OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2023: Question 11

✅ Correct Answer: C

Compound C contains a phenol group (aromatic O–H) and an aldehyde group (–CHO), both of which account for the key diagnostic absorbances seen in the spectrum.

💡 Key Knowledge (Diagnostic Peaks)

• Broad peak around 3200–3600 cm⁻¹: Indicates hydrogen-bonded O–H stretching (present in phenols/alcohols/carboxylic acids).
• Sharp, deep peak around 1650–1750 cm⁻¹: Indicates C=O stretching (carbonyl group).
• Peaks just below 3000 cm⁻¹: C–H aliphatic stretching.

🧠 Exam Technique

Process of elimination is your best tool here:

• Look at the very broad absorption stretching from ~3200 cm⁻¹ downwards. This rules out options lacking an O–H group.
• Check for the sharp carbonyl (C=O) peak around 1700 cm⁻¹. Compound A has a carboxylic acid (broad O–H from 2500–3300 cm⁻¹ which is not present here), whereas C has an aldehyde C=O and a clean phenolic O–H.

❌ Common Errors

• Confusing O–H types: Mistaking the very wide carboxylic acid O–H absorption envelope (which spreads heavily across 2500–3300 cm⁻¹) for a standard alcohol/phenol O–H peak.
• Ignoring the fingerprint region: Spending too much time below 1500 cm⁻¹ instead of focusing on the diagnostic functional group region above 1500 cm⁻¹.