Aromatic chemistry
Benzene and electrophilic substitution
Benzene C₆H₆ has a ring of six sp² carbons with a delocalised π system above and below the plane; all C–C bonds are equal (0.139 nm). Evidence against Kekulé's alternating C=C: equal bond lengths, enthalpy of hydrogenation 152 kJ mol⁻¹ less exothermic than expected (delocalisation stability), and resistance to addition. Benzene undergoes electrophilic substitution, keeping the stable ring: nitration (conc. HNO₃ + conc. H₂SO₄, < 55 °C; electrophile NO₂⁺), halogenation (Br₂ with FeBr₃ or AlCl₃ halogen carrier), Friedel–Crafts alkylation/acylation (RCl or RCOCl with AlCl₃). Methylbenzene reacts faster and directs to the 2- and 4-positions; its side chain is oxidised by hot alkaline KMnO₄ to benzoic acid.
≈ 55 minPhenol
Phenol C₆H₅OH is a weak acid, stronger than water and ethanol but weaker than carboxylic acids: the phenoxide ion is stabilised by delocalisation of the O⁻ charge into the ring. It reacts with Na and NaOH but not with carbonates. A lone pair on oxygen also delocalises into the ring, activating it: phenol decolourises bromine water at room temperature without a catalyst, giving a white precipitate of 2,4,6-tribromophenol, and nitrates with dilute HNO₃. Halogenoarenes (e.g. chlorobenzene) are unreactive to nucleophiles because the halogen lone pair delocalises into the ring, strengthening the C–X bond.
≈ 40 min