Mechanism, hydroperoxides oxides

It seems very probable that the epoxidation reaction proceeds through a two-stage mechanism. Hydroperoxide oxidizes the catalyst to peroxo complex and the this complex performs epoxidation of olefins. 

The surface oxidation products are mainly carbonyl and/or carboxyl groups, with lower level of hydroperoxide groups. The mechanisms of oxidation by ozone and atomic oxygen have been proposed. 

Antioxidants shift the autoaccelerating increase of chemiluminescence intensity to higher time. This is due to reactions 12 and 13 of the Bolland-Gee mechanism (Section 1, Scheme 2) in which peroxyl radicals and hydroperoxides are scavenged until antioxidants InFl and D are consumed. A typical example of such a behavior occurs for samples of PP containing 0.1 %wt. of Irganox 1010 (a sterically hindered phenol) (Figure 18 and Table 4). The presence of antioxidants usually reduces the maximum chemiluminescence intensity [61,62]. This may be explained by the quenching effect of the antioxidant on excited carbonyls, but it may be also related to the mechanism of oxidation of stabilized PP. Stabilizers in... 

The traditional chain oxidation with chain propagation via the reaction RO/ + RH occurs at a sufficiently elevated temperature when chain propagation is more rapid than chain termination (see earlier discussion). The main molecular product of this reaction is hydroperoxide. When tertiary peroxyl radicals react more rapidly in the reaction R02 + R02 with formation of alkoxyl radicals than in the reaction R02 + RH, the mechanism of oxidation changes. Alkoxyl radicals are very reactive. They react with parent hydrocarbon and alcohols formed as primary products of hydrocarbon chain oxidation. As we see, alkoxyl radicals decompose with production of carbonyl compounds. The activation energy of their decomposition is higher than the reaction with hydrocarbons (see earlier discussion). As a result, heating of the system leads to conditions when the alkoxyl radical decomposition occurs more rapidly than the abstraction of the hydrogen atom from the hydrocarbon. The new chain mechanism of the hydrocarbon oxidation occurs under such conditions, with chain... 

Quasistationary (with respect to hydroperoxide) oxidation is possible for the mechanisms with a critical behavior (at [InH] > [InH]cr) as well as for ordinary mechanisms (at kdr > 1). 

Hydroperoxides oxidize aromatic amines more readily than analogous phenols. Thus, at 368 K cumyl hydroperoxide oxidizes a-naphthylamine and a-naphthol with ku = 1.4 x 10 4 and 1.7 x 10 5L mol-1 s 1, respectively [115,118], The oxidation of amines with hydroperoxides occurs apparently by chain mechanism, since the step of free radical generation proceeds much more slowly. This was proved in experiments on amines oxidation by cumyl hydroperoxide in the presence of /V,/V -diphcnyl-l, 4-phcnylcnediamine (QH2) as a radical acceptor [125]. The following reactions were supposed to occur in solution (80% decane and 20% chlorobenzene) ... 

The stoichiometry of this reaction is usually close to unity [6-9]. Thus, cumyl hydroperoxide oxidizes triphenyl phosphite in the stoichiometry A[ROOH]/A[Ph3P] from 1.02 1 to 1.07 1, depending on the proportion between the reactants [6], The reaction proceeds as bimolecular. The oxidation of phosphite by hydroperoxide proceeds mainly as a heterolytic reaction (as follows from conservation of the optical activity of reaction products [5,11]). Oxidation is faster in more polar solvents, as evident from the comparison of k values for benzene and chlorobenzene. Heterolysis can occur via two alternative mechanisms... 

Keywords polypropylene, mechanical destruction, oxidation rate, hydroperoxides. 

Khandwala, A., and J. B. L. Gee. Unoleic acid hydroperoxide Impaired bacterial uptake by alveolar macrophages, a mechanism of oxidant lung injury. Science 182 1364-1365, 1973. 

As oxidation processes were clarified, it was observed in other chain extension reactions that R02 radicals reacted with oxidation products hydroperoxides, alcohols, and ketones. The high reactivity of hydroperoxides and alcohols strongly influences the mechanism of oxidation processes. Chain rupture results from recombination of R02 radicals. 

There is further evidence for the role played by methyl hydroperoxide in the low temperature combustion of acetone. Knox (23) showed that if one assumes a simple basic chain mechanism for oxidation, then the acceleration constant, < , which characterizes the exponential acceleration to maximum rate, is given by... 

Ebselen acts as a GPx mimic by reducing hydroperoxides to water or the corresponding alcohol, and a mechanism involving oxidation of thiols to the disulfides has been postulated [20],... 

At American University, Harriet Frush and Horace Isbell worked on the mechanism of oxidation of carbohydrates by peroxides. They discovered that, in aqueous alkaline hydrogen peroxide, aldoses are quantitatively degraded to formic acid, so that hexoses produce six moles of this acid and pentoses produce five moles. A detailed study of the mechanism of the reaction revealed that degradation takes place by several pathways, the most rapid one involving the formation of peroxy radicals and hydroxy radicals. Thus, when a hydroperoxide-aldose adduct reacts with hydrogen peroxide, a peroxy radical is formed, which decomposes to a hydroxy radical, formic acid, and the next lower aldose. It was also found that, under basic conditions, hydroxy radicals oxidize alditols and aldonic acids to carbonyl compounds in much the same way they do with Fe2+ in the Fenton reaction. During the years she spent at American University, Dr. Frush was able to publish 10 papers without help from any research assistant or laboratory technician. This brought her total to more than 70 papers. 

A typical redox system for cold SBR production employs sodium formaldehyde sulfoxylate (reducing agent), a hydroperoxide (oxidant), and ferrous sulfate, plus a chelating agent or a chelated iron salt. A simplified kinetic mechanism for this redox couple follows (Wright and Tucker, 1977). 

The origin of the catalytic oxidative activity of the the Sn-siljcalites is not clear at the moment. It may be due to the reduction of isolated Sn " to Sn, which is then oxidised back with H2O2. Also, many hydroperoxides of tin have been known from the action of H2O2 upon solutions of Sn2+ and Sn4+. With our Sn-silicalites, however, there was no evidence for the dissolution of Sn under the reaction conditions as they have been regenerated after the reaction and reused several times without significant loss of catalytic activity. Surface tin hydroperoxides may be the active species but further detailed studies are required before possible mechanisms of oxidation involving Sn could be discussed. 

As it was mentioned above, polypropylenes are more prone to oxidation, hence, requiring significantly higher amounts of antioxidants and UV stabilizers compared to PE. It was shown that oxygen intake is much faster in polypropylene compared to that in PE [10], The primary reason is in the microbranched chemical structure of PP (see above), containing tertiary hydrogens that makes formation of hydroperoxides in PP much easier compared to that in polyethylenes. Overall, the mechanisms of oxidation (both photo- and thermooxidation) in PP and PE are quite different. For example, the termination reaction rates for oxidation in PE are 100-1000 times faster compared to PP [11]. 

Of particular interest for this review are those reactions of 02 that closely resemble those found in the autoxidation. Since both 02 and R02 react with many olefins to form hydroperoxides, some basis for distinguishing between these reactants can be important in understanding the detailed mechanism of oxidation of a specific compound, particularly in photooxidations where both type I and II processes can occur. Many simple substituted olefins give very similar mixtures of hydroperoxides by the two pathways however, certain structural units do give markedly different hydroperoxides from R02 and 02 and are useful as criteria for mechanism. Examples are 1,2-dimethylcyclohexene [190]... 

Nevertheless, side chain oxidation also takes place with toluene, sumesting that the mechanism of oxidation is not influenced by the catalyst structure. With tert-butyl hydroperoxide, results are totally different. 

---