Free radical dissociation-recombination mechanism

Scheme 2.24 Free radical dissociation-recombination mechanism...

The migrating group migrates with nearly complete retention of configuration. Thus, the reaction is intramolecular and concerted. However, there is evidence which supports the free radical dissociation-recombination mechanism (Scheme 3.27). 

Baechler and coworkers204, have also studied the kinetics of the thermal isomerization of allylic sulfoxides and suggested a dissociative free radical mechanism. This process, depicted in equation 58, would account for the positive activation entropy, dramatic rate acceleration upon substitution at the a-allylic position, and relative insensitivity to changes in solvent polarity. Such a homolytic dissociative recombination process is also compatible with a similar study by Kwart and Benko204b employing heavy-atom kinetic isotope effects. 

Secondary rearrangements apparent isomerizations through radical recombination reactions. In the rearrangement reactions considered so far, the isomerization step is the primary photochemical process, except when a biradical is formed as an intermediate for in that case the primary photochemical process is really a dissociation, even though the fragments cannot separate. There are however cases of overall isomerizations which result from the recombinations of separated free radicals formed through a process of photodissociation. The photo-Fries reaction is an important example of this mechanism, and is illustrated in Figure 4.43. 

In high-intendty ultrasonics in liquids, the local absorption of elastic energy is closely related to the phenomenon of cavitation. In depolymerization reactions free radicals are usuaUy formed. In this care the shear stresses not only dissociate the bonds in the main chain, but also separate the radicals formed, preventing in this way their recombination. An extensive review of the experimental woik on the mechanical degradation of polymers has been given by Porter and Casale (77). 

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