SN-Ar-( NUCLEOPHILIC AROMATIC SUBSTITUTION:

S_N2 (Aromatic) Type: Chloro benzene does not under goes Nucleophilic substitution reaction in ordinary condition due to following reason.

C–X bond in aryl halide is stable due to delocalisation of electrons by resonance. Also (C–X) bond possesses a double bond character like vinyl chloride and is stronger than C–X bond in alkyl halide.

Case-(1): Hence, SN reaction is not possible in benzene nucleus under ordinary conditions. However, under high temperature and pressure, SN reaction is made possible.

 Case-(2): However Nucleophilic substitutions occur in the presence of one or more electron withdrawing (EWG) at ortho and para position. These electron-withdrawing groups must be positioned ortho or para to the leaving group. The greater the number of electron-withdrawing substituents, the easier it will be to carry out the nucleophilic aromatic substitution reactions. 

EXAMPLE:

While electron releasing group (ERG) destabilizes carbanium ion and deactivated.

OTHER EXAMPLES:

SN-Ar-(SUBSTITUTION OF GROUP OTHER THAN HYDROGEN:

Nucleophilic substitution not occurs in aromatic compound but few Nucleophilic substitutions occur in the presence of strong nucleophile. These reactions may be S_N1 -Aromatic unimolecular and S_N2 –Aromatic bimolecular.

S_N1 (Aromatic) Type: The decomposition of diazonium salt in polar medium and formation of different product in the presence of different nucleophile are example of Nucleophilic Aromatic Substitution (S_N1-Ar). The rate of reaction depends upon only concentration of Aryl cation only hence it is unimolecular reaction.

Aryl cation is very reactive and takes up any nucleophile present recombined with elimination of N₂. Therefore reaction is reversible.

Although aryl cation is very unstable , the driving force for it formation of elimination of very stable due to very high bond dissociation of  nitrogen molecule.