The Configuration of Free Radicals

 Since the difference in energy between a planar radical and a rapidly inverting pyramidal radical is small, radicals generated at chiral centers generally lead to racemic products. However, unlike carbocation intermediates, which prefer to be planar, radicals tolerate being restricted to a pyramidal configuration. The following illustration shows the decomposition of a bicyclic bridgehead acyl peroxide. Initial formation of a carboxyl radical is followed by loss of carbon dioxide to give a pyramidal bridgehead radical. This radical abstracts a chlorine atom from the solvent, yielding the bridgehead chloride as the major product. Although this is a 3º-alkyl halide, it does not undergo S_N1 solvolysis reactions because of the strain imposed on the carbocation intermediate by its pyramidal confinement.

The concurrent formation of ester and dimeric cycloalkane products from acyl peroxides is common, and reflects a cage effect in homolysis reactions. When a pair of radicals is formed by homolysis, they are briefly held in proximity by the surrounding solvent molecules (the cage). Rapid decomposition to other radicals may occur, but until one or both of these radicals escape the solvent cage a significant degree of coupling (recombination) may occur. A general description of the cage effect will be displayed above by clicking on the diagram.
Cage recombination of radicals may be sufficiently rapid to preserve the configuration of the generating species. An example will be shown above in the above diagram a second time. Ester formation is clearly a cage product, whereas 2-chloro-1-phenylpropane comes largely from radicals that have escaped the cage and lost configurational identity. The chiral centers in these compounds are marked by asterisks.