Mechanism of hydroperoxides

The free radical mechanism of hydroperoxide formation is close to that proposed by Nalbandyan. 

Different chain mechanisms of hydroperoxide decomposition are known with the participation of alkyl, alkoxyl, and peroxyl radicals [9]. 

AM Tolks. Kinetics and Mechanism of Hydroperoxide Groups Decay in Polymers. PhD Thesis, Polytechnic Institute, Riga, 1973 [in Russian]. 

Free radicals formed in polymers due to thermomechanical stress appear not only during the polymer use but also during the polymer processing and shaping to final products [46], The kind of initiation which prevails in a certain polymer depends not only on initial conditions of oxidation but also on the extent of a previous oxidation as well as on the occurrence of additional interactions among oxidation products. Increasing extent of oxidation is usually characterized by higher concentration of hydroperoxides which are secondary sources of initiation. The products of oxidation formed may alter the kinetics and mechanism of hydroperoxide decomposition so that the rate of initiation is the result of several mutually coupled processes. 

Discussion. In the autoxidation of organic compounds that takes place through the mechanism of hydroperoxide formation, one finds that following the induction stage steady state mathematics can be applied to account for the observed dependence of the rate of oxidation upon the... 

It is well known that the free radical mechanism of hydroperoxide formation involves the abstraction of a hydrogen atom from the a-methylene group of a lipid... 

A. Hroch, G. Gemmecker, W. R. Thiel, Metal-catalyzed oxidations, 10 New insights into the mechanism of hydroperoxide activation by investigation of dynamic processes in the coordination sphere of seven-coordinated molybdenum peroxo complexes, Eur. J. Inorg. Chem. (2000) 1107. 

The numerous schemes suggested for this type of hydroperoxide decomposition are not always well substantiated. The chemical structure of polyalkene, as well as the process conditions (presence of admixtures, additives, temperature) play a decisive part in realising one or other mechanisms of hydroperoxide decomposition. 

In the liquid phase their decomposition to radicals occurs more rapidly and with lower activation energy. For example, tert-butyl hydroperoxide in the gas phase decomposes with kp = 5.010 exp(-158//f7) = 510 s and in a chlorobenzene solution 23 4.0-10 exp(-138/R7) = 1.8-10" s" (393 K). Mechanisms of hydroperoxide decomposition in organic compounds are diverse. 

Chromium compounds decompose primary and secondary hydroperoxides to the corresponding carbonyl compounds, both homogeneously and heterogeneously (187—191). The mechanism of chromium catalyst interaction with hydroperoxides may involve generation of hexavalent chromium in the form of an alkyl chromate, which decomposes heterolyticaHy to give ketone (192). The oxidation of alcohol intermediates may also proceed through chromate ester intermediates (193). Therefore, chromium catalysis tends to increase the ketone alcohol ratio in the product (194,195). 

A. Preparation of hydroperoxide. In a 2-1. three-necked round-bottomed flask, equipped with a mechanical stirrer (Note 1), a dropping funnel, and a thermometer, is placed 800 g. of 30% hydrogen peroxide (Note 2). The flask is surrounded by an ice bath and rapid stirring is started. In the meantime, 800 g. of 95-96% sulfuric acid is added to 310 g. of cracked ice and the solution is cooled to 10°. When the temperature of the hydrogen peroxide reaches 5-10°, the cold sulfuric acid is added slowly from the dropping funnel during a period of about 20 minutes (Note 3). The temperature of the solution should not exceed 20° during the... 

Alkyl hydroperoxides give alkoxy radicals and the hydroxyl radical. r-Butyl hydroperoxide is often used as a radical source. Detailed studies on the mechanism of the decomposition indicate that it is a more complicated process than simple unimolecular decomposition. The alkyl hydroperoxides are also sometimes used in conjunction with a transition-metal salt. Under these conditions, an alkoxy radical is produced, but the hydroxyl portion appears as hydroxide ion as the result of one-electron reduction by the metal ion. ... 

Double bonds in a,/3-unsaturated keto steroids can be selectively oxidized with alkaline hydrogen peroxide to yield epoxy ketones. In contrast to the electrophilic addition mechanism of peracids, the mechanism of alkaline epoxidation involves nucleophilic attack of hydroperoxide ion on the con-... 

Henbest and Jackson have rationalized these remote directive effects on the basis of the well-supported mechanism of alkaline epoxidations. The initial step in the reaction is the reversible addition of the hydroperoxide ion... 

