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Halogenation of Benzene

halogenation of arenes

Reaction type: Electrophilic Aromatic Substitution

Summary.

  • Overall transformation :  Ar-H to Ar-X
  • Reagent : normally the halogen (e.g. Br2) with a Lewis acid catalyst
  • The active catalyst is not Fe (0) but the FeX3 formed by reaction of Fe with X2
  • Electrophilic species :  the halonium ion (i.e. X +) formed by the removal of a halide ion by the Lewis acid catalyst
  • Restricted to Cl2 and Br2.  I- or F- are usually introduced using alternative methods
MECHANISM FOR HALOGENATION OF BENZENE
Step 1:
The bromine reacts with the Lewis acid to form a complex that makes the bromine more electrophilic.
bromination of benzene
Step 2:
The p electrons of the aromatic C=C act as a nucleophile, attacking the electrophilic Br, and displacing iron tetrabromide. This step destroys the aromaticity giving the cyclohexadienyl cation intermediate.
Step 3:
Removal of the proton from the sp3 C bearing the bromo- group reforms the C=C and the aromatic system, generating HBr and regenerating the active catalyst.
 

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