Hydroperoxides formation/decomposition

Thus, the overall reaction may be written as RH + 02 1 ROOH. The G values for hydroperoxide formation at 50°C range from -16 for 2,2,4-trimethylpentene-l to -400 for cyclohexene (Wagner, 1969). Although this temperature is somewhat lower than the temperature of decomposition of the hydroperoxide, in practice the reactions are conducted at elevated temperatures. In such cases, the radition-induced initiation either eliminates the induction period or allows the recations to proceed at somethat lower temperatutes than would be otherwise required. 

The kinetic curve has a sigmoidal shape and can be intuitively divided into four stages (a) induction period, (b) stage of accelerated hydroperoxide formation, (c) stage of retarded ROOH formation, and (d) stage with prevalence of ROOH decomposition after the point... 

The complex and incompletely understood phenomena of cool flames and then-close relationship with autoignition processes is discussed in considerable detail. As the temperature of mixtures of organic vapours with air is raised, the rate of autoxidation (hydroperoxide formation) will increase, and some substances under some circumstances of heating rate, concentration and pressure will generate cool flames at up to 200° C or more below their normally determined AIT. Cool flames (peroxide decomposition processes) are normally only visible in the dark, are of low temperature and not in themselves hazardous. However, quite small changes in thermal flux, pressure, or composition may cause transition to hot flame conditions, usually after some delay, and normal ignition will then occur if the composition of the mixture is within the flammable limits. 

Trioxolaquines, antimalarial drugs, 1320 Trioxycarbonates, hydrofluorocarbon decomposition, 604 Triperoxides, thermochemistry, 166 Triphenylantimony, peroxyoxalate chemiluminescence, 1265-6 Triphenylbromogermane, germyl hydroperoxide formation, 822 Triphenylgermanol, germyl hydroperoxide formation, 822... 

Hydroperoxide formation is characteristic of alkenes possessing tertiary allylic hydrogen. Allylic rearrangement resulting in the formation of isomeric products is common. Secondary products (alcohols, carbonyl compounds, carboxylic acids) may arise from the decomposition of alkenyl hydroperoxide at higher temperature. 

Cobaltn-Schiff base complexes, e.g. Co(salen),567 Co(acacen)568 and cobalt(II) porphyrins,569 e.g. Co(TPP), are effective catalysts for the selective oxygenation of 3-substituted indoles to keto amides (equation 249), a reaction which can be considered as a model for the heme-containing enzyme tryptophan-2,3-dioxygenase (equation 21).66 This reaction has been shown to proceed via a ternary complex, Co-02-indole, with probable structure (175), which is converted into indolenyl hydroperoxide (176). Decomposition of (176) to the keto amide (174) readily occurs in the presence of Co(TPP), presumably via formation of a dioxetane intermediate (177).569,56 Catalytic oxygenolysis of flavonols readily occurs in the presence of Co(salen) and involves a loss of one mole of CO (equation 251).570... 

Hydroperoxide Levels. In thermally oxidized fats hydroperoxides are usually very low. At higher temperatures, oxidation proceeds rapidly and the rate of hydroperoxide decomposition exceeds that of hydroperoxide formation (17,18). For example, when ethyl linoleate was oxidized at 70°C, peroxide content reached a maximum of 1777 meq/kg after 6 hr then decreased to 283 meq/kg after 70 hr. At 250°C, on the other hand, peroxide value reached a maximum of only 198 meq/kg after 10 min, and was zero after 30 min. 

The now classic Farmer-type hydrogen-abstraction Initiation of free radical autoxldatlon accounts for a large portion of the nonenzymlc oxidations of n-3 fatty acids (45). Because fish lipids contain substantial concentrations of EPA and DHA (47-48), they provide many allowed sites (18, 22, 45, 46, 49) of hydroperoxide formations, and thus can account for a large array of decomposition products. Oxidizing model systems of unsaturated methyl esters of fatty acids yielded monohydroperoxides, but also produce dlhydroperoxldes that are formed by cycllzatlon of Intermediate hydroperoxy radicals when suitable H-donatlng antioxidants are not present to quench the free radical reaction (45, 50, 51). Decomposition of monohydroperoxides of fatty acids In model systems yields a very different profile of lower molecular weight products than observed for similar decompositions of dlhydroperoxldes of the same fatty acids (45, 46). 

The importance of alkylperoxy radicals as intermediates had long been realized (see Sect. 2) and their subsequent reaction to yield the alkyl-hydroperoxide or decomposition products such as aldehydes and alcohols had been reasonably successful in describing the mechanism of the autocatalytic oxidation of alkanes. However, even though 0-heterocycles (which cannot be derived from intermediate aldehydes) had been found in the products of the oxidation of n-pentane as early as 1935 [66], the true extent of alkylperoxy radical isomerization reactions has been recognized only recently. Bailey and Norrish [67] first formulated the production of O-heterocycles in terms of alkylperoxy radical isomerization and subsequent cyclization in order to explain the formation of 2,5-dimethyl-tetrahydrofuran during the cool-flame oxidation of n-hexane. Their mechanism was a one-step process which involved direct elimination of OH. However, it is now generally formulated as shown in reactions (147) and(I67)... 

Calculations Use the induction period, percentage inhibition or rates of hydroperoxide formation or decomposition, or IC50 value (concentration required to achieve 50% inhibition) based quantification (this is discussed further in a later section). 

During the inhibited self-initiated autoxidation of methyl linoleate by a-Toc in solution, Niki and coworkers made the interesting observation that a-Toc acts as an antioxidant at low concentrations, but high concentrations (up to 18.3 mM) actually increased hydroperoxide formation due to a pro-oxidant effect. The pro-oxidant effect of a-Toc was observed earlier by Cillard and coworkers in aqueous micellar systems and they found that the presence of co-antioxidants such as cysteine, BHT, hydroquinone or ascor-byl palmitate inverted the reaction into antioxidant activity, apparently by reduction of a-To" to a-Toc . Liu and coworkers ° found that a mixture of linoleic acid and linoleate hydroperoxides and a-Toc in SDS micelles exhibited oxygen uptake after the addition of a-Toc. The typical ESR spectrum of the a-To" radical was observed from the mixture. They attributed the rapid oxidation to decomposition of linoleate hydroperoxides, resulting in the formation of linoleate oxy radicals which initiated reactions on the lipid in the high concentration of the micellar micro-environment. Niki and coworkers reported pro-oxidant activity of a-Toc when it was added with metal ions, Fe3+25i Qj. jjj (jjg oxidation of phosphatidyl choline liposomes. a-Toc was found... 

In static oven aging at 40°C (104°F) of a rosin ester tackifier (Fig. 10) the rate of hydroperoxide formation was reduced significantly using AO-2, with even better results using AO-3. The hydroperoxides are fairly stable at room temperature. At temperatures associated with hot-melt compounding or drying of solvent- and water-based formulations, hydroperoxides decompose spontaneously. The decomposition products initiate further reactions, which can result in the formation of color species. The addition of AO-2 and AO-3 which reduces the level of hydroperoxides formed, subsequently reduces the level of tackifier discoloration after oven aging (Fig. 11). 

The route of benzaldehyde BAL and benzyl alcohol BZA formation is changed from successive (v/o benzyl hydroperoxide BH decomposition at catalysis with Ni(II)(acac)2 MP (Scheme 1, reactions 1.3-1.5) on parallel with BH - at catalysis with Ni2(OAc)3(acac) MP ( A ) complexes, formed during oxidation. 

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