Bateman, Gee, Barnard, and others at the British Rubber Producers Research Association [6,7] developed a free radical chain reaction mechanism to explain the autoxidation of rubber which was later extended to other polymers and hydrocarbon compounds of technological importance [8,9]. Scheme 1 gives the main steps of the free radical chain reaction process involved in polymer oxidation and highlights the important role of hydroperoxides in the autoinitiation reaction, reaction lb and Ic. For most polymers, reaction le is rate determining and hence at normal oxygen pressures, the concentration of peroxyl radical (ROO ) is maximum and termination is favoured by reactions of ROO reactions If and Ig. 

The early work of Kennerly and Patterson [16] on catalytic decomposition of hydroperoxides by sulphur-containing compounds formed the basis of the preventive (P) mechanism that complements the chain breaking (CB) process. Preventive antioxidants (sometimes referred to as secondary antioxidants), however, interrupt the second oxidative cycle by preventing or inhibiting the generation of free radicals [17]. The most important preventive mechanism is the nonradical hydroperoxide decomposition, PD. Phosphite esters and sulphur-containing compounds, e.g., AO 13-18, Table la are the most important classes of peroxide decomposers. 

Sulphur compounds, e.g., thiopropionate esters and metal dithiolates (Table la, AO 16 and 17), decompose hydroperoxides catalytically, i.e., one antioxidant molecule destroys several hydroperoxides through the intermediacy of sulphur acids [19,20]. Scheme 6 shows a simplified scheme for the antioxidant mechanism of simple alkyl sulphides. 

Figure 16.6 MECHANISM Mechanism of the electrophilic hydroxylation of p-hydroxyphenyl acetate, by reaction with FAD hydroperoxide. The hydroxyiating species is an "0H+ equivalent that arises by protonation of FAD hydroperoxide, RO-OH + H+ — ROH -+ 0H+.

Mechanism of the formation of phenol by acid-catalyzed rearrangement of cumene hydroperoxide. 

Figure 27.13 Proposed mechanism of the oxidation of squalene by flavin hydroperoxide.

Eckstein, J. W., Hastings, J. W., and Ghisla, S. (1993). Mechanism of bacterial bioluminescence. 4a,5-Dihydroflavin analogs as models for luciferase hydroperoxide intermediates and the effect of substituents at the 8-position of flavin on luciferase kinetics. Biochemistry 32 404 111. 

Watanabe, H., et al. (1992). Chemiluminescence in the crude extracts of soybean seedlings. Postulated mechanism on the formation of hydroperoxide intermediates. Biochim. Biophys. Acta 1117 107-113. 

The mechanism of secondary stabilization by antioxidants is demonstrated in Figure 15.5. TnT-nonylphenyl phosphites, derived from PCI3 and various alcohols, and thio-compounds are active as a secondary stabilizer [21], They are used to decompose peroxides into non-free-radical products, presumably by a polar mechanism. The secondary antioxidant is reacting with the hydroperoxide resulting in an oxidized antioxidant and an alcohol. The thio-compounds can react with two hydroperoxide molecules. 

The mechanism of NPYR formation has been studied by Coleman (37) and Bharucha et al. ( ). Coleman (37) reported that the requirement for a high temperature, the inhibitory effects of water and antioxidants, and the catalytic effect of a lipid hydroperoxide are consistent with the involvement of a free radical in the formation of NPYR. Similarly, Bharucha et al. (29) suggested that, since both NPYR and NDMA increase substantially towards the end of the frying process, N-nitros-amine formation during frying of bacon occurs essentially, if not entirely, in the fat phase after the bulk of the water is removed and therefore by a radical rather than an ionic mechanism. These authors speculated that, during the frying of... 

The Possible Mechanisms of the Oxidation of LDL in the Production of Lipid Hydroperoxides... 

Mechanisms of lipid peroxidation that have been implicated in atherosclerosis may be pertinent to RA. Cellular lipoxygenase enzymes may promote LDL modification by inserting hydroperoxide groups into unsaturated fetty-acid side chains of the LDL complex (Yla-Herttuala etal., 1990). 15-Lipoxygenase has been implicated as an initiator of LDL oxidation (Cathcart etal., 1991) whilst 5-lipoxygenase does not appear to be involved (Jessup et al., 1991). Products of activated lipoxygenase enzymes within inflammatory synovial fluid surest that this pathway could be activated in RA (Costello etal., 1992). 

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