Arenes

Chemistry A2 · Hydrocarbons. 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

Benzene and methylbenzene are arenes. The delocalised π system makes the ring very stable, so arenes react by electrophilic substitution, not addition. Addition would destroy the delocalised system; substitution keeps it.

In the mechanism a strong electrophile (Br⁺ or NO₂⁺) accepts a pair of π electrons from the ring. This gives a positive intermediate in which the delocalisation is partly broken. The intermediate then loses H⁺ and the full delocalised ring forms again.

In methylbenzene the conditions decide where a halogen goes: a catalyst gives substitution in the ring, ultraviolet light gives free-radical substitution in the side chain. A group already on the ring directs the next group to particular positions.

Rules to remember

Halogenation of the ring: Cl₂ or Br₂ with AlCl₃ or AlBr₃, e.g. C₆H₆ + Br₂ → C₆H₅Br + HBr. The catalyst makes the electrophile: Br₂ + AlBr₃ → Br⁺ + AlBr₄⁻.
Nitration: concentrated HNO₃ and concentrated H₂SO₄, 25 °C to 60 °C. HNO₃ + H₂SO₄ → NO₂⁺ + HSO₄⁻ + H₂O.
Friedel–Crafts, AlCl₃ and heat: CH₃Cl gives methylbenzene (alkylation); CH₃COCl gives phenylethanone, C₆H₅COCH₃ (acylation). HCl also forms.
Side-chain oxidation: hot alkaline KMnO₄, then dilute acid, turns any alkyl side chain into –COOH (benzoic acid).
Hydrogenation: H₂ with Pt or Ni, heat, turns the benzene ring into a cyclohexane ring (3 mol of H₂ per ring).
Halogen + ultraviolet light, no catalyst: substitution in the side chain, e.g. C₆H₅CH₂Cl.
Directing effects: –NH₂, –OH and –R (alkyl) direct to positions 2, 4 and 6; –NO₂, –COOH and –COR direct to positions 3 and 5.

Common mistake

Doing the steps of a synthesis in the wrong order. The group already on the ring decides the position of the next group, so decide whether you need a 2,4-director or a 3-director on the ring before you substitute.

Worked example

Methylbenzene is to be converted into 4-nitrobenzoic acid in two steps. In which order should nitration and oxidation of the side chain be done?

In the product, –NO₂ is at position 4 relative to the carbon that carried the methyl group.
–CH₃ directs to positions 2 and 4, but –COOH directs to position 3.
So nitrate while the group is still –CH₃ (concentrated HNO₃ and H₂SO₄), and separate the 4-nitromethylbenzene from the 2- isomer.
Then oxidise the methyl group with hot alkaline KMnO₄, followed by dilute acid.

Answer: Nitrate first, then oxidise. Oxidising first would give benzoic acid, which nitrates at position 3.

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Watch: 4 | Hybridization | Alkenes | Structure of Benzene

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