OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2023: Question 6

✅ Correct Answer: B

The carbon-carbon bond lengths are all the same.

The Kekulé model features alternating single (154 pm) and double (134 pm) carbon-carbon bonds, meaning bond lengths should vary. X-ray diffraction shows all C-C bonds in benzene are identical in length (approx. 139 pm), perfectly supporting the delocalised model where all six electrons are shared equally.

💡 Key Knowledge

• Kekulé model: Localised single and double bonds with alternating lengths.
• Delocalised model: Overlapping adjacent p-orbitals above and below the plane of the carbon rings creating a cloud of electron density.
• Hydrogenation evidence: Enthalpy of hydrogenation is 152 kJ mol⁻¹ less exothermic than the theoretical Kekulé model predicts, proving benzene is much more thermodynamically stable.

🧠 Exam Technique

Read the negative constraint carefully: "and not the Kekulé model". Ask yourself: Which of these properties does a localized ring fail to explain? Alternating structures would have alternating bond lengths, so uniform bond lengths actively disprove Kekulé.

❌ Common Errors

• Choosing C: Students often misremember enthalpy changes. Benzene is less exothermic than expected (more stable), not more exothermic.
• Choosing D: Benzene is actually less reactive than alkenes because adding across the ring would destroy the stable delocalised ring system.
• Choosing A: Sigma bonds form the initial single bonds in the ring skeleton; the pi-system is formed from the overlap of p-orbitals, not sigma bonds.