Alkyl hydroperoxides initiation steps

The irradiation of an alkane solution in acetonitrile with visible light in the presence of catalytic amounts of quinone and copper(II) acetate gives rise to the formation of almost pure alkyl hydroperoxide which is decomposed only very slowly under these conditions to produce the ketone and alcohol [64c,d]. It has been proposed [64d] that the first step of the reaction is a hydrogen atom abstraction from the alkane, RH, by a photoexcited quinone species to generate the alkyl radical R and semiquinone. The former is rapidly transformed into ROO and then alkyl hydroperoxide, while the latter is reoxidized by Cu(II) into the initial quinone (Scheme IX. 10). 

One product of the second chain-propagating step is R — OOH, called an alkyl hydroperoxide. The oxygen-oxygen bond of an alkyl hydroperoxide is quite weak, and it can break and produce radicals that can initiate other chains ... 

Hindered amine light stabilizers have attracted enormous scientific and commercial interest due to their efficiency as photo-antioxidants. The stabilization process in the presence of HALS involves the interference of secondary >NH or tertiary (>NCH3) groups with hydroperoxides POOH generated by oxidation chain mechanism of polymer alkyl radicals P. HALS-derived nitroxides >NO, formed through scavenging peroxyls POO by hydroxylamine species >NOH, are the key intermediates in the HALS stabilization mechanism because they are able to quench alkyl radicals arising in the initiation step of oxidation [100, 101]. The involvement of HALS in the stabilization process can be attributed to their ability to remain an... 

The polymer radicals thus produced interact with oxygen to form alkyl peroxy radicals (P i—O2) that can abstract hydrogen of the neighboring molecules in various ways, as shown in the mechanism of the auto-oxidation process of Table 2.2. The formation of hydroperoxide in step C of the sequence of reactions is the most important source of initiating radicals. In practice, the following three kinds of antioxidant and stabilizer are used. Peroxide decomposers are materials that form stable products with radicals formed in the auto-oxidation of Table 2.2 ... 

Nucleophilic attack hy the hydroperoxide anion produces initially a tetrahedral borale (26) in which one alkyl group, here (-)-Ipc 31. migrates from boron to oxygen, in the course of which a hydroxide anion is eliminated and a stable boron-oxygen bond develops in compound 27. This step proceeds with retention of configuration at the migrating carbon atom. The same step are repealed with the second residue to give trialkoxyborane 28. Finally, hydrolysis results in release of alcohol 4, two molecules of (-)-Ipc alcohol 30. and one equivalent of borale 29. 

Step 3 is accelerated by the decomposition of the hydroperoxide products ROOH to form additional free radicals, i.e., ROOH —> RO + OH. Degradation is also accelerated by the presence of even a small number of reactive tertiary H atoms but sometimes secondary H atoms. Evidence indicates that a plethora of free-radicals including peroxy RO2, hydroperoxy HO2, oxyradicals RO, hydroxy HO, and alkyl R are capable of formation and thereby initiating thermal-oxidative degradation of the polymer [1-11]. 

The oxidation of alkanes by r-butyl hydroperoxide (TBHP) has been catalysed by titanium alkoxides, producing the corresponding alcohols and ketones. A radical mechanism is proposed in which r-butoxyl radical formed from TBHP and titanium alkoxide initiates the reaction. The evolution of oxygen (from the decomposition of peroxide) and the abstraction of hydrogen from alkane to form alkyl radical occur competitively. A method for the determination of both the primary and secondary KIEs at a reactive centre based on starting-material reactivities allows the determination of the separate KIEs in reactions for which neither product analysis nor absolute rate measurements are applicable. It has been applied to the FeCls-catalysed oxidation of ethylbenzene with TBHP, which exhibits both a primary KIE and a substantial secondary KIE the findings are in accordance with previous mechanistic studies of this reaction. The oxidation of two l-arylazo-2-hydroxynaphthalene-6-sulfonate dyes by peroxy-acids and TBHP catalysed by iron(III) 5,10,15,20-tetra(2,6-dichloro-2-sulfonatophenyl)porphyrin [Fe(ni)P] is a two-step process. In single turnover reactions, dye and Fe(in)P compete for the initially formed OFe(IV)P+ in a fast reaction and OFe(IV)P is produced the peroxy acid dye stoichiometry is 1 1. This is followed by a slow phase with 2 1 peroxy acid dye stoichiometry [equivalent to a... 

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