SN-Ar-:Meisenheimer complex Intermediate:

S_N2-Ar Mechanism:

Nucleophilic aromatic substitution takes place by a two-step reaction known as an S_N2-Ar (S_N2 –Aromatic bimolecular.) reaction. The S_N2-Ar mechanism and involves a resonance-stabilized anionic intermediate called the Meisenheimer complex.

ILLUSTRATIVE EXAMPLE:

Meisenheimer complex Intermediate:

Step-(1):  An Addition

Step-(2): An Elimination:

Thus, the overall mechanism is an Addition–Elimination Mechanism. the Meisenheimer complex is stable enough to form only if an electron-withdrawing group therein can stabilize the negative charge by resonance.

Characteristics of S_N2-Ar reaction:

(1) It is a Nucleophilic aromatic Substitution reaction by Addition–Elimination Mechanism.

(2) Rate of this reaction depends upon concentration of halide and nucleophile both

(3) Order of reaction is (2) Bimolecular reaction.

(4) Carbanion is intermediate. Hence rearrangement not possible.

(5) Rate of reaction increase on increasing number of EWG which stabilized Meisenheimer complex.

(6) Rate of fluorine derivative (fluoro benzene) is most reactive because of high (–I) inductive effect

.

(7) Only those halogen replace which are ortho or para position with compared to nitro group.

Other Examples:

 (3) Substitution of unactivated hydrogen: (Benzyne Mechanism):