Hydrogen abstraction alkoxyl radicals

Most organic compounds have more than one type of C-H bond, and hydrogen abstraction by radicals usually proceeds in an unselective manner, giving complex mixtures of products. An exception is when the abstraction is intramolecular, when for example a long-chain alkoxyl... 

It is usually postulated that the final product in the accepted mechanism, the alkoxyl radical (4), cleaves (eqs. 14 and 15) before or after hydrogen abstraction, and that this accounts for the drop in molecular weight of the... 

Figure 17.2 Lipid peroxidation scheme. LH, a polyunsaturated fatty acid LOOM, lipid hydroperoxide LOH, lipid alcohol L, lipid radical LOO, lipid hydroperoxyl radical LO, lipid alkoxyl radical. Initiation the LH hydrogen is abstracted by reactive oxygen (e.g. lipid alkyl radical, lipid alkoxy radical, lipid hydroperoxyl radical, hydroxy radical, etc.) to produce a new lipid alkyl radical, L. Propagation the lipid alkyl, alkoxyl or hydroperoxyl radical abstracts hydrogen from the neighbouring LH to generate a new L radical.

Alkoxyl radicals are very active in reactions of hydrogen atom abstraction (see Table 2.20). The problems of their reactivity will be discussed in Chapter 6. 

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... 

The IPM parameters for hydrogen transfer atom in alkoxyl radicals are presented in Table 6.12. Isomerization proceeds via the formation of a six-membered activated complex, and the activation energy for the thermally neutral isomerization of alkoxyl radicals is equal to 53.4 kJ mol-1. These parameters were used for the calculation of the activation energies for isomerization of several alkoxyl radicals via Eqns. (6.7, 6.8, 6.12) (see Table 6.14). The activation energies for the bimolecular reaction of hydrogen atom (H-atom) abstraction by the alkoxyl radical and intramolecular isomerization are virtually the same. 

More relevant to our consideration now is the radical addition of hydrogen bromide to an alkene. Radical formation is initiated usually by homolysis of a peroxide, and the resultant alkoxyl radical may then abstract a hydrogen atom from HBr. 

Figure 22.7. Schematic depiction of the TS for hydrogen atom abstraction from methylcuhane (6) hy an alkoxyl radical. The polarity of the TS, depicted in the bottom resonance stmcture, was confirmed by the results of population analyses. " ...

The rate constants for hydrogen abstraction from Rh H, O H, and C-H bonds by chromyl ions and Craq002+ are summarized in Table VI. Also listed in the table are selected relative rate constants for hydrogen abstraction by tert-butoxyl and tert-butylperoxyl radicals, expressed as .buo/AbuOO- The difference between the two sets of data is striking in that alkoxyl radicals are 105-107 times more reactive than alkylperoxyl radicals, but in the chromium series the ratio kcrolkcrOO is only about 102 for all the reactions studied. This ratio is preserved over about 103-fold change in absolute rate constants within each series. 

Figure 10.10. Orbitals for a hydrogen atom abstraction reaction by a) alkoxyl radical from H—OR b) methyl radical from CH4 (one bond shown) (c) silyl radical from SiH4 (one bond shown).

The photolysis of n-octyl nitrite in n-heptane provides a favorable situation for comparing the tendency of an alkoxyl radical to undergo the Barton-type intramolecular reaction or, alternatively, the intermolecular reaction as indicated in eqs. 1 and 2. Calculations23 reveal that both intramolecular and intermolecular hydrogen abstraction involving a second-... 

In contrast to aminyl radicals, alkoxyl (and acyloxy) radicals are highly reactive. As illustrated in equation (7), their cyclization reactions are extremely rapid and irreversible. However, the rapidity of such cyclizations does not guarantee success because alkoxyl radicals are also reactive in inter- and intramolecular hydrogen abstractions, and -fragmentations (see Section 4.2.S.2). This lack of selectivity may limit the use of alkoxyl radicals in cyclizations, but S-exo cyclizations are so rapid that they should succeed in many cases, and other types of cyclizations may also be possible. 

The photolysis of organic nitrites of appropriate constitution and conformation in solvents such as benzene or acetonitrile transforms them into A-nitroso alcohols via the sequence (1) a homolytic fission of the O-N bond of their nitrosoxy group (2) an intramolecular d-hydrogen abstraction of the resulting alkoxyl radicals to generate a d-carbon radical and (3) formation of d-nitroso alcohols by combining of the d-carbon radical with the generated nitric oxide. The nitroso alcohols are isolated as d-hydroxyimino alcohols as a result of spontaneous thermal isomerization or as nitroso-dimers [1] (Scheme 1). This transformation has been named the Barton reaction [2, 3],... 

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-... 

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