30.1 Arenes

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Describe the reactions of benzene and methylbenzene — including halogenation, nitration, Friedel–Crafts alkylation, Friedel–Crafts acylation, complete oxidation of the side-chain, and hydrogenation.
  2. Describe the mechanism of electrophilic substitution, as shown by the formation of bromobenzene and nitrobenzene, and explain why substitution is preferred over addition due to aromatic stabilisation.
  3. Predict whether halogenation occurs in the side-chain or in the ring, depending on the reaction conditions.
  4. Describe how different substituents (–NH₂, –OH, –R, –NO₂, –COOH, –COR) direct incoming groups to specific positions on the ring.

1. Introduction to Arenes

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Arenes are a family of organic compounds that contain a benzene ring — a ring of six carbon atoms joined together in a special way.

The benzene ring is unusually stable. This stability comes from something called delocalisation of electrons. In benzene, the electrons in the ring are not fixed between just two carbon atoms — instead, they are spread out (delocalised) across all six carbon atoms in the ring, forming a "cloud" of electron density above and below the ring. This is called aromatic stabilisation.

Because of this stability, benzene does not react the same way as alkenes. Alkenes tend to undergo addition reactions (where atoms add across a double bond). Benzene, however, strongly prefers substitution reactions (where one atom or group replaces another, leaving the ring intact). This is because breaking the aromatic ring costs a lot of energy and is unfavourable.

2. Reactions of Benzene and Methylbenzene

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2a. Halogenation (Substitution with Cl₂ or Br₂)

In this reaction, a halogen atom (chlorine or bromine) replaces a hydrogen atom on the benzene ring.

Reagents and conditions:

  • Chlorine (Cl₂) with an anhydrous AlCl₃ catalyst, or
  • Bromine (Br₂) with an anhydrous AlBr₃ catalyst

Overall reaction (bromination example):

Benzene + Br₂ → Bromobenzene + HBr (with AlBr₃ catalyst)

The catalyst is essential — without it, the reaction does not happen because benzene is too stable to react on its own.

For methylbenzene: The –CH₃ (methyl) group on the ring donates electron density into the ring, making it more reactive. This means halogenation of methylbenzene occurs at the 2-position or 4-position (the positions next to, or directly opposite, the methyl group), giving a mixture of two products.

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