Halogen compounds

Chemistry A2 · Halogen compounds. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

A halogenoarene has a halogen atom joined directly to a benzene ring, as in chlorobenzene, C₆H₅Cl. It is made by electrophilic substitution: the arene reacts with Cl₂ or Br₂ in the presence of AlCl₃ or AlBr₃.

Halogenoarenes are much less reactive than halogenoalkanes towards nucleophiles. Chloroethane is hydrolysed by hot NaOH(aq), but chlorobenzene is not. The reason is that a lone pair on the chlorine atom overlaps with the delocalised π system of the ring. This gives the C–Cl bond some double-bond character, so it is shorter and stronger and harder to break.

The electron-rich ring also repels nucleophiles such as OH⁻.

Rules to remember

C₆H₆ + Cl₂ → C₆H₅Cl + HCl, with AlCl₃ catalyst.
Catalyst role: Cl₂ + AlCl₃ → Cl⁺ + AlCl₄⁻; AlCl₃ accepts a lone pair from chlorine.
Methylbenzene + Cl₂ with AlCl₃ gives 2-chloromethylbenzene and 4-chloromethylbenzene.
Chloroethane: CH₃CH₂Cl + NaOH → CH₃CH₂OH + NaCl (nucleophilic substitution, heat). Chlorobenzene: no reaction under these conditions.
C–Cl bond in chlorobenzene: shorter and stronger than in chloroethane (partial double-bond character).
Test: warm with NaOH(aq), acidify with dilute HNO₃, add AgNO₃(aq). A white precipitate of AgCl forms from a chloroalkane only.
A halogen on a side chain (e.g. C₆H₅CH₂Cl) behaves like a halogenoalkane and is hydrolysed.

Common mistake

Saying chlorobenzene is unreactive because the C–Cl bond is not polar. The bond is still polar; it is unreactive because the chlorine lone pair is delocalised into the ring, which strengthens the C–Cl bond.

Worked example

Chloroethane heated with ammonia in ethanol under pressure gives ethylamine. Would chlorobenzene give phenylamine under the same conditions?

The reaction of chloroethane is nucleophilic substitution: NH₃ attacks the δ+ carbon and the C–Cl bond breaks.
In chlorobenzene a chlorine lone pair overlaps with the ring π system, so the C–Cl bond is stronger and has partial double-bond character.
The delocalised electrons of the ring also repel the lone pair of the NH₃ nucleophile.

Answer: No. Chlorobenzene does not undergo nucleophilic substitution under these conditions, so no phenylamine forms.

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Watch: 6 | Electrophilic Substitution of Benzene | Halogenation & Nitration

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