Abstraction of hydrogen atoms

In this section we shall restrict ourselves to abstraction of hydrogen atoms by oxy radicals since comparative data are available. Many aspects of the conclusions reached will be valid for nitrogen radicals as well. 

The competitive method employed for determining relative rates of substitution in homolytic phenylation cannot be applied for methylation because of the high reactivity of the primary reaction products toward free methyl radicals. Szwarc and his co-workers, however, developed a technique for measuring the relative rates of addition of methyl radicals to aromatic and heteroaromatic systems. - In the decomposition of acetyl peroxide in isooctane the most important reaction is the formation of methane by the abstraction of hydrogen atoms from the solvent by methyl radicals. When an aromatic compound is added to this system it competes with the solvent for methyl radicals, Eqs, (28) and (29). Reaction (28) results in a decrease in the amount... 

If a vinyl monomer is polymerized in the presence of cellulose by a free radical process, a hydrogen atom may be abstracted from the cellulose by a growing chain radical (chain transfer) or by a radical formed by the polymerization catalyst (initiator). This leaves an unshared electron on the cellulose chain that is capable of initiating grafting. As cellulose is a very poor transfer agent [10], very little copolymer results from the abstraction of hydrogen atoms by a growing chain radical. The... 

Many compounds that damage DNA via radical intermediates have been identified. Some of the agents, such as bleomycin and the enediynes, damage DNA primarily through abstraction of hydrogen atoms. ° In these cases, chemical reactions are directed to certain positions on the DNA backbone by noncovalent binding that places the reactive intermediates in close proximity to particular deoxyribose sugar residues. Similar to the reactions of HO described above, small radicals, such as... 

Will the solvent react with the excited state to yield undesirable side-products Often there is a real possibility that the solvent will enter into the picture through reaction with the excited solute. A common example of this is the abstraction of hydrogen atoms from solvents by excited ketones. Several solvents often used for a preliminary examination due to their relative inertness are benzene, /-butanol, carbon disulfide, carbon tetrachloride, and cyclohexane. 

These observations are consistent with the reactive species being constituted from tight ion pairs between cations and the alkoxide anions resulting from abstraction of hydrogen atoms in A, B and C (Scheme 3.11). 

The reactions of intramolecular isomerization occur and are important in the oxidation of natural and synthetic rubbers. The peroxyl radical addition to the double bond occurs very rapidly. For example, the peroxyl radical adds to the double bond of 2-methylpropene by 25 times more rapidly than abstraction of hydrogen atom from this hydrocarbon (see Chapter 4). Therefore, the oxidation of polymers having double bonds proceeds as a chain process with parallel reactions of P02 with double and C—H bonds including the intramolecular isomerization of the type [12] ... 

All the examples presented under Sect. 4.1 used an iodine atom transfer to generate the desired radicals. Another approach involving abstraction of hydrogen atom is also reported. For instance, ethers and acetals undergo direct intermolecular addition to aldehydes under treatment with Et3B/air... 

Amorphous fluoropolymers have many applications in the areas of advanced materials where they are used in applications requiring thermal and chemical resistance. Their manufacture is hindered by their low solubility in many solvents. Many fluoropolymerizations cannot be carried out in hydrocarbon solvents because the radical abstraction of hydrogen atoms leads to detrimental side reactions. Chlorofluorocarbons (CFCs) were thus commonly used, but their use is now strictly controlled due to their ozone depleting and greenhouse gas properties. Supercritical carbon dioxide is a very attractive alternative to CFCs and it has been shown that amorphous fluoropolymers can be synthesized by... 

Steacie, E. W. R., and A. F. Trotman-Dickenson The Reactions of Methyl Radicals. IV. The Abstractions of Hydrogen Atoms from Cyclic Hydrocarbons, Butynes, Amines, Alcohols, Ethers and Ammonia. J. chem. Physics 19, 329 (1951)-... 

The aromatic-hydroxyl radical reaction has been studied by Davis et They reported rate constants for benzene and toluene and concluded that hydroxyl additions to the aromatic ring compete favorably with the abstraction of hydrogen atom from the alkyl substituent. Doyle et al recently published hydroxyl reaction rate constants for a series of alkylbenzenes. 

The mechanism of cytochrome P450 catalysis is probably constant across the system. It is determined by the ability of a high valent formal (FeO) species to carry out one-electron oxidations through the abstraction of hydrogen atoms or electrons. The resultant substrate radical can then recombine with the newly created hydroxyl radical (oxygen rebound) to form the oxidized metabolite. Where a heteroatom is the (rich) source of the electron more than one product is possible. There can be direct recombination to yield the heteroatom oxide or radical relocalization within the... 

An example of carbonyl compound which illustrate Photodecarbonylation and example of Abstraction of hydrogen atom to form alcohol... 

Similar chain decompositions were observed in the photolyses of perfluofopropionaldehyde and perfluoro-n-butyraldehyde69 but the energies of activation for the abstraction of hydrogen atoms are unexpectedly much lower, that for abstraction by -CsHs radicals being 4.5 kcal. mol.-1 and that for abstraction by n-C3H radicals4.0kcal. mol.-1. 

Comparative studies indicate that abstraction of hydrogen atoms from the methyl group of 1-methylpyrrole is easier than from the methyl group of toluene. Subsequent dimerization and polymerization of the 1-pyrrolylmethyl radicals, initiated by the addition of r-butoxy radicals, occurs and low yields (0.5%) of l,2-bis(l-pyrrolyl)ethane and l-methyl-3-(l-pyrrolylmethyl)pyrrole have been isolated from the tars (B-77MI30500). The radical dimerization of pyrrole initiated with r-butoxy radicals produces 2-(2-pyrrolyl)-A1-pyrroline (B-77MI30500). 

Both of the above reaction pathways are suppressed if 443-D and perdeuteriated solvents are used simultaneously in cyclization reaction 265. Mainly oligomer and less than 5% of alkane 450 were then produced. This has been explained by different reactivities of radical 445 formed directly from 444 or formed by abstraction of hydrogen atom from 443 through complex 453. The more energetic radicals formed by hydrogen atom transfer from 443, add intermolecularly to an alkene and initiate preferentially oligomerization512, while... 

Triplet oxygen is a stable and surprisingly unreactive electrophilic diradical. Its reactions with organic compounds (autoxidation) include abstraction of hydrogen atoms or single electrons and addition to reactive C-C double bonds (Scheme 3.14). 

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