Approaches to Radical Mechanisms

Because radicals are in minute concentration, the usual radical mechanisms involve a radical colliding with an even electron molecule in a chain process. A common error is to have a termination step instead of creating a regenerating loop. The three common radical path combinations, Sh2, Adn2, and radical polymerization, all have propagation steps in which radicals collide with an even electron species, creating a new radical. 

Usually, the last propagation step creates the starting radical for the first propagation step, thus making a loop. 

Initiators are added to the reaction in trace amounts and are just to get the radical chain loop going. The loop often goes for thousands of cycles before termination. Do not include this trace initiator in your propagation steps. Make your radical loop out of reactants present in good amounts. 

balancing the reaction and numbering the carbons of the reactant and product will give us more information about the reaction. Since carbons 1,5, and 6 in the reactant were functionalized in the reactant, the most logical numbering of the product is  

In figuring out what bonds have changed, it may be helpful to draw in all the hydrogens. Changes are carbon 6 has gone from a methylene to a methyl, so a new C-H bond was formed there the bromine atom has been lost from carbon 1 finally, a new single bond between carbon 1 and carbon 5 has formed. 

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The radical chain dehalogenation with Bu3Sn-H is a very useful reaction. The ease of dehalogenation follows the C-X bond strength I > Br > Cl > F the weakest C-X bond is preferred. The initiator for this reaction is commonly AIBN. An example mechanism involving this reaction is shown in Section 11.7, Approaches to Radical Mechanisms. 

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