Hydroperoxide, reactions

Properly end-capped acetal resins, substantially free of ionic impurities, are relatively thermally stable. However, the methylene groups in the polymer backbone are sites for peroxidation or hydroperoxidation reactions which ultimately lead to scission and depolymerisation. Thus antioxidants (qv), especially hindered phenols, are included in most commercially available acetal resins for optimal thermal oxidative stabiUty. 

An important descriptor of a chain reaction is the kinetic chain length, ie, the number of cycles of the propagation steps (eqs. 2 and 3) for each new radical introduced into the system. The chain length for a hydroperoxide reaction is given by equation (10) where HPE = efficiency to hydroperoxide, %, and 2/ = number of effective radicals generated per mol of hydroperoxide decomposed. For 100% radical generation efficiency, / = 1. For 90% efficiency to hydroperoxide, the minimum chain length (/ = 1) is 14. 

Mn (IT) is readily oxidized to Mn (ITT) by just bubbling air through a solution in, eg, nonanoic acid at 95°C, even in the absence of added peroxide (186). Apparently traces of peroxide in the solvent produce some initial Mn (ITT) and alkoxy radicals. Alkoxy radicals can abstract hydrogen to produce R radicals and Mn (ITT) can react with acid to produce radicals. The R radicals can produce additional alkylperoxy radicals and hydroperoxides (reactions 2 and 3) which can produce more Mn (ITT). If the oxygen feed is replaced by nitrogen, the Mn (ITT) is rapidly reduced to Mn (IT). 

Characteristic reactions of singlet oxygen lead to 1,2-dioxetane (addition to olefins), hydroperoxides (reaction with aHyhc hydrogen atom), and endoperoxides (Diels-Alder "4 -H 2" cycloaddition). Many specific examples of these spectrally sensitized reactions are found iu reviews (45—48), earlier texts (15), and elsewhere iu the Engchpedia. 

Also for the reaction that was described as dimerization of the chromanol methide radicals 10 to the ethano-dimer of a-tocopherol 12, the involvement of the C-centered radicals has been disproven and these intermediates lost their role as key intermediates in favor of the o-QM 3. It was experimentally shown that ethano-dimer 12 in hydroperoxide reaction mixtures of a-tocopherol was formed according to a more complex pathway involving the reduction of the spiro dimer 9 by a-tocopheroxy 1 radicals 2, which can also be replaced by other phenoxyl radicals (Fig. 6.10).11 Neither the hydroperoxides themselves, nor radical initiators such as AIBN, nor tocopherol alone were able to perform this reaction, but combinations of tocopherol with radical initiators generating a high flux of tocopheroxyl radicals 2 afforded high yields of the ethano-dimer 12 from the spiro dimer 9. 

Thus in mixtures with various model hydroperoxides (reaction (24)), neither amine II nor nitroxide I had any effect on the iodometrically determined peroxide content after standing for a few days at RT. 

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 decompositions of hydroperoxides (reactions 4 and 5) that occur as a uni-or bimolecular process are the most important reactions leading to the oxidative degradation (reactions 4 and 5). The bimolecular reaction (reaction 5) takes place some time after the unimolecular initiation (reaction 4) provided that a sufficiently high concentration of hydroperoxides accumulates. In the case of oxidation in a condensed system of a solid polymer with restricted diffusional mobility of respective segments, where hydroperoxides are spread around the initial initiation site, the predominating mode of initiation of free radical oxidation is bimolecular decomposition of hydroperoxides. 

The parameters used in the IPM are presented in Table 4.16 and Table 4.17. In these tables, the additional parameters for the reactions of hydroperoxides with molecules and free radicals are given. The reactions of hydroperoxide with free radicals are important for the chain processes of the decomposition of hydroperoxides (see later). The results of the calculation of rate constants of various hydroperoxide reactions are collected in Tables 4.18-4.21. The comparison of the calculated values with the experimental values helped to introduce a few corrections in the traditional view on the bimolecular reactions of hydroperoxides. 

The values of rate constants of the hydroperoxide reaction with phosphites are given in Table 17.1. 

As mentioned earlier, when NO concentration exceeds that of superoxide, nitric oxide mostly exhibits an inhibitory effect on lipid peroxidation, reacting with lipid peroxyl radicals. These reactions are now well studied [42-44]. The simplest suggestion could be the participation of NO in termination reaction with peroxyl radicals. However, it was found that NO reacts with at least two radicals during inhibition of lipid peroxidation [50]. On these grounds it was proposed that LOONO, a product of the NO recombination with peroxyl radical LOO is rapidly decomposed to LO and N02 and the second NO reacts with LO to form nitroso ester of fatty acid (Reaction (7), Figure 25.1). Alkoxyl radical LO may be transformed into a nitro epoxy compound after rearrangement (Reaction (8)). In addition, LOONO may be hydrolyzed to form fatty acid hydroperoxide (Reaction (6)). Various nitrated lipids can also be formed in the reactions of peroxynitrite and other NO metabolites. 

In the propagation steps, this radical then reacts with oxygen, producing a peroxyl radical, which then abstracts hydrogen from a further molecule of the substrate. The product is thus the hydroperoxide, reaction having occurred at the allylic position of the alkene. Two possible chain-termination steps might... 

Hydrogen Atom Transfer from Hydroperoxides to Peroxy Radicals. The reaction of cumylperoxy radicals with Tetralin hydroperoxide (Reaction 10) can be studied at hydroperoxide concentrations below those required to reduce the oxidation rate to its limiting value. The rate of oxidation of cumene alone can be represented by ... 

The main features of this hydroperoxidation reaction are that in any case a shift of the double bond is connected with this reaction, and that no free radicals are involved i.e., no hydrogen abstraction from the carbon atom a to the double bond prior to the C—O bond formation occurs as is the case in the well-studied autooxidation reactions. In the latter reactions two different hydroperoxides are formed as... 

A3-Carene 168) delivers only three frans-alcohols (169-171) after reduction of the primarily formed hydroperoxides.199-200 The observed product distribution, which practically is 169 170 171 — 2 1 1, is explained by assuming that only the boat conformation 168a takes part in the hydroperoxidation reaction.200 This being the case, the ratio of 169 (170 + 171) should be 1 1, and the ratio of 170 171 should also be 1 1, since attack of oxygen at C3 and C4 should follow a statistical... 

Mushrush, George W. 1992. Fuel instability 2 Organo-sulfur hydroperoxide reactions. Fuel Science and Technology International 10(10) 1563-1600. 

Competition between Homolytic and Heterolytic Catalytic Decompositions of Hydroperoxides Reactions of Transition Metals with Free Radicals Reactions of Transition Metal Ions with Dioxygen Catalytic Oxidation of Ketones Cobalt Bromide Catalysis Oscillating Oxidation Reactions... 

Hydroperoxides that are initially present can result from the radicals formed in thermolysis or in side reactions produced during synthesis or processing. Since the chain oxidation generates hydroperoxides (reaction 14.III), it is expected to be autoaccelerated and needs very low initial POOH concentrations to occur (Audouin et al., 1995) ... 

Turner JO. The acid-catalyzed decomposition of aliphatic hydroperoxides reactions in the presence of alcohols. Tetrahedron Lett 1971 14 887-890. 

In aqueous solution outer-sphere electron transfer between metal ions and alkyl hydroperoxides [reactions (95) and (96)] is expected to be favorable. In nonpolar solvents, electron transfer probably proceeds via the formation of inner-sphere, covalently bonded complexes. The overall reaction constitutes a catalytic decomposition of the hydroperoxide into alkoxy and alkylperoxy radicals ... 

Other workers348 d 124 have reported that commencement of the cobalt-catalyzed autoxidation of pure hydrocarbons, i.e., nonpolar solvent, is accompanied by oxidation of Co(II) to Co(III). The transformation is easily observed by the change in color from pale violet or pink [Co(II)] to intense green [Co(III)]. Similarly, manganese-catalyzed autoxidations were observed to start when Mn(II) was converted to Mn(III). The concentration of Co(III) reached a maximum during the course of autoxidation and then decreased. This maximum coincided with the appearance of aldehydes in the reaction mixtures. The authors also showed by calculation124 that reduction of Co(III) by a secondary product accounted for the observed kinetics much better than reduction by hydroperoxide. Hence, the decrease in concentration of Co(III) after it had reached a maximum was attributed to reduction by aldehydes (see Section II.B.3.e), which was much more facile than reduction by alkyl hydroperoxide [reaction (96)]. 

Iron(III) weso-tetraphenylporphyrin chloride [Fe(TPP)Cl] will induce the autoxidation of cyclohexene at atmospheric pressure and room temperature via a free radical chain process.210 The iron-bridged dimer [Fe(TPP)]2 0 is apparently the catalytic species since it is formed rapidly from Fe(TPP)Cl after the 2-3 hr induction period. In a separate study, cyclohexene hydroperoxide was found to be catalytically decomposed by Fe(TPP)Cl to cyclohexanol, cyclohexanone, and cyclohexene oxide in yields comparable to those obtained in the direct autoxidation of cyclohexene. However, [Fe(TPP)] 20 is not formed in the hydroperoxide reaction. Furthermore, the catalytic decomposition of the hydroperoxide by Fe(TPP)Cl did not initiate the autoxidation of cyclohexene since the autoxidation still had a 2-3 hr induction period. Inhibitors such as 4-tert-butylcatechol quenched the autoxidation but had no effect on the decom-... 

---