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.
  
  
  
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• 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:

• Nucleophilic substitution can lead to substitution on either
• the same carbon that bore the leaving group (see addition mechanism above)
• or on an adjacent carbon (see addition mechanism below)

 
• This is most readily apparent when the benzyne is substituted:

•