Reactions of Free Radicals

For many years organic chemists considered free radical reactions to have limited applications, and to be of little interest outside some fields of industrial chemistry. This view has changed markedly, and important examples of substitution, addition and elimination reactions proceeding by way of radical intermediates have been developed and used in the synthesis of complex molecules.
Since most free radical intermediates are very reactive and have short lifetimes, all the steps in a practical chain reaction sequence must be fast compared with possible competing reactions. This means that atom abstractions and radical additions should be exothermic, or only mildly endothermic. Both C–H and C–X abstractions satisfy this requirement, but the strong O–H bond does not. Also addition to C=C is energetically more favorable than addition to C=O; and when radicals add to certain C=O functions, they bond to the oxygen, not the carbon. Another characteristic of radical reaction sequences is fragmentation with expulsion of a small stable molecule, such as CO₂, CO & N₂.