OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2022: Question 11

✅ Correct Answer

B ( CH₃COOCH₂CH₃ )

💡 Key Knowledge

• C=O Stretch: A very strong, sharp dip around 1680–1750 cm⁻¹ indicates a carbonyl group. Here, a deep absorption peak is clearly visible around 1740 cm⁻¹.
• Absence of O-H: There is no broad absorption peak in the 3200–3600 cm⁻¹ region, ruling out alcohols and carboxylic acids.
• C-H Stretch: Strong absorptions just below 3000 cm⁻¹ indicate aliphatic C-H stretching.

🧠 Exam Technique

• Elimination Strategy: Scan for major diagnostic peaks first. The sharp peak at ~1740 cm⁻¹ instantly confirms a C=O bond, eliminating compound A (which has an O-H stretch instead).
• Check Remaining Options: Options B, C, and D all contain carbonyl or similar functional groups, so look closer at the specific structure and other regions (like C-O stretches around 1000–1300 cm⁻¹ found in esters).

❌ Common Errors

• Confusing the sharp C-H aliphatic stretch (just below 3000 cm⁻¹) with a broad O-H alcohol stretch (which appears at higher wavenumbers, 3200–3600 cm⁻²).
• Failing to check the Data Sheet absorption values, leading to misidentification of the strong 1740 cm⁻¹ peak.