Free Radical or Chain Mechanisms

Some of the elementary concepts of free radical mechanisms were presented in Chapter 1. Reactions following free radical mechanisms have reactive intermediates containing unpaired electrons which are produced by homolytic cleavage of covalent bonds. A method of detecting free radicals was published in 1929, and it is based on the fact that metals such as lead react with free radicals. When heated, tetramethyl lead decomposes. 

A lead mirror is produced in a heated glass tube when tetramethyl lead is passed through it. Also, the lead mirror in a cool portion of the tube can be 

However, if the flow system is arranged so that a long tube is used and considerable distance separates the point where the CHa radicals are generated and they react with the cool lead mirror, the reaction is hindered because of radical recombination. 

Perhaps the best known example of a chain process, certainly it is the classic case, is the reaction 

This reaction was studied by Bodenstein and Lind nearly 90 years ago, and the rate law found was written as 

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