Electrophilic Substitution Reactions
In this chapter, the authors study the electrophilic substitution reactions that occur in aromatic systems and look at those that occur in aliphatic systems. The best leaving groups in an electrophilic substitution reaction will be weak Lewis acids. One of the commonest electrophilic substitution reactions of benzene is the simple nitration of the aromatic ring. The substitution of an unsubstituted benzene ring by an electrophile to give a monosubstituted aromatic system proceeds by the arenium ion mechanism, which involves the Wheland intermediate. By studying the effects of inductive and mesomeric groups upon the Wheland intermediate, it is possible to predict the orientation of substitution that the second incoming electrophile would adopt. Another common aromatic electrophilic substitution reaction is chlorination. The mechanisms that occur in aliphatic electrophilic substitution reactions are less well defined than those that occur in aliphatic nucleophilic substitution and aromatic electrophilic reactions.