Substitution: Nucleophilic Aromatic
Nucleophilic aromatic substitution occurs when a nucleophilic Lewis base displaces a nucleofuge from an sp2 aromatic carbon. Aromatic systems are generally electron rich moieties in which the Ļ-system inhibits the substitution. (The electron rich nucleophile has to pierce the electron rich pi-system to reach the centre at which the nucleophilic substitution is to occur.) However, if the Nfg function —for example, a halogen— has electron withdrawing functions ortho or para to the Nfg, the reaction proceeds far more readily. Many halogenated nitrogen heterocycles are susceptible to SNAr attack.