Flashcards · 7405 · A Level Topic 28

Aromatic chemistry (A Level) flashcards

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Bonding in benzene: A planar ring of six carbons, bond angles 120°. Each C has one p electron; the p orbitals overlap to form a ring of six delocalised electrons above and below the plane. All C–C bonds are the same length, between single and double.
How does hydrogenation data show benzene’s extra stability?: Predicted ΔH for cyclohexa-1,3,5-triene = 3 × ΔH(cyclohexene). Benzene’s measured value is less exothermic, so benzene is more stable, because of delocalisation.
Why does benzene undergo substitution rather than addition?: Addition would break the delocalised ring and lose its extra stability. Substitution keeps the delocalised ring.
Nitration of benzene: reagents and electrophile: Concentrated HNO₃ and concentrated H₂SO₄. Electrophile NO₂⁺: HNO₃ + 2H₂SO₄ → NO₂⁺ + H₃O⁺ + 2HSO₄⁻
Friedel–Crafts acylation: reagents and electrophile: An acyl chloride with an AlCl₃ catalyst (dry). Electrophile RCO⁺: RCOCl + AlCl₃ → RCO⁺ + AlCl₄⁻. The product is a phenyl ketone.
Why is nitration of benzene important?: It is a step in the manufacture of explosives, and reducing nitrobenzene gives aromatic amines used to make dyes.
Product of benzene with propanoyl chloride and AlCl₃: 1-phenylpropan-1-one, C₆H₅COCH₂CH₃, plus HCl.