Hydrogen free-radical migration

Free radicals, unlike carbocations, do not normally undergo isomerization by methyl or hydrogen migration. However, hydrogen transfer (chain transfer) occurs when a free radical reacts with other hydrocarbons. 

The results observed in this system unambiguously support free-radical stability as a significant governing principle. By far the major product (62) is derived by attack on CS a minor product (63) is due to attack on C6 and hydrogen migration is effectively eliminated. These results show clearly that steric hindrance is of some importance in these reactions, although the transition states for some of these reactions q>pear to be remarkably early. ... 

It was further found that the trapped radicals in polyethylene decayed more rapidly in a hydrogen atmosphere. The migration of the free valencies was therefore explained on the basis of a process similar to Reactions 13 and 14, assuming an exchange reaction involving the hydrogen which results from the radiolysis of the polymer. However, Reaction 13 is endothermic and therefore unlikely to occur at room temperature. 

On heating, migration of a hydrogen atom, alpha to a polyene group, to an alkyl free radical site would also convert a polyene to a polyenyl free radical, a transformation similar to transformation of the allyl free radical described above ... 

Very little skeletal rearrangement occurs via pyrolysis, a fact inherent in the failure of free radicals to readily isomerize by hydrogen atom or alkyl group migration. As a result, little branched alkanes are produced. Aromatization through the dehydrogenation of cyclohexanes and condensation to form polynuclear aromatics can take place. Additionally, olefin polymerization also can occur as a secondary process. 

The most important chain transfer antioxidants are phenols and aromatic amines (B-81MI11502). They act by donating hydrogen to the peroxy radical with the formation of a stable free radical which does not take part in further chain reactions (equation 4). Two amine antioxidants, used in the rubber industry, are 6-dodecyl-l,2-dihydro-2,2,4-trimethylquinoline (1) and polymeric l,2-dihydro-2,2,4-trimethylquinoline (2). Because of its polymeric structure (2) is thermally stable, has low volatility and is non-blooming, i.e. it shows little tendency to migrate. 

In the 1950s, Semenov and Voevodskii [148] made an attempt to apply the concepts of the branching-chain reaction theory to the kinetics of heterogeneous catalysts. They applied the concept of free valencies migrating over the catalyst surface and of "semi-chemisorbed radicals. But their attempt was criticized (see, for example, ref. 149 where Temkin, using hydrogenation of ethylene on palladium as an example, proved experimentally the inapplicability of the chain theory concepts). 

---