Radical-nucleophilic aromatic substitution propagation steps

Equation 2.1). Three propagation steps then follow, including dissociation of the radical anion to an aryl radical and X- (Equation 2.2). In contrast, the corresponding alternative SN1 reaction would lead to the much less stable aryl cation (the empty p-orbital is part of the a-framework, and so cannot be stabilised by the n -electrons). The aryl radical then reacts rapidly with another nucleophile (Y in general or NH2- in this case) to give another radical anion (Equation 2.3) then electron transfer from one radical anion to another reactant molecule (Equation 2.4) initiates another chain. The overall consequence of the three propagation steps is nucleophilic aromatic substitution (Equation 2.5). 

The revealed mechanism of ter Meer reaction is well-founded. It helps us to understand the peculiarities of nucleophilic substitution reactions having the chain ion-radical mechanism and involving the interaction of radicals with anions at the chain propagation steps. It also demonstrates how the knowledge of kinetics and mechanism can be used to find new ways of initiating and optimizing the reactions important for technical practice. The ter Meer reaction turns out to be a reaction having one name and mechanism. This differs from, say, aromatic nitration, which has one name bnt different mechanisms. 

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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