Chapter 23: Aryl Halides

Electrophilic Aromatic Substitution of Aryl Halides

Summary

• Aryl halides are themselves reactive towards electrophilic aromatic substitution but they are less reactive than benzene.
• This is because halides are weak deactivators
• Halides direct subsequent reactions  ortho, para.
• This makes them a little unusual (activators are usually ortho, para-directing, deactivators meta-directing).
• The weak deactivation is due to the electronegativity of the halogen making the intermediate cations less stable than those produced when benzene undergoes substitution:

• The directing effect is due to the resonance stabilisation of the cationic intermediates derived by ortho or  para attack but not by meta attack. For example, the stabilisation during ortho attack is shown below :

• However, aryl halides can undergo many of the same electrophilic aromatic substitution reactions that benzene can (review) including nitration, sulfonation,  further halogenation and Friedel-Crafts alkylation or acylation reactions.

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© Dr. Ian Hunt, Department of Chemistry