Abstraction by free radicals

Besides hydrogen, other atoms and groups are susceptible to abstraction by free radicals. The most important from a synthetic point of view are bromine, iodine, sulfur. 

Additional evidence was obtained when the oxidation was carried out in the presence of copper(II) acetate, a common radical terminator. The oxidation proceeded unhindered and is not, therefore, dependent on hydrogen abstraction by free radicals. It can be represented by Eq. (15) or better by... 

Upon exposure to air, animal and vegetable fats and oils become rancid (i.e., develop color changes and a musty, rank taste and odor). Here, the hydrogen atoms of the —CH2—groups located between alternating double bonds (i.e., —CH=CH—CH2—CH=CH—) of a polyunsaturated phospholipid or fatty acid (LH) are very susceptible to abstraction by free radicals. This process can then lead to a general reaction known as autoxidation, which results in the formation of a lipid hydroperoxide (LOOH) and the generation of a new free radical hence, an autocatalytic reaction results (lipid peroxidation). 

The characteristic thing about macromolecules, we have said, is their great size. This size has little effect on chemical properties. A functional group reacts much as we would expect, whether it is in a big or little molecule an ester is hydrolyzed, an epoxide undergoes ring-opening, an allylic hydrogen is susceptible to abstraction by free radicals. 

In order to compare the C-H bonds, AMI wavefunctions were calculated for methane and for the hydrofluoromethanes, orientating each molecule such that a C-H bond lay along the z-axis. Localised molecular orbitals were produced using the population localisation method and values of >4b(—1) calculated (see Table 6). The AMI calculations reproduce the experimental observation that the C-H bond length is insensitive to increasing fluorination. Nevertheless, the C-H bonds in these molecules do show a marked variation in chemical reactivity. This is evident from the rate constants and from the activation energies for H-atom abstraction by free radicals. The reaction with OH is of particular... 

In contrast to the examples cited above, alkylated naphthalenes oxidized considerably slower (approximately 1/30 the rate) compared to other hydrocarbons and this effect is believed to be due to formation of phenolic antioxidants. Unsubstituted positions on aromatic rings are not liable to hydrogen abstraction by free radicals due to high C-H bond strengths. Such positions in naphthalene... 

Two important elementary step reactions were described in these radical chains of Rice. One was the ubiquity of the metathetical reaction of H atom abstraction by free radicals from hydrocarbons. The second was the rapid unimolecular decomposition of free radicals into olefins and secondary atoms or radicals, as a chain step competitive with metathesis. 

Other atoms and groups apart from hydrogen are susceptible to abstraction by free radicals. The most important from a synthetic point of view are bromine, iodine, sulfur, and selenium substituents. Group transfer reactions can occur inter- or intramolecularly. Indeed, we have already encountered one example in the addition of polyhalogenated methanes to alkenes. The chain is propagated by a bromine atom transfer. 

The effects of structure on reactivity found in oxidation are very similar to some reported in the literature for hydrogen abstraction by free radicals. These include abstraction by methyl radicals in the gas phase (159), abstraction by methyl radicals in solution (140), and abstraction by e/ <-butoxy radicals in liquid phase (141). The oxidation reactivities are widely different from those for carbanion intermediates. 

O-CH -CH-CHg-O in the backbone. There are, therefore, also 3 X 5 = 170 hydrogens that can be abstracted by free radical to form a radical site where initiation of monomers can occur. 

The elastomers from the high molecular weight silicone polymers must be crosslinked to obtain rubber-like properties. One way to accomplish this is through hydrogen abstraction by free radicals that are generated by decomposition of added peroxides. 2,4-Dichlorobenzoyl peroxide is often used for this purpose. It is decomposed between 110-150 C. The reaction can be shown as follows ... 

The major limitation of the all GBH reactions is that the arene has to be in the liquid form to serve as the solvent. All attempts to arylate solid arenes under the GBH reaction conditions were unsuccessful. Furthermore, the presence of radical-sensitive substituents in either diazonium salts or in arene results in lower yields, or the reaction fails completely. Generally, some alkyl, alkoxycarbonyl, formyl, or iodo-substituted arenes or diazonium salts are susceptible for hydrogen- or iodo-abstraction by free-radicals and are not common GBH reactants. For example, 2-tolyldiazonium tetrafluoroborate in the ptGBH reaction with benzene gives indazole in 74% yield, and only 2% of biaryl product [61]. 

Substitution Reactions. Substitution reactions can occur on the methyl group by free-radical attack. The abstraction of an aHybc hydrogen is the most favored reaction, followed by addition to that position. 

As mentioned in an earlier section (cf. Chapter 1, Section III), allylic positions are subject to attack by free radicals resulting in the formation of stable allyl radicals. A-Bromosuccinimide (NBS) in the presence of free-radical initiators liberates bromine radicals and initiates a chain reaction bromination sequence by the abstraction of allylic or benzylic hydrogens. Since NBS is also conveniently handled, and since it is unreactive toward a variety of other functional groups, it is usually the reagent of choice for allylic or benzylic brominations (7). 

RE Pliss, VA Machtin, EM Pliss. In Abstracts of Conference Regulation of Biological Processes by Free Radicals Role of Antioxidants, Free Radical Scavengers, and Chelators. Moscow, Yaroslavl, Yaroslavl State Technical University, 1998, p 17. 

Bedard et al. [7] studied quantitatively the initiation of the peroxidation of human low-density lipoproteins (LDL) with H00702 . In accord with the above findings the initiation rate increased when pH decreased from 7.6 to 6.5. It was suggested that initiation occurred via hydrogen atom abstraction by perhydroxyl radical from endogenous a-tocopherol, which in this process exhibited prooxidant and not antioxidant properties. Neutral, positively, and negatively charged alkyl peroxyl free radicals were the more efficient initiators of LDL peroxidation compared to superoxide. 

Attempts to polymerise isobutene by free radical catalysis have all failed [16,17] and copolymerisation experiments show that the t-butyl radical has no tendency to add to isobutene. The reasons for these facts are not at all obvious. Evidently, they cannot be thermodynamic and therefore they must be kinetic. One factor is probably that the steric resistance to the formation of polymer brings with it a high activation energy [17], and that the abstraction by a radical of a hydrogen atom from isobutene, to give the methallyl radical, has a much smaller activation energy. This reaction will also be accelerated statistically by the presence of six equivalent hydrogen atoms. 

The most common strategy for laboratory scale hydrocarbon functionalization is hydrogen atom abstraction followed by free radical recombination. Although the conversions are often low, the simplicity of the approach can in some cases make this the method of choice for preparing a particular target molecule. A representative procedure is the cyanation of 2,3-dimethylbutane to give a 77% yield of 2,2,3-trimethylbutanenitrile1. 

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