Halogen Compounds (A Level)

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Cambridge International AS & A Level Chemistry (9701)

Halogen Compounds (A Level)

The big idea: This builds on the AS chapter Halogen Compounds. A halogen on a benzene ring behaves very differently from one on an alkyl chain: the ring makes the C–X bond stronger and keeps nucleophiles away, so halogenoarenes resist the substitution reactions that halogenoalkanes undergo easily.

Summary — What This Chapter Covers

Halogenoarenes are made by electrophilic substitution of an arene with Cl₂ or Br₂ and a halogen carrier.
Methylbenzene gives a mixture of 2-chloromethylbenzene and 4-chloromethylbenzene.
Chloroethane is hydrolysed by aqueous OH⁻; chlorobenzene is not, under the same conditions.
The difference comes from overlap of a halogen lone pair with the ring's π system.

1. Halogen Compounds

31.1

Making halogenoarenes

Arenes react with Cl₂ or Br₂ in the presence of an anhydrous halogen carrier (AlCl₃ or AlBr₃), at room temperature and without UV light, by electrophilic substitution.

Halogenation of the ring C₆H₆ + Cl₂ → C₆H₅Cl + HCl (chlorobenzene)
C₆H₅CH₃ + Cl₂ → ClC₆H₄CH₃ + HCl (2-chloromethylbenzene and 4-chloromethylbenzene)

The methyl group directs the incoming chlorine to the 2- and 4-positions, so methylbenzene gives a mixture of two isomers.

Practice Question

State the reagents and conditions to make bromobenzene from benzene, and write the equation.

Halogenoalkane vs halogenoarene

	Chloroethane, CH₃CH₂Cl	Chlorobenzene, C₆H₅Cl
Warm with NaOH(aq)	Nucleophilic substitution: CH₃CH₂Cl + OH⁻ → CH₃CH₂OH + Cl⁻	No reaction under these conditions
Warm, then acidify with HNO₃ and add AgNO₃(aq)	White precipitate of AgCl	No precipitate

Why chlorobenzene is so unreactive

A lone pair on the Cl atom (in a p orbital) overlaps with the delocalised π system of the ring.
This gives the C–Cl bond partial double-bond character, so it is stronger and harder to break than the C–Cl bond in chloroethane.
The electron-rich π system repels an approaching nucleophile such as OH⁻.

Practice Question

Memorise

Making halogenoarenes

₂ ₂ ₃ ₃

Methylbenzene

Stronger C–Cl

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Halogen Compounds (A Level)