Nucleophilic aromatic substitution

A nucleophilic aromatic substitution (S_NAr) is a substitution reaction in organic chemistry in which the nucleophile displaces a good leaving group, such as a halide, on an aromatic ring. Aromatic rings are usually nucleophilic, but some aromatic compounds do undergo nucleophilic substitution. Just as normally nucleophilic alkenes can be made to undergo conjugate substitution if they carry electron-withdrawing substituents, so normally nucleophilic aromatic rings also become electrophilic if they have the right substituents.

This reaction differs from a common S_N2 reaction, because it happens at a trigonal carbon atom (sp² hybridization). The mechanism of S_N2 reaction does not occur due to steric hindrance of the benzene ring. In order to attack the C atom, the nucleophile must approach in line with the C-LG (leaving group) bond from the back, where the benzene ring lies. It follows the general rule for which S_N2 reactions occur only at a tetrahedral carbon atom.

The S_N1 mechanism is possible but very unfavourable unless the leaving group is an exceptionally good one. It would involve the unaided loss of the leaving group and the formation of an aryl cation. In the S_N1 reactions all the cations employed as intermediates were planar with an empty p orbital. This cation is planar but the p orbital is full (it is part of the aromatic ring) and the empty orbital is an sp² orbital outside the ring.^[1]