OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2024: Question 8

✅ Correct Answer: C

CCl₂F₂ → •Cl + •CClClF₂

UV light causes homolytic fission of a weak C-Cl bond to form a chlorine free radical ( •Cl ) and a carbon-containing radical.

💡 Key Knowledge

• Initiation: Formation of free radicals, usually by UV light providing energy for homolytic bond fission.
• Bond Enthalpies: C-Cl bonds have a lower bond enthalpy than C-F bonds, making C-Cl bonds much easier to break homolytically under UV radiation in the stratosphere.
• Radical Notation: Always look for the unpaired electron dot ( • ) on generated species.

🧠 Exam Technique

• Count your atoms carefully across the reaction arrow to ensure mass balance.
• Eliminate options that break strong unreactive bonds first (like C-F).
• Remember that initiation starts with a stable molecule (no radicals on the reactant side) and produces two radicals.

❌ Common Errors

• Choosing option B (breaking a C-F bond) — C-F bonds require much higher energy UV radiation to break than is available in the stratosphere.
• Choosing option A — falsely assuming a carbon radical ( •C ) forms directly from the backbone.
• Failing to check stoichiometry or missing radical dots.