31.1 Halogen Compounds

2026 Syllabus Objectives

By the end of these notes, you should be able to:

  1. Recall the reactions by which halogenoarenes can be produced — specifically, the substitution of an arene (benzene or methylbenzene) with Cl₂ or Br₂ in the presence of an AlCl₃ or AlBr₃ catalyst, giving products such as chlorobenzene, 2-chloromethylbenzene, and 4-chloromethylbenzene.
  2. Explain the difference in reactivity between a halogenoalkane and a halogenoarene, using chloroethane and chlorobenzene as examples.

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1. What Are Halogenoarenes?

A halogenoarene is a molecule that contains a benzene ring (an arene) with a halogen atom — such as chlorine (Cl) or bromine (Br) — directly attached to it.

Think of benzene as a six-carbon ring with a cloud of electrons spread evenly around it (this cloud is called the delocalised π system — basically a shared pool of electrons above and below the ring). When a halogen atom bonds directly onto this ring, the result is a halogenoarene.

Examples:

  • Chlorobenzene — benzene with one Cl atom attached to the ring
  • Bromobenzene — benzene with one Br atom attached to the ring

2. How Are Halogenoarenes Produced?

Halogenoarenes are made through a type of reaction called electrophilic substitution. This means an attacking particle (the electrophile) replaces a hydrogen atom on the benzene ring.

🔑 Electrophile = a particle that is attracted to electrons. It is electron-hungry. It attacks areas that are rich in electrons, like the benzene ring.

What you need for the reaction:

  • An arene (benzene or methylbenzene)
  • Cl₂ gas or Br₂ liquid/gas (the halogen)
  • An anhydrous catalyst: AlCl₃ (aluminium chloride) when using Cl₂, or AlBr₃ (aluminium bromide) when using Br₂

🔑 Anhydrous = completely dry, with no water present. Water would destroy the catalyst, so this condition is essential.

🔑 Catalyst = a substance that speeds up a reaction without being used up itself. AlCl₃ and AlBr₃ are sometimes called halogen carriers because they help carry the halogen into the reaction.

3. Reaction 1 — Benzene to Chlorobenzene

Reactants: Benzene + Cl₂ + AlCl₃ catalyst

Product: Chlorobenzene + HCl

Word equation: Benzene + chlorine → chlorobenzene + hydrogen chloride

Overall equation: C₆H₆ + Cl₂ → C₆H₅Cl + HCl (in the presence of AlCl₃)

How the reaction works — step by step:

Step 1 — Generating the electrophile The AlCl₃ catalyst reacts with Cl₂. This weakens (polarises) the Cl–Cl bond, producing a very reactive Cl⁺ particle (the electrophile).

AlCl₃ + Cl₂ → Cl⁺ + [AlCl₄]⁻

Step 2 — Electrophilic attack on the benzene ring The Cl⁺ electrophile is strongly attracted to the electron-rich benzene ring. It attacks the ring and bonds to one of the carbon atoms. At this moment, the special stable ring structure (aromaticity) is temporarily broken — the delocalised π system is disrupted. An unstable, positively charged intermediate forms.

Step 3 — Restoring the ring (and regenerating the catalyst) The [AlCl₄]⁻ ion (from Step 1) removes the H atom from the carbon that now has both H and Cl attached. When H is removed, the electrons go back into the ring, restoring the stable delocalised π system. This also reforms AlCl₃ (so the catalyst is regenerated — it is not used up). HCl gas is released as a by-product.

Overall result: One H atom on the benzene ring has been substituted (replaced) by one Cl atom → chlorobenzene is formed.

The same reaction happens with Br₂ and an AlBr₃ catalyst, giving bromobenzene.

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