Intermediate peroxyl

This reaction plays an important role in the oxidation of hydrocarbons, as illustrated in Scheme 1,2 5-267 self-reaction of the intermediate peroxyl radicals to form transient tetraoxide intermediates 44 was shown by the direct observation of these intermediates at low temperatures, and isotope scrambling when 2 and 02 was used in auto-oxidation with formation of Tetraoxides with secondary hydrogens (45) can undergo... 

Under aerobic conditions, the alkyl-type macroradical - A - reacts rapidly with the molecule of dioxygen (O ) yielding a peroxyl type macroradical, hereafter denoted as AOO. The intermediate peroxyl type macroradical formed may react with an adjacent HA macro molecule (HA), and thus the radical chain reaction propagates quickly. 

The presence of oxygen can modify the course of a fiee-radical chain reaction if a radical intermediate is diverted by reaction with molecular oxygen. The oxygen molecule, with its two unpaired electrons, is extremely reactive toward most free-radical intermediates. The product which is formed is a reactive peroxyl radical, which can propagate a chain reaction leading to oxygen-containing products. 

Clearly kinetics alone will not distinguish the two schemes. To gain this distinction one can deliberately add a reagent that, judging from its independent chemistry, will react with one of the possible chain-carrying radicals. If the suspected radical is indeed an intermediate, and it reacts with the addend, the overall reaction will be slowed or halted. The added substance is a chain-breaker. In this case Fe2+ and Cu2+ (separately) were added. The first of these would very likely react with either of the peroxyl radicals, ROO or MOO. Indeed, Fe2+ dramatically inhibits the reaction. This evidence confirms the chain nature of the process, but does not distinguish between the mechanisms since both ROO and MOO would be scavenged by Fe2+. 

The formation and reaction of peroxyl radicals derived by reaction of tervalent phosphorus compounds with oxygen have attracted interest. Photolysis of trialkyl phosphites in oxygenated solutions of aromatic hydrocarbons gives phenols. " Phosphorus trichloride reacts with 1,2-dichloroethylene, in the presence of oxygen, to give (17). It is tempting to suggest that both reactions occur via similar intermediates, e.g. (15) and (16). 

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). 

Free radicals P generated during the initiation process (reaction 1) are, in the presence of oxygen, converted to peroxyl radicals POj (reaction 2), and subsequently to hydroperoxides (reaction 3) intermediate hydroperoxides provoke further chain reaction unless stabilizers (InH or D) are used to interrupt it (reactions 12 and 13). Respective reaction of the scheme is completed by the method that monitors it. 

The elementary reaction step, which involves the formation of singlet oxygen, is a reaction of superoxide anion radicals CO ), which are the reaction intermediates of the above oxidation when performed in an alkaline medium. In the presence of water they may be converted to hydrogen peroxyl radicals HOO as follows ... 

Peroxyl radicals are the species that propagate autoxidation of the unsaturated fatty acid residues of phospholipids (50). In addition, peroxyl radicals are intermediates in the metabolism of certain drugs such as phenylbutazone (51). Epoxidation of BP-7,8-dihydrodiol has been detected during lipid peroxidation induced in rat liver microsomes by ascorbate or NADPH and during the peroxidatic oxidation of phenylbutazone (52,53). These findings suggest that peroxyl radical-mediated epoxidation of BP-7,8-dihydrodiol is general and may serve as the prototype for similar epoxidations of other olefins in a variety of biochemical systems. In addition, peroxyl radical-dependent epoxidation of BP-7,8-dihydrodiol exhibits the same stereochemistry as the arachidonic acid-stimulated epoxidation by ram seminal vesicle microsomes. This not only provides additional... 

Due to the ability of tertiary peroxyl radicals to disproportionate with the formation of alkoxyl radicals, the chain decomposition of tertiary hydroperoxides proceeds via the action of intermediate alkoxyl radicals [9,135]. 

In the initial period the oxidation of hydrocarbon RH proceeds as a chain reaction with one limiting step of chain propagation, namely reaction R02 + RH. The rate of the reaction is determined only by the activity and the concentration of peroxyl radicals. As soon as the oxidation products (hydroperoxide, alcohol, ketone, etc.) accumulate, the peroxyl radicals react with these products. As a result, the peroxyl radicals formed from RH (R02 ) are replaced by other free radicals. Thus, the oxidation of hydrocarbon in the presence of produced and oxidized intermediates is performed in co-oxidation with complex composition of free radicals propagating the chain [4], A few examples are given below. 

A molecule of linear alkyl ether possesses a very convenient geometry for intramolecular hydrogen atom abstraction by the peroxyl radical. Therefore, chain propagation is performed by two ways in oxidized ethers intermolecular and intramolecular. As a result, two peroxides as primary intermediates are formed from ether due to oxidation, namely, hydroperoxide and dihydroperoxide [62],... 

This Bayer-Villiger reaction was supposed to proceed via the intermediate formation of the peroxyl adduct between peracid and the carbonyl group [25,43]. 

Antioxidants that break chains by reactions with peroxyl radicals. These are reductive compounds with relatively weak O—H and N—H bonds (phenols, naphthols, hydro-quinones, aromatic amines, aminophenols, diamines), which readily react with peroxyl radicals forming intermediate radicals of low activity. 

Nitroxyl radicals are formed as intermediates in reactions of polymer stabilization by steri-cally hindered amines as light stabilizers (HALS) [30,34,39,59]. The very important peculiarity of nitroxyl radicals as antioxidants of polymer degradation is their ability to participate in cyclic mechanisms of chain termination. This mechanism involves alternation of reactions involving alkyl and peroxyl radicals with regeneration of nitroxyl radical [60 64],... 

Peroxyl radicals are not only ones, which are able to react with ubihydroquinones. Poderoso et al. [245] showed that the short-chain ubihydroquinones Q0 and Q2 are oxidized by nitric oxide with the rate constants of 0.49 x 104 and 1.6x 1041 mol-1 s 1, respectively. The reaction apparently proceeded by one-electron transfer mechanism because the formation of intermediate semiquinone radicals has been registered. 

Marnett LJ. Prostaglandin synthase-mediated metabolism of carcinogens and a potential role for peroxyl radicals as reactive intermediates. Environ Health Perspect 1990 88 5-12. 

Antioxidants are compounds that inhibit autoxidation reactions by rapidly reacting with radical intermediates to form less-reactive radicals that are unable to continue the chain reaction. The chain reaction is effectively stopped, since the damaging radical becomes bound to the antioxidant. Thus, vitamin E (a-tocopherol) is used commercially to retard rancidity in fatty materials in food manufacturing. Its antioxidant effect is likely to arise by reaction with peroxyl radicals. These remove a hydrogen atom from the phenol group, generating a resonance-stabilized radical that does not propagate the radical reaction. Instead, it mops up further peroxyl radicals. In due course, the tocopheryl peroxide is hydrolysed to a-tocopherylquinone. 

Lipid hydroperoxides are also generated in singlet molecular oxygen mediated oxidations and by the action of enzymes such as lipoxygenases and cyclooxygenases. Chemiluminescence (CL) arising from lipid peroxidation has been used as a sensitive detector of oxidative stress both in vitro and in vivo . Several authors have attributed ultra-weak CL associated with lipid peroxidation to the radiative deactivation of O2 and to triplet-excited carbonyls (63, 72) (equations 35 and 36) " . It has been proposed that the latter emitters arise from the thermolysis of dioxetane intermediates (61, 62) (equation 35), endoperoxide (73) (equation 37) and annihilation of aUtoxyl, as well as peroxyl radicals ... 

The peroxyl radicals formed combine via a tetraoxide intermediate, which decomposes generating O2 and the corresponding alcohol and a ketone (equation 38a). Alternatively, the tetraoxide intermediate can also decompose, by generating a triplet ketone in the excited state (LAO, equation 38b), which produces O2 by energy transfer to the ground state oxygen (equation 48). 

The conversion of benzaldehyde in the presence of air to benzoic acid was reported in 1832 by Wohler and Liebig, and in 1900 Baeyer and Villiger proposed perbenzoic acid as an intermediate in the reaction. The currently accepted free radical chain mechanism for the process was proposed by Backstrom in 1934 (equation 34). Bates and Spence already in 1931 had proposed that photolysis of CH3I forming CHs in the presence of O2 led to peroxyl radicals CHaOO-. ... 

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