chemistry world: nucleophilllic aromatic substitution

A nucleophilic aromatic substitution is a substitution reaction in organic chemistry in which the nucleophile displaces a goodleaving group, such as a halide, on an aromatic ring. There are 6 nucleophilic substitution mechanisms encountered with aromatic systems:

the S_NAr (addition-elimination) mechanism

Summary
- The generally accepted mechanism for nucleophilic aromatic substitution in nitro-substituted aryl halides is shown by example below:
- Attack of the strong nucleophile on the halogen substituted aromatic carbon forming an anionic intermediate.
- Loss of the leaving group, the halide ion restores the aromaticity.
- Kinetics of the reaction are observed to be second order.
- The addition step is the rate determining step (loss of aromaticity).
- Nucleophilic substitution, and therefore reaction rate, is facilitated by the presence of a strong electron withdrawing group (esp. NO₂) ortho or para to the site of substitution, which stabilize the cyclohexadienyl anion through resonance.
the aromatic SN1 mechanism encountered with diazonium salts

The wide utility of aryl diazonium ions as synthetic intermediates results from

the excellence of N2 as a leaving group. There are several general mechanisms by

which substitution can occur.One involves unimolecular thermal decomposition of

the diazonium ion, followed by capture of the resulting aryl cation by a nucleophile.

The phenyl cation is very unstable and therefore Either the solvent or an anion can act as the nucleophile. to react with it and completes nucleophillic aromatic substitution.

the benzyne mechanism

Elimination-Addition Mechanism: Benzyne

Summary:

This pathway is followed when the nucleophile is an exceptionally strong base (e.g. amide ion, NH₂^-) and the absence of the strong electron withdrawing groups: