ames test, using tester strain TA 104 (Chesis et al. 1984) or TA 102, strains which are particularly sensitive to reactive oxygen species. In male Sprague-Dawley rats, a rapid increase of unmetabolised benzo[fl]pyrene was observed in sera 3h after benzo [a] pyrene treatment followed by a sharp decrease (Kim et al. 2000). The time-dependent pattern of serum lipid peroxidation and the level of erythrocyte antioxidant enzymes were shown to be related to the concentrations of the formation of benzo[ ]pyrene-quinones, oxidatively altered lipids and antioxidant enzymes in the blood. 

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