Recombination of Alkyl Radicals

The rate constants of the recombination of alkyl radicals with peroxyl radicals... 

If we compare the rate constants for the recombination of alkyl radicals ( 1010,5 liter/mole-sec.) with collision frequencies of these same radicals (1011-3 liter/mole-sec.) we are struck by the very high efficiency (1 in 6 collisions) of these recombination processes. For the younger generation of kineticists these values are by now well established and occasion no surprise. However, one has only to turn back in the literature some 15 or more years to discover that the older generation was quite prepared for recombination efficiencies of the order of 10 a to 10 4 while a number of respected workers anticipated activation energies of the order of 3 to 15 kcal. What was the origin of such speculations and why was the range so broad ... 

Summarizing, one can say that 75-90 % of the strain enthalpy released in the dissociation process is found as a reduction in AG or AH. The missing 10-25 % is either a tribute to inadequacies of the compensation effect discussed before or also the recombinations of alkyl radicals pass, in contradiction to Fig. 1, a small activation barrier which is equal to 10-25 % of the strain enthalpy of the dimer 9). 

According to our solid-state, 9F- and 13C-NMR spectroscopy, FT-IR spectroscopy, electron spectroscopy for chemical analysis (ESCA), it has been made clear that Y-type and modified Y-type (Y -type) crosslinking structures are formed in PTFE by its molten state irradiation [6, 10, 11]. In addition, it has been found that the various sorts of double bonds structure excluding crosslinking site are produced. Y-type crosslinking through recombination of alkyl radical and methylene (end-chain) type radical can be easily expected by probability of combinations for both radicals in its molten state. 

Table S Recommended values for the rate constants for recombination of alkyl radicals...

The high pressure rate constants for a large number of radical-radical association reactions have been studied theoretically by Klippenstein, Harding, Miller and co-workers.They have focused especially, but not exclusively, on the recombination of alkyl radicals, such as CH3 -I- CHs, of resonance-stabilised radicals such as C3H3 + CsHs, and of the association of H atoms with alkyl and aryl radicals such as CH3 + H and CgHs -I- H. These reactions are particularly important in high temperature systems and experimental information about their rate constants is limited, generally to room temperature and above. 

The use of a very labile SGl-based alkoxyamine drastically improved the controlled character of a bulk styrene potymer-ization. In that case, a crowded tertiary alkyl radical moiety instead of a secondary alkyl one increased the value from 5.0 X 10 to 1.7 X 10 s at 90 °C. Additionally, Benin and Charleux research groups showed that a very labile SGl-based alkoxyamine (BlocBuilder, Table 4, 71) allowed the polymerization of nBA to be successfully performed without any initial addition of free nitroxide. In this case, high leads to the in situ production of free nitroxide in the early stage of the polymerization and this forces the recombination of alkyl radicals with nitroxides even at low monomer conversion. Chauvin et propagation rate constant of the monomer ... 

Recombination of alkyl radicals, as that of atoms, occurs practically without an activation energy. In the gas phase at a sufficiently high pressure the recombination of methyl radicals is bimolecular with the rate constant close to (l/4)Zo (where Zo is the frequency factor of bimolecular collisions, and the factor 1/4 reflects the probability of collisions of particles with the antiparallel orientation of s ins). The theoretical estimation of the constant at a collision diameter of 3.5-10 m agrees with the experimental value = 2-10 ° l/(mol-s) (300 K). This k value agrees with the estimation by the theory of absolute reaction rates under the assumption that the free rotation of methyl groups is retained in the transition state. In the liquid the recombination of meth)d radicals is bimolecular with the rate constant of difiiision collisions (see Qiapter 5). For example, in water 2k = 3.2-10 l/(mol s) (298 K). Ethyl radicals react with each other by two methods recombine and disproportionate... 

However, unlike the recombination of alkyl radicals, which occurs with the dif-fiisional rate constant, radical anions recombine more slowly due to their repulsion upon bringing together. The recombination rate depends on the counterion (tetrahy-droluran, 273 K)... 

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 values of rate constants of alkyl radical recombination in solution are collected in Table 2.19. 

The simultaneous photolyses of two different species of dithiocarbamates (R - DC and R2 - DC) in the absence of monomers and at high concentrations result in enforced recombination between alkyl radicals produced by photolysis to produce three types of alkyl-alkyl recombination product namely Ri - Ri, R2 - Ri and Ri- R2, as shown in Scheme 5. If the concentration of one species (Ri - DC) is much higher than the other R2 - DC), a relative fraction of heterotopically recombined product, Ri - R2, must increase. If such a heterotopic cross-recombination reaction predominantly occurs at a solution/solid interface, an it2-alkylated dithiocarbamated surface is converted into an it 1-alkylated surface, as schematically shown in Scheme 6. 

The past decade has been an extremely fruitful one in the field of quantitative free radical kinetics. Two papers can be identified as the starting point of much of this work. The first of these is the acetone photolysis study by Noyes and Dorfman1 which gave confidence to the use of acetone as a reproducible source of methyl free radicals in a fairly simple kinetic environment. The second is the study of Gomer and Kistiakow-sky2 of the absolute rate of recombination of CH3 radicals. The latter study made it possible to give absolute values for the Arrhenius parameters for the reactions of alkyl free radicals with stable molecules. It also opened the way for putting the reactions of methyl radicals with other alkyl radicals on an absolute basis. 

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. 

The efficiency of alkyl radical recombination is very high (about 1 in 6 collisions), and Benson12 has shown that the high A factor for alkyl recombination requires a transition complex in which the alkyl fragments rock almost freely against each other. The attractive potential between the loosely associated radicals is ascribed by Benson to partial contribution of ionic states such as... 

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 ... 

Lithium aluminium hydride markedly accelerates the photoreductive dehalogenation of chlorobenzene, bromobenzene and para-bromochlorobenzene362. Presumably, lithium aluminium hydride suppresses the recombination of alkyl and halogen radicals by means of efficient capture of the latter. 

Fig. 3. Autoxidation of polyunsaturated fatty acids in phospholipid membranes. Addition of oxygen to lipid free radicals is extremely fast. It yields peroxyl radicals ROO which will tend to capture labile hydrogen atoms of neighbouring polyunsaturated lipids. Accidentally produced free radicals will therefore initiate a chain reaction of lipid peroxidation which will propagate along membranes. This process can result in several dozen propagation steps before it is stopped by a termination reaction. Examples of such termination reactions are the recombination of peroxyl radicals and the formation of a stable free radical from a free radical scavenger (scavH). Termination through recombination of low steady-state concentration of alkyl radicals is unlikely in aerobic medium.

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