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,

Problems

4. Write the possible isomers of the aromatic compound having molecular formula C₇H₇Cl. Which of these have weakest C-Cl bond?

Solution

The possible isomers of the aromatic compound having molecular formula C₇H₇Cl are benzyl chloride and ortho chlorotoluene and para chlorotoluene.

Benzyl chloride has the weaker bond because the chlorine atom is at the substituted side chain of the alkyl group. In o- and p- chlorotoluene the C-Cl bond is stronger because it is directly part of the benzene ring. This makes the C-Cl bond less polar due to electron releasing resonance effect. p-chloro toluene is more stable than o-chloro toluene.

5. Which of the following is an aryl halide?

Solution

(c) is chlorobenzene, an aryl halide because the chlorine is directly attached to the benzene ring. (a), is benzyl chloride which is a substituted haloarene, while (b) is chlorocyclohexane which is a haloalkane.