General Aspects of Radical Chain Reactions

Organosilanes in Radical Chemistry C. Chatgilialoglu 2004 John Wiley Sons, Ltd ISBN 0-471-49870-X 

Generally, radical chain reactions are carried out in nonpolar solvents although strong solvent effects on propagation steps are rare. Apart from the polarity, a much more important criterion for the solvent choice is the solvent s inertness towards the chain propagating radicals involved. 

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The majority of radical reactions of interest to synthetic chemists are chain processes [3,4]. For those readers who are not familiar with this chemistry, some general aspects of radical chain reactions are discussed here. Scheme 1 represents the simple addition of a thiol to a carbon-carbon double bond as an example of a chain process. Thus, RS radicals, generated by some initiation processes, undergo a series of propagation steps generating fresh radicals. The chain reactions are terminated by radical combination or disproportionation. In order to have an efficient chain process, the rate of chain transfer steps must be greater than that of chain termination steps. Since the termination rate constants in the liquid phase are controlled by diffusion (i.e. 10 M s ) and radical... 

O. Ito, in Z. B. Alfassi (ed.), Free Radical Polymerization and Chain Reactions in General Aspects of the Chemistry of Radicals, John Wiley and Sons, Inc., New York, 1999, p. 209. 

Radical reactions are looked upon by many chemists as uncontrollable processes which yield intractable mixtures of products. Even so, the ingenuity of synthetic chemists has resulted in many useful synthetic schemes via radical pathways. These include chain lengthening, ring formation and ring expansion processes. This synthetic work has seen dramatic steps forward over the last few decades and has been the subject of several general reviews recently664"669. Other notable reviews have also been written10,11,61,670 which cover more specific aspects of radical reactions and the reader is referred to all these sources for further and in-depth information. 

The fact that the typical furyl radicals in Scheme 6.6 are unable to propagate in the classical mode of free radical polymerization because of their resonance stabilization, has profound consequences in terms of the reactivity of furan monomers in this type of polymerization and the role of furan derivatives as possible perturbing agents in free radical chain reactions in general. These specific aspects will be illustrated below. 

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