Nucleophilic substitution reactions, aliphatic leaving group effects

The Skv I radical chain mechanism lor nucleophilic substitution [I70. as illustrated generally in eqs (2.64a-c), has been absent from our discussions. This mechanism has been shown to occur itt malty displacement reactions of leaving groups front both aromatic and aliphatic substrates. I low ever, in most of the aliphatic eases the substrates have a nitro or nilrophcnyl group (or other effective electron acceptor) in the a-position to the leaving group. The combination of I he nucleophile with the radical from the substrate must also lot m a relatively stable radical-anion in step 2.6lb. which is capable of propagating the chain... 

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