Oxygen peroxide radical mechanism

Because of the irreversible and not well-understood change of the electrocatalyst surface above 1.0 V, early mechanistic studies were conducted under ill-defined conditions. Thus, while anodic evolution of Oj takes place always in the presence of oxygen-covered electrodes, the cathodic reaction proceeds on either oxygen-covered or oxygen free surfaces with different mechanisms (77,158). The electrochemical oxide path, proposed for oxide-covered platinum metals in alcaline electrolytes (759,160), has been criticized by Breiter (7), in view of the inhibition of oxygen reduction by the oxygen layers. Present evidence points to the peroxide-radical mechanism (77,... 

Mechanisms of metal-catalyzed oxidations by hydrogen peroxide may be different for alkanes and arenes as well as for different metal complexes. Lor example, for the oxidation of alkanes by the complex [Ru(dmp)2(S)2 (PF6)2 (where S = MeCN or H2O) a mechanism analogous to the oxygen rebound radical mechanism, assumed for cytochrome P450 and its models (see Chapter XI), has been proposed (Scheme X.I) [I3h]. 

As mentioned earlier, ethers are normally rather inert. They do, however, react slowly with oxygen by radical mechanisms to form hydroperoxides and peroxides. Because peroxides can decompose explosively, extreme care should be taken with samples of ethers that have been exposed to air for several days. 

Inspired by Cozzi s studies on the imine Reformatsky reaction [164] and guided by Noyori s investigation of reactive zinc species [165], the authors propose an oxygen-initiated radical mechanism for the formation of the Reformatsky reagent and a catalytic cycle that is included in a modified version in Scheme 5.87. First, it is assumed that the reaction of dimethyl zinc with oxygen produces - via the peroxide MeOOZnMe - a-methyl radical that enters the first catalytic cycle A. 

The reaction follows a free radical mechanism and gives a hydroperoxide a compound of the type ROOH Hydroperoxides tend to be unstable and shock sensitive On stand mg they form related peroxidic derivatives which are also prone to violent decomposi tion Air oxidation leads to peroxides within a few days if ethers are even briefly exposed to atmospheric oxygen For this reason one should never use old bottles of dialkyl ethers and extreme care must be exercised m their disposal... 

The addition of hydrogen halides to simple alkenes, in the absence of peroxides, takes place by an electrophilic mechanism, and the orientation is in accord with Markovnikov s rule. " When peroxides are added, the addition of HBr occurs by a free-radical mechanism and the orientation is anti-Markovnikov (p. 985). It must be emphasized that this is true only for HBr. Free-radical addition of HF and HI has never been observed, even in the presence of peroxides, and of HCl only rarely. In the rare cases where free-radieal addition of HCl was noted, the orientation was still Markovnikov, presumably beeause the more stable product was formed. Free-radical addition of HF, HI, and HCl is energetically unfavorable (see the discussions on pp. 900, 910). It has often been found that anti-Markovnikov addition of HBr takes place even when peroxides have not been added. This happens because the substrate alkenes absorb oxygen from the air, forming small amounts of peroxides... 

Inhibition and stimulation of LOX activity occurs as a rule by a free radical mechanism. Riendeau et al. [8] showed that hydroperoxide activation of 5-LOX is product-specific and can be stimulated by 5-HPETE and hydrogen peroxide. NADPH, FAD, Fe2+ ions, and Fe3+(EDTA) complex markedly increased the formation of oxidized products while NADH and 5-HETE were inhibitory. Jones et al. [9] also demonstrated that another hydroperoxide 13(5)-hydroperoxy-9,ll( , Z)-octadecadienoic acid (13-HPOD) (formed by the oxidation of linoleic acid by soybean LOX) activated the inactive ferrous form of the enzyme. These authors suggested that 13-HPOD attached to LOX and affected its activation through the formation of a protein radical. Werz et al. [10] showed that reactive oxygen species produced by xanthine oxidase, granulocytes, or mitochondria activated 5-LOX in the Epstein Barr virus-transformed B-lymphocytes. 

The decomposition of aliphatic peroxides produces oxygen radicals too unstable for paramagnetic measurement. These radicals initiate the polymerization of olefins and give the complex mixtures of decomposition products associated with radical mechanisms. On the other hand, aliphatic peroxides are also capable of polar decomposition reactions, a subject to be taken up in Chapter VIII. The characteristic reactions of the less stable oxygen free radicals are /3-cleavage to form... 

The decomposition of the peroxide IX, which gives acetophenone when the conditions are such as to favor the radical mechanism, gives methyl ethyl ketone if the reaction is run in acid. This is because of the superior migration aptitude of phenyl groups in real or incipient oxygen cations.112... 

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. 

For MDI based polyurethanes we have provided evidence for formation of a diphenylmethyl radical by direct excitation (248 nm) of the carbamate moiety as well as hydrogen abstraction by a tert-butoxy radical which is produced by excitation (351 nm) of tert-butyl peroxide. The diphenylmethyl radical readily reacts with oxygen. A proposed mechanism which accounts for the production (direct or indirect) and subsequent reaction with oxygen of the diphenylmethyl radical is shown in Scheme IV. The hydrogen peroxide product depicted in Scheme IV has been previously identified by FT-IR (7) we have simply provided a plausible mechanism for its formation. 

Oxidation to CO of biodiesel results in the formation of hydroperoxides. The formation of a hydroperoxide follows a well-known peroxidation chain mechanism. Oxidative lipid modifications occur through lipid peroxidation mechanisms in which free radicals and reactive oxygen species abstract a methylene hydrogen atom from polyunsaturated fatty acids, producing a carbon-centered lipid radical. Spontaneous rearrangement of the 1,4-pentadiene yields a conjugated diene, which reacts with molecular oxygen to form a lipid peroxyl radical. 

It was postulated [152, 153] that the aryl amine is oxidized by direct oxygen transfer from Compound I to the substrate. In contrast, for the oxidation of alkaloids, e.g. morphine, codeine and thebaine (Eq. 12), to the corresponding N-oxi-des by hydrogen peroxide in the presence of HRP or crude enzyme preparation from poppy seedlings, a radical mechanism was proposed [154]. 

Fig. 21. Detailed mechanism for HO-1 catalysis. In 1, oxygenation and electron transfer forms the ferric (Fe +)-peroxy complex. Steric factors and H-bonding help to bend the peroxide toward the a-meso-heme position for regio-selective hydroxylation. One proposed mode of forming verdoheme is shown in part 2. A key part of step 2 is the resonance structures between Fe + and Fe +/radical, which enable the porph3rrin ring to be oxygenated. Although the mechanism shown does not require any reducing equivalents (176), there remain experimental inconsistencies on the requirement of an additional electron in step 2. However, reduction of the verdoheme iron is necessary to prepare the substrate for step 3, verdoheme to bihverdin.

The liquid-phase oxidation of acrolein is estimated to proceed through the free radical mechanism since adding hydroquinone stopped oxygen absorption. The primary molecular product is peracrylic acid. This reacts with acrolein to form the peroxide complex (not isolated),... 

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