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 min