Arenes — practice questions

Step 1: Calculate the theoretical enthalpy of hydrogenation for a structure with three double bonds (Kekulé structure). Expected ΔH = 3 × (Enthalpy of hydrogenation of cyclohexene) Expected ΔH = $3 \times (-120 \text{ kJ mol}^{-1}) = -360 \text{ kJ mol}^{-1}$

Step 2: Compare the theoretical value with the experimental value. Experimental ΔH for benzene = -208 kJ mol^{-1}

Step 3: Calculate the difference, which represents the delocalisation energy. Delocalisation Energy = Theoretical ΔH - Experimental ΔH Delocalisation Energy = $(-360 \text{ kJ mol}^{-1}) - (-208 \text{ kJ mol}^{-1}) = -152 \text{ kJ mol}^{-1}$

This means that benzene is 152 kJ mol^{-1} more stable than the hypothetical cyclohexa-1,3,5-triene due to the delocalisation of its pi electrons.

The sulfuric acid is a stronger acid than nitric acid. It protonates the nitric acid, which then eliminates water to form the nitronium ion. Step 1: H₂SO₄ + HNO₃ ⇌ H₂NO₃⁺ + HSO₄⁻ Step 2: H₂NO₃⁺ → NO₂⁺ + H₂O Overall: 2H₂SO₄ + HNO₃ → NO₂⁺ + 2HSO₄⁻ + H₃O⁺ (often simplified as shown above for clarity).

This synthesis is achieved via a Friedel-Crafts acylation reaction.

• Reaction Type: Friedel-Crafts Acylation
• Reagents: Ethanoyl chloride (CH₃COCl) and anhydrous aluminium chloride (AlCl₃) catalyst.
• Conditions: Heat under reflux (approx. 60°C) in a non-aqueous solvent (e.g., dry ether).
• Equation: C₆H₆ + CH₃COCl --(AlCl₃, heat)--> C₆H₅COCH₃ + HCl
• Product: Phenylethanone (also known as acetophenone).