Electrophilic substitution reactons of haloarenes

Electrophilic substitution reactons of haloarenes

The halogen atom is ortho and para 

directing and therefore, substitution takes 

place at ortho and para positions giving 

electrophilic substitution reactions at the benzene ring.

In the resonance structures of chlorobenzene the ortho and para positions get negatively charged, i.e. the electron density is relatively more at ortho and para positions. The incoming electrophile is more likely to attack these positions. But, because of steric hindrance at the ortho position, the para-product usually predominates over the ortho product. Also, the halogen atom has -I effect because it is a electron withdrawing group. As a result, it tends to deactivate the benzene ring. Therefore, the electrophilic substitution reactions of chlorobenzene occur slowly and under drastic conditions compared to benzene. Thus,

• Haloarenes undergo electrophilic substitution reactions slowly as compared to benzene.

• Halogen group is ortho and para directing (para-product usually predominates over the ortho product).

Halogenation

Haloarenes react with halogens in the presence of ferric salt as catalyst to give ortho para isomers.

Nitration

Haloarenes with concentrated HNO₃ in the presence of concentrated H₂SO₄.

Sulphonation

Sulphonation occurs when haloarene is treated with concentrated H₂SO₄.

Alkylation and acylation

The alkylation and acylation reaction, known as 'Friedel-Craft reaction', is carried by treating haloarene with alkyl chloride or acyl chloride in the presence of a catalyst like anhydrous aluminium chloride. For example,