Disproportionation of alkyl radicals

The recombination and the disproportionation of alkyl radicals play an important role in many other chain reactions, for example, pyrolysis, photolysis, and radiolysis of organic... 

The decay of free radicals taking part in oxidation of a polymer may occur as a recombination or disproportionation of alkyl radicals, alkyl and peroxyl radicals, or peroxyl radicals ... 

Let us assume that only the reaction steps listed in this scheme participate in the radical chain halogenations of hydrocarbons. Let us thus disregard the fact that chain termination can also occur owing to the radical-consuming reactions R- + Hal-— R—Hal and 2R- —> R—R and possibly also by disproportionation of alkyl radicals R- to give the alkane, which has one H atom more, and the olefin, which has one H... 

Figure 4 indicates that the yields of olefins produced by radiolysis are independent of the absorbed dose. These unsaturated products are pentene, light hydrocarbons, and eventually decenes produced by Reaction 15. Cage disproportionation of alkyl radicals produced by C—C bond scission and molecular fragmentation account for the formation of light olefins. Pentene would be formed either by elimination of molecular hydrogen (Reaction 5) or by disproportionation of radicals when the temperature of the sample is raised (Reactions 9 and 16). The latter process is thus a postirradiation production of pentene. 

Reactions of recombination and disproportionation of alkyl radicals are diffusion-controlled... 

Entry 3 has only alkyl substituents and yet has a significant lifetime in the absence of oxygen. The tris(/-butyl)methyl radical has an even longer lifetime, with a half-life of about 20 min at 25°C. The steric hindrance provided by the /-butyl substituents greatly retards the rates of dimerization and disproportionation of these radicals. They remain highly reactive toward oxygen, however. The term persistent radicals is used to describe these species, because their extended lifetimes have more to do with kinetic factors than with inherent stability." Entry 5 is a sterically hindered perfluorinated radical and is even more long-lived than similar alkyl radicals. 

The alkyl radicals possess reducing activity as well. In the disproportionation reaction, one alkyl radical reacts as an acceptor and another as a donor of the hydrogen atom. This is the reason for the reducing action of alkyl radicals in reactions with peroxides. 

We present the enthalpies of disproportionation of aminyl radicals with hydroxyalkyl and alkyl peroxyl radicals in Table 16.2 [22-24],... 

Fig. 4. Relation between the statistical corrected ratio of rates of disproportionation and dimerization of alkyl radicals and their Es-constants Ig = -0.48 Eg - 1.73 (r = 0.9901)...

If we assiime that there is no activation energy for the disproportionation or recombination, then fcj 109-5 liter/mole-sec. (see Table III). This is about a factor of 10 higher than the values to be expected of H-abstraction reactions of alkyl radicals. It is furthermore anomalous in having a negligible activation energy compared to the expected 8 3 kcal. Note that if we assign 1 kcal. of activation energy to the disproportionation then Ad 101 U liter/mole-sec. 

A number of authors have studied the reductive termination step. The classical work by Hermann and Nelson (121) showed that the reduction of the titanium in alkyltitanates occurred readily with the more alkylated species. The same authors (122) showed that the stability of the alkyl metal decreased markedly when the number of alkyls per metal atom increased. This is parallel to increased anionicity of the alkyl. Vanheerden (123) pointed out that the disproportionation of alkyl-titanium was bimolecular. Cotton (124) showed that the coupling occurs when phenyl Grignard reagents are mixed with various transition metal salts. This is through disproportionation or combination of the alkyl radicals. 

Selective trapping of alkyl radicals from the alkyl halide component during the course of the catalytic disproportionation is the same as the previous observation with silver, and it indicates that the prime source of radicals in the Kharasch reaction lies in the oxidative addition of alkyl halide to reduced iron in Equation 47. Separate pathways for reaction of i-propyl groups derived from the organic halide and the Grignard reagent are also supported by deuterium labelling studies which show that they are not completely equilibrated.(49) Furthermore, the observation of CIDNP (AE multiplet effect) In the labelled propane and propene... 

The behavior of diethylamino radicals is akin to that of alkyl radicals, in that both dimerization and disproportionation occur and at approximately equal rates. However we have no accurate numerical data for the corresponding sec-pentyl radical which may be used to provide the required contrast. The occurrence of DEMA, methane, and ethane in the reaction products suggests that the methyl radical is an essential precursor and the decomposition of diethylamino radicals... 

Under thermal conditions, hydroxylamine ethers can reversibly decompose (Reaction 15). The radicals formed disproportionate to eliminate olefins and yield hydroxylamine (Reaction 16). In the presence of sufficiently effective acceptors of alkyl radicals (e.g., oxygen), the reaction rate of peroxy radical formation is much higher than that of hydroxylamine formation. Thus, in the process of polymer photooxidation, nitroxyl radicals regenerate and can break multiple oxidative chains. 

Kabaskalian and Townley (1962a) also showed that the primary process gave RO + NO in a series of alkyl nitrites photolyzed in benzene solution. They deduced this from the products formed which were aldehydes and alcohols, the products expected om the disproportionation of RO radicals. For RO radicals... 

To establish the effect of structure on the ratio of recombination and disproportionation, the knowledge of reactions of low-molecular analogs may be use. It may be deduced from the ratios of rate constants of disproportionation and combination determined for different types of alkyl radicals that primary alkyl radicals enter combination preferably while tertiary radicals dispoiporti[Pg.153]

Following C—H bond scission, the H atoms formed remain trapped in the vicinity of the alkyl radicak only at 4 K. A slight increase of temperature will induce their further reactions and decay. At scanewhat higher temperature (e.g. at 77 K), disproportionation and transfer reactions occur within a radical pair of alkyl radicals and hydrogen atoms, and in ESR spectrum hydrogen atoms cannot be traced. 

Table 10. Rate constants (in cm mol s ) and activation energies of alkyl radical disproportionation reactions [232]...

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