OCR A-Level Chemistry Synthesis and analytical techniques (02), November 2021: Question 14

✅ Correct Answer: C (Only 2 and 3)

The correct option is C, meaning statements 2 and 3 provide evidence specifically supporting the delocalised model rather than the Kekulé model.

💡 Key Knowledge: Analysing the Statements

• Statement 1 (Incorrect): The enthalpy change of hydrogenation is actually less exothermic (more stable) than expected for a Kekulé structure. A hypothetical cyclohexatriene has an expected hydrogenation value of -360 kJ mol⁻¹. Real benzene is -208 kJ mol⁻¹, meaning it is 152 kJ mol⁻¹ more stable (less exothermic).
• Statement 2 (Correct): Kekulé's localized double bonds would readily undergo electrophilic addition with bromine (like alkenes) without a catalyst. Benzene requires a halogen carrier because its delocalised ring is stable and undergoes substitution instead.
• Statement 3 (Correct): Kekulé predicted alternating single (0.154 nm) and double (0.134 nm) bond lengths. X-ray diffraction shows all carbon-carbon bonds in benzene are identical in length (intermediate at ~0.140 nm).

🧠 Exam Technique

When tackling multi-statement questions:

• Evaluate each statement independently before looking at the letter options.
• Watch out for reversed terminology (e.g., "more exothermic" vs "less exothermic").
• Eliminate options quickly once you find a definitive true or false statement.

❌ Common Errors

• Misinterpreting Enthalpy Data: Many students confuse "more stable" with "more exothermic". Because real benzene releases less heat when hydrogenated than a triene model, it is more stable (delocalisation energy lowers its overall energy state).
• Confusing Reaction Types: Forgetting that substitution vs addition is a hallmark of delocalisation stability.