Alkoxy radicals recombination

Basically, three reactions were evoked to support the occurrence of 5a-C-centered radicals 10 in tocopherol chemistry. The first one is the formation of 5a-substituted derivatives (8) in the reaction of a-tocopherol (1) with radicals and radical initiators. The most prominent example here is the reaction of 1 with dibenzoyl peroxide leading to 5a-a-tocopheryl benzoate (11) in fair yields,12 so that a typical radical recombination mechanism was postulated (Fig. 6.6). Similarly, low yields of 5a-alkoxy-a-tocopherols were obtained by oxidation of a-tocopherol with tert-butyl hydroperoxide or other peroxides in inert solvents containing various alcohols,23 24 although the involvement of 5 a-C-centered radicals in the formation mechanism was not evoked for explanation in these cases. 

Hypohalites IRO-Hal) are similar to nitrates (see p. 155 in their photochemical behaviour. Ultraviolet irradiation gives an (n,Ji > excited state that cleaves to form an alkoxy radical and a halogen atom. The radical may undergo alpha-cleavage before recombination with the halogen atom occurs, and this accounts for the formation of 5-iodopentanal (5.69) from the hypoiodite of cydopentanol such hypoiodites are generated in situ from the alcohol. Iodine and merturyfll oxide. In open-chain systems the alkoxy radical can... 

Table VII. Disproportionation-Recombination Ratios Involving Alkoxy Radicals...

Rearrangement is also observed in the irradiation of Formula 450 which gives the hydroxamic acid (Formula 451) (203). This interesting rearrangement probably involves ring opening in the alkoxy radical (Formula 452) followed by recombination with nitrous oxide and cyclization (203). Oxidative fission of alkoxy radicals produced in nitrite... 

The photochemical reactions of arenecarboxylic acid esters with alkenes has received recent attention by Cantrell. - For example, irradiation of 2,3-dimethyl-2-butene and methyl benzoate gave a mixture of alkoxyoxetane (56), carbonyl-alkene metathesis product (57) and ketone (58), resulting from alkoxy radical allylic hydrogen abstraction and radical recombination. Such alkoxyoxetane photoproducts are... 

In general, alkoxy radicals generated in cellulose are stable as compared to carbon radicals. The carbon radicals readily undergo secondary termination reactions. Carbon radicals in vacuo have an affinity for recombination and hydrogen abstraction to stabilize themselves in the presence of oxygen, and they are transformed rapidly into hydroperoxide radicals to build up hydroperoxide. This rapid oxygenation reaction is further accelerated when excited oxygen is presented (S3). 

Peroxy radicals dimerise at diffusion rates to tetroxides, which in turn, above 150 K, decompose into alkoxy radicals and oxygen. Low-temperature ESR has established the linear tetroxides as real molecules, with mM to pM dissociation constants at 110-150 The combination is subject to a very large magnetic isotope effect, caused by the coupling to O to the electronic spin, which relaxes some of the spin-symmetry prohibitions on radical recombination a value of knjki of 1.8 is observed for combination of with of R- 0- 0. If, however, the peroxy radical is a to a hydroxyl... 

Termination. Just as peroxy radicals are key to the propagation sequence, so the bimolecular recombination of these radicals is the major termination process in the unstabilized polymer. The existence of an intermediate tetroxide has been established in solution (25). Several factors influence the competitive pathways of subsequent decomposition to form alcohols, ketone and singlet oxygen or to form alkoxy radicals which can couple before separation from the reaction center to form a peroxide. This latter process is a route to crosslinking in the case of polymeric peroxy radicals. The effect of steric control, viscosity and temperature have been studied in solution. However, in the solid phase the rates of bimolecular processes which require the mutual diffusion of the reactant groups will be limited by the diffusion process. As a standard, we have assumed a value close to that determined from oxygen absorption (26) and by ESR spectra (27) for oxidized polypropylene films. 

Figure 6.35). The proposed mechanism for the dominant reaction involves an unremarkable hydroxylation at C17 of the steroid nucleus (Figure 6.36). This is then followed by an attack of the ferric peroxo moiety on the carbonyl to yield a species that fragments to an alkoxy radical and a one-electron oxidized ferryl species. The alkoxy radical subsequently decomposes to produce acetic acid and a carbon radical that recombines with the ferryl species to yield a gem-diol, which dehydrates to the C17 carbonyl of the product. This mechanism is in accord with a wealth of labeling studies and can be modified simply to explain the origin of the other observed products. 

Alkanox 240 and other sterically hindered noncyclic phosphites react with alkyl peroxy radicals to produce the corresponding phosphate and an alkoxy radical (Scheme 2.105) [126]. The latter can attack another phosphite molecule, where under recombination finally a quinoidal structure is produced. This step... 

In the solid phase, the recombination of radicals in the cage is highly efficient, whereas the escape of NO and RO radicals from the cage is quite low. Alkoxy radicals RO generated in reactions (Equation 3.77) decompose or enter into substitution reactions with neighbouring macromolecules to form chain R and end R alkyl macroradicals as well as low-molecular alkyl radicals r. The appearance of the latter radicals in the reaction of PP hydroperoxide with NO was verified in [66] ... 

The aUcyl radicals formed by the abstraction reaction react exclusively with O2 in the atmosphere to form alkylperoxy radicals (pathways (c) (h)), and most of them are then transformed to alkoxy radicals by oxidizing NO to NO2 in the presence of NOx (pathways (d) and (i)). However, some alkylperoxy radicals react with NO by the recombination isomerization reactions (e) and (j), to form alkyl nitrate (see Sect. 5.3.3). Although the yields of 2-butyl and 1-butyl nitrate are not large in the case of 2-butyl and 1-butyl radicals, 0.083 and <0.04, respectively, the production yields of alkyl nitrates increase with the increase of carbon number of alkyl radicals as described in Sect. 5.3.3, and it is as large as >0.2 for the Cg, and C7 secondary n-aUcyl radicals (2-hexyl, 3-hexyl, 2-heptyl, and 3-heptyl radicals) (Lightfoot et al. 1992 Arey et al. 2001). Since these reactions act as termination reactions for the OH chain reaction, they are important as parameters in determining the ozone formation efficiency in model calculations. The production yields of RONO2... 

The 2-butoxy, and 1-butoxy radicals formed in the reactions of 2-butyl and 1-butyl peroxy radicals with NO (path (d) and (j)), react with O2 to produce HO2 together with carbonyl compounds such as methyl ethyl ketone and butanal (path (g) and (1)), and complete the HO cycle. Under the typical NO concentrations in the polluted atmosphere, the formation rate of alkyl nitrates by the reactions of alkoxy radicals with NO2 is negligible, the recombination isomerization reaction by the alkylperoxy radicals with NO is the major pathway as the formation process of alkyl nitrate. 

The formation of a cage should be especially favored by coordination of one of the oxygen atoms to boron. Radical recombination would lead to the alkoxy compounds commonly observed, whereas diffusion from the cage could lead to the products derived from free radicals. 

Few examples have been reported (5, 8, 9, 10, 12, 24) of cage recombination of simple alkoxy or acyloxy radicals to form O—O bonds in isolable molecules. This paper explores further the implications of the observed (17, 22, 23) scrambling of label seen in acetyl peroxide carbonyl-18O recovered after partial decomposition. 

The quantum efficiency of this reaction is reported to be about unity (//). The -OH radicals thus produced may abstract hydrogen atoms to form alkyl radicals, which cause crosslinking by recombination, or take part in the general sequence of oxidation, as will be described later. Alkoxy... 

Polymers containing terminal photoactive benzoin groups have been synthesized using azo-benzoin initiators [210,211]. The thermal treatment of these initiators in the presence of styrene leads to benzoin groups at both ends of the polystyrene chain, as polystyryl radicals tend to terminate via recombination. Upon irradiation of the styrene-based photoinitiators, benzoyl and alkoxy-benzyl radicals are produced, both capable of initiating polymerizations to give mixtures of homopolymers and block copolymers [212]. 

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