// CHEMISTRY L5 — AROMATIC CHEMISTRY
The ring that refuses to break.
Benzene's six carbons share their electrons in a doughnut of delocalised charge — so stable that benzene reacts by substitution, keeping its ring, where a normal alkene would add across the double bond. Flip between the two ways of drawing it, then run the classic reactions.
// FIG. 01 — TWO MODELS OF BENZENE (C₆H₆)
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// FIG. 02 — ELECTROPHILIC SUBSTITUTION BENCH
General reaction: C₆H₆ + E⁺ → C₆H₅E + H⁺ (the ring survives — one H is swapped for E)
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Every reaction here follows the same three beats: (1) a catalyst generates a strong electrophile E⁺; (2) the ring's π electrons attack E⁺, briefly breaking aromaticity to form an unstable intermediate; (3) the intermediate loses H⁺, restoring the stable ring.
// WHY SUBSTITUTION, NOT ADDITION?
An alkene adds across its C=C double bond because that swaps a weak π bond for two strong σ bonds. Benzene could add too — but doing so would destroy the delocalised ring and lose its extra stability (about 150 kJ mol⁻¹, measured from enthalpies of hydrogenation). Substitution keeps the ring intact, so it's the favoured path. That's the single big idea of aromatic chemistry.
// THE EVIDENCE FOR DELOCALISATION
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// REFERENCES & FURTHER READING
Cross-check against your own exam board's specification (AQA, Edexcel, OCR, SQA).