Disproportionation alkoxyl radicals

The chain termination is a result of tertiary alkylperoxyl radical recombination in the solvent cage. The values of the rate constants for chain termination through the disproportionation of tertiary peroxyl radicals are collected in Table 2.15. They vary in the range 103 to 105 L mol 1 s 1 at room temperature. The probability of a pair of alkoxyl radicals to escape cage recombination is sufficiently higher than that of cage recombination. The values of rate constants of the reaction 2 R02 > 2 RO + 02 measured by the EPR technique are presented in Table 2.16. 

Dialkyl peroxides decompose with splitting of the weakest O—O bond [3,4]. The pair of forming alkoxyl radicals recombine or disproportionate in the cage or go out the cage ... 

Due to the ability of tertiary peroxyl radicals to disproportionate with the formation of alkoxyl radicals, the chain decomposition of tertiary hydroperoxides proceeds via the action of intermediate alkoxyl radicals [9,135]. 

Hydroxyl and substrate-derived alkoxyl radicals, produced via Eq. (3), are the two main oxidants in Gif chemistry which are capable of effecting H-atom abstraction from unfunctionalized alkanes (Eq. 5). In contrast, Eq. (4) serves as a provider of Fe(II) ions and peroxyl radicals, the latter being responsible for the increased amounts of dioxygen observed with Fe(III) reagents by virtue of disproportionation paths. 

When the alkoxyl radical and the hydrogen to be abstracted are not properly disposed for the Barton reaction, the reactions of the alkoxyl radical, for example -fragmentation, intramolecular addition to the double bond, disproportionation or a-hydrogen fission, and intermolecular hydrogen abstraction, compete with the Barton reaction or result in an exclusive reaction. Among these reactions, /l-frag-... 

Table 5. Arrhenius parameters and rate constants for disproportionation of alkoxyl radicals and oxygen...

The flrst study of the disproportionation-to-combinadoii ratio of alkoxyl radicals with NO was made by McMillan (1961), who photolyzed diisopropyl peroxide at 230-290 nm in the presence of NO. From the ratio of (CH3)2CO to 1-C3H7ONO produced he could deduce His values are given in Table 8. 

Similarly, two allylic hydroperoxides can undergo disproportionation, by transfer of hydrogen from two alkoxyl radical intermediates, into a mixture of ketone and hydroxy products (11). 

The photolysis of cyclic secondary alcohol nitrites is sometimes accompanied by the formation of the corresponding cyclic ketones via loss of the a-hydrogen of the alkoxyl radicals arising from either disproportionation or, less likely, a-hydrogen fission. 

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