Radical peroxyl alkoxyl

Reactivity of the Hydrocarbons in Reactions with Peroxyl, Alkoxyl, and Alkyl Radicals... 

The results of the calculation of the activation energies and the rate constants of peroxyl, alkoxyl, and alkyl radicals with alkanes and cycloalkanes are presented in Table 6.3-Table 6.5. 

INTRAMOLECULAR HYDROGEN TRANSFER REACTIONS IN PEROXYL, ALKOXYL, AND ALKYL RADICALS... 

Examples for frequently encountered intermediates in organic reactions are carbocations (carbenium ions, carbonium ions), carbanions, C-centered radicals, carbenes, O-centered radicals (hydroxyl, alkoxyl, peroxyl, superoxide anion radical etc.), nitrenes, N-centered radicals (aminium, iminium), arynes, to name but a few. Generally, with the exception of so-called persistent radicals which are stabilized by special steric or resonance effects, most radicals belong to the class of reactive intermediates. 

Lipid peroxidation may beinitiated by any primary free radical which has sufficient reactivity to extract a hydrogen atom (Fig. 2.10) from a reactive methylene group of an unsaturated fatty acid. For example, species such as hydroxyl radicals OH, alkoxyl radicals RO peroxyl radicals ROO and alkyl radicals R may be involved. The formation of the initiating species is accompanied by bond rearrangement that results in stabilization by diene conjugate formation. The lipid radical then takes up oxygen to form the peroxyl radical. Peroxyl radicals can... 

We also have attempted to inhibit these free radical reactions with 2,6-di-t-butyl-4-methylphenol and found no effect on the formation of cyclohexanol or cyclohexanone using catalyst 4. This latter result strongly suggests that peroxyl, alkoxyl, or hydroxyl radicals are not intermediates in these reactions. The intermediacy of a putative oxo-manganese complex is further strengthened by the reaction of 1-4 with cyclohexene in the presence of TBHP or iodosylbenzene to provide cyclohexene epoxide and our proposed mechanism is shown in the Equation (2). w H... 

Lipid peroxidation is the primary mechanism by which food deteriorates upon storage in the presence of oxygen. This process of oxidation can be initiated enzymatically, by metal ion catalysis, or by photochemical processes, to name a few. Free radicals including peroxyl, alkoxyl, and hydroxyl have been implicated in the mechanism of lipid peroxidation. The changes in the quality of processed foods are manifested by... 

Other termination reactions can proceed by condensation of peroxyl, alkoxyl or alkyl radicals. At low temperatures, peroxyl radicals can combine to produce peroxyl-linked dimers (LOOL) with the formation of oxygen (15). 

PD—S) to yield phosphates and alcohols, see Scheme 5 reaction a. Sterically hindered aryl phosphites (e.g., AO 14) have an additional chain breaking activity, i.e. they react with peroxyl and alkoxyl radicals during their function as antioxidants (reactions 5b and 5c) [18]. 

The accumulation of hydroperoxides and their subsequent decomposition to alkoxyl and peroxyl radicals can accelerate the chain reaction of polyunsaturated fatty-acid p>eroxidation leading to oxidative damage to cells and membranes as well as lipoproteins. It is well-recognized that transition metals or haem proteins, through their... 

Injury to cells and tissues may enhance the toxicity of the active oxygen species by releasing intracellular transition metal ions (such as iron) into the surrounding tissue from storage sites, decompartmentalized haem proteins, or metalloproteins by interaction with delocalized proteases or oxidants. Such delocalized iron and haem proteins have the capacity to decompose peroxide to peroxyl and alkoxyl radicals, exacerbating the initial lesion. 

Organic peroxides such as cumene hydroperoxide and t-butyl hydroperoxide have extensively been used as experimental agents. They provoke lipid peroxidation in hepatocytes, probably by the generation of alkoxyl and peroxyl radical intermediates after reaction with cytochrome P450. Other cytotoxic mechanisms are probably involved including protein thiol and non-protein thiol oxidation and deranged calcium homeostasis (Jewell et al., 1986). In fact, the addition of cumene hydroperoxide to isolated bUe duct cells, devoid of cytochrome P450 activity, still results in cell death but lipid peroxidation is not detectable (Parola et al., 1990). 

In Scheme 7, the peroxidic 0-0 bond of the hydroperoxyl group is broken together with /1-scission of the formed alkoxyl radical, and, further, ring closure of alkyl peroxyl diradical may occur. The process generates a hydroxyl radical, methylcarbonyl terminal groups (-CH2-CO-CH3) and dioxetane. The latter is unstable and decomposes into an excited triplet state of formaldehyde and/or excited triplet state of methylcarbonyls (Scheme 8). 

In the case of the hematin-catalyzed reaction we have proposed that peroxyl radicals are the epoxidizing agents (40). The mechanism is illustrated in Figure 8. Hematin reduces the hydroperoxide to an alkoxyl radical that cyclizes to the adjacent double bond. 

InH acceptor reacting with alkoxyl and peroxyl radicals... 

Along with this reaction, the alkoxyl radicals are formed by the recombination of the tertiary peroxyl radical (see Chapter 2). 

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