A level Chemistry · Edexcel 9CH0 · Topic 18A
Arenes - benzene compounds
Key points
- Kekulé proposed alternating C=C and C–C bonds; evidence against: all C–C bonds are the same length, the enthalpy of hydrogenation is less exothermic than expected (about −208 not −360 kJ mol⁻¹), and benzene resists addition with bromine.
- The delocalised model: each carbon has a p orbital that overlaps sideways to form a ring of π electrons above and below the plane.
- Electrophilic substitution mechanism: the electrophile attacks the ring, an intermediate forms, and H⁺ is lost to restore the ring.
- Reactions: nitration (conc. HNO₃ + conc. H₂SO₄, 55 °C; electrophile NO₂⁺); halogenation (Br₂ with FeBr₃ or AlCl₃ catalyst); Friedel–Crafts alkylation and acylation (AlCl₃ catalyst).
- Benzene burns with a smoky flame (high C:H ratio).
- Phenol reacts with bromine water without a catalyst to give a white precipitate of 2,4,6-tribromophenol, because the O lone pair increases electron density in the ring; it is a weak acid.
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Benzene, C₆H₆, looks as if it should behave like a very unsaturated alkene, but it does not. Understanding why it is so stable explains why it reacts by substitution rather than addition, and why groups such as –OH on the ring change its…
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