Combination of free radicals

An excited product is to be expected from radical-radical combination. The ethane produced in the recombination of methyl radicals must contain the net of the energy released by the carbon-carbon bond formation and the change in configurations of the methyl group. Unless stabilized, this hot molecule will revert to the reactants, viz. 

This is a case for which only stabilization is of interest. It has not been treated explicitly as an excitation reaction. Its rate has been measured directly103-105 at low temperatures (300-450 °C) as have those for several other radical combinations. Without doubt, the combination of methyl radicals is the most important reaction of this type for chemical kinetics. It serves as a reference standard for the measurement of rates of many hydrogen abstraction reactions, but very little is known about the temperature dependence of its rate coefficient. 

Radical combinations become important for chemi-excitation only when the product can undergo reactions other than stabilization and reformation of the reactants. The alternative reactions require that weaker bonds be present in the new molecule than in the one formed in the combination. The alkyl halides are useful in this respect. Elimination of HX is a competitive secondary reaction106 -110. Our final example of the use of chemi-excitation is drawn from this field111. It shows the application of three techniques group transfer reactions, radical combination, and methylene insertion 

Rates have been determined and analyzed in terms of the type of C-X bond 

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This simplified approach is analogous to the more rigorous absolute rate treatment. The important conclusion is that the bimolecular rate constant is related to the magnitude of the barrier that must be surmounted to reach the transition state. Note that there is no activation barrier (/.e., that AG = 0) in cases where no chemical bond is broken prior to chemical reaction. One example is the combination of free radicals. (In other cases where electrons and hydrogen ions can undergo quantum mechanical tunneling, the width of the reaction barrier becomes more important than the height.)... 

An exhaustive treatment of this topic can be found in (H2). Hannan makes the observations that three consequences are to be expected when fats are irradiated (1) combination of free radicals to form peroxides, (2) breaking of hydrocarbon chains with recombination of the free radical fragments to form long or branched chains, and (3) decarboxylation. 

This secondary carbon position offers the best combination of free radical stability and ability to approach the enzyme s reactive site. This addition product could be further transformed to yield 2-carboxy-substituted compounds (So and Young, 1999), derivatives that are subsequently used in pathways involving fatty acids. 

The key observation in the case of 152 is that photolysis in benzene conforms to the expected a-cleavage and decarbonylation reactions to form diphenylmethyl (A") and benzyl radicals (B"), which are free to diffuse apart. The statistical combination of free radicals A" and B gives a 1 2 1 mixture of products 154,155, and 156. In contrast, photochemical excitation in the crystalline phase led to the exclusive formation of 153 by combination of the geminate radical pair A B with a 100% cage effect. 

Figure 11 shows that the decay rate decreases with the lapse of time. This would naturally follow, if the mechanism of decay is associated with reactions listed at the end of the previous section. After geographically easy combinations of free radical species occur, it becomes increasingly difficult for the unpaired electrons to meet and combine. The decay rate consequently declines with time. 

By-products. The presence in the products of small quantities of compounds which would arise from combination of free radical intermediates can provide evidence for a free radical process. For example, the explosive reaction of methane with fluorine gives mainly hydrogen fluoride and a mixture of mono-, di-, tri- and tetrafluoromethanes, but small quantities of fluorinated ethanes, including C2F6, are also produced. These two-carbon products cannot be readily explained on the basis of possible molecular reactions (see reaction 6.16), but would arise naturally as combination products of the fluorinated methyl radicals produced in a radical chain reaction sequence (reaction 6.17). 

The Macallum polymerization (6, 7, 8) for preparing poly(arylene polysulfides) has been shown by Lenz and co-workers (9) to involve a combination of free-radical and nucleophilic substitutions. This polymerization involves the reaction of a polyhaloaromatic compound with an alkali-metal sulfide or an alkaline-earth metal sulfide catalyzed by sulfur and carried out at a high temperature without solvent. Related to the Macallum reaction are two nucleophilic processes for the preparation of poly(phenylene sulfide). One of these, reported by Lenz and co-workers (10, 11), involves heating cuprous or sodium p-bromothio-phenoxide at 250° to 305°C ... 

Termination may occur through the chance combination of free radicals, i.e. 

Combination of free radicals values of the steric factors... 

During the initial stages of drying, the chief method of cross-linking is by direct combination of free radical sites on different oil molecules (see Fig. 32). Peroxy, ether and carbon-carbon links can be formed. 

It should be remembered that the majority of the products obtained from the thermal decomposition of coal are formed mostly by the random combination of free-radical species that are generated during the process (Gun et al., 1979) ... 

Polymers That Cross-link by Dimerization of Nitrenes and by Other Combinations of Free-Radicals... 

Combinations of Free-Radicals to Form Covalent Bonds.223... 

Nearly all of the materials described in this monograph were prepared by some combination of free-radical and/or condensation reactions. Most recently, however, Lipatova et obtained grafted sequential IPNs on the basis of matrices from living network polymers, using anionic polymerizations for both networks I and II. 

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