Chapter 12 :  Reactions of Arenes. Electrophilic Aromatic Substitution

Halogenation of Benzene

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.
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.

© Dr. Ian Hunt, Department of Chemistry