High-energy processes alkyl radicals

At temperatures lower than approximately 900 K, high activation energies for alkyl radical decomposition make these processes relatively slow. Under such conditions, the most important reactions for alkyl radicals R consist of addition of... 

It has been mentioned that phenol is formed via Path A, by diffusion of radicals from the solvent cage and hydrogen abstraction from the solvent. This process is undoubtedly favored (and the yield of phenol is increased) when the phenoxy radical 65 already loses its counterpart in the solvent cage, i.e., when it loses the acyl radical 68 as a consequence of its decarbonylation. From the hitherto reported results it can be assumed that decarbonylation is significant and proceeds very readily under two conditions. It occurs (1) if the acyl radical formed possesses excess energy ( hot radical) due to excitation of high energy, e.g., by y-radiolysis,41,46 and (2) if the alkyl or aryl radicals formed by the decarbonylation of the acyl radical are exceptionally stable.61... 

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 ... 

Such a process is structurally very unlikely. It would be expected to have a very small A factor as well as a fairly high activation energy. If we rule this type of process out then we are forced to assume that the CH2 attacks the H atom initially in very much the same way that it might for a metathetical process. If we follow this process through, then we see that if the H-atom abstraction is successful, we have still a transition state in which two radicals, the newly formed methyl and the residual alkyl, arc in very close proximity, being essentially H-bonded. However, we have already noted as an empirical fact that the cross sections for alkyl radical recombinations are extremely high. We should thus like to propose that the H-bonded radical pair simply rotates into the favorable position for C-C bond formation before separation can take place. The pathway for the insertion reaction would then take the form ... 

Although BDE is by far not the only factor that determines the kinetics of H-transfer reactions, within a given series of simple alkyl radical a correlation seems to hold (Table 6.4). In polymers, where the lifetime of the polymer-bound radicals may be long, radical transfer reactions by intramolecular H-abstrac-tion (primary - secondary —> tertiary) are common (Chap. 9.4). In general, whenever a system starts with a mixed radical system (e.g., in the reaction of OH with 2-PrOH 2-hydroxy-prop-2-yl and 2-hydroxypropyl) a steady-state is approached which is dominated by the lower-energy radical [here 2-hydroxy-prop-2-yl, cf. reaction (21)]. This process is favored by low initiation rates and high substrate concentrations, and these two factors determine whether such an H-transfer manifests itself is also in the final products. 

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