OCR A-Level Chemistry AS Breadth in chemistry (01), June 2022: Question 19

✅ Correct Answer

A: CH₃COCH₂CH₃ (Butan-2-one)

Mark Awarded: 1 / 1 (AO2.5 - Applying chemical knowledge)

💡 Key Knowledge

• C=O peak: Strong, sharp absorption trough found around 1680–1750 cm⁻¹ (present in ketones, aldehydes, carboxylic acids).
• C-H stretch: Strong absorption just below 3000 cm⁻¹ representing aliphatic C-H bonds.
• O-H stretch (alcohol): Broad absorption around 3200–3600 cm⁻¹ (absent here).
• O-H stretch (acid): Very broