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Preparation of aryl halides

  1. From diazonium salts
     
    Description: 45275.png
     
    Description: 45826.png
  2. By halogenation of arenes or substituted arenes
     
    Description: 45286.png
     
    where X2 = Cl2 or Br2; Lewis acid = FeCl3, AlCl3, etc.

Chemical properties

Nucleophilic aromatic substitution
  1. SNAr mechanism: Consider the following reaction:
     
    Ar – X + ZAr – Z + X
     
    For facile reaction, Ar must contain strongly electron-withdrawing groups at ortho and/or para position to the halogen atom. The reaction involves the formation of carbanion as an intermediaten
     
    Description: 45834.png
     
    Reaction proceeds through carbanion formation as intermediate. The rate of the reaction increases with the increase in the number of electron-withdrawing groups at ortho and para positions, since the cabanion formed would be readily stabilized.
     
    Step 1:
     
    Description: 45842.png
     
    Step 2:
     
    Description: 45851.png
  2. Benzyne mechanism: Unactivated and deactivated aryl halides undergo nucleophilic substitution by benzyne mechanism. In benzyne mechanism, the first step involves elimination, while the second step involves addition of nucleophile.
     
    Description: 45860.png
     
    For example,
     
    Description: 45870.png




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