Nucleophilic Aromatic Substitution
Nucleophilic aromatic substitution (also called addition-elimination), requires an electron withdrawing substituent to be present on the ring ortho or para to the halide being substituted. The reaction is difficult to achieve with aryl halides for which halogen is the lone ring substituent. If a strongly electron-withdrawing substituent is present, however, ortho or para to the halogen, the carbanion intermediate will be more stable. A substituent such as nitro, while deactivating electrophilic aromatic substitution (which forms a carbocation intermediate), activates ortho and para positions, if they contain halogen, for nucleophilic aromatic substitution. Basicity of the nucleophile also facilitates the reaction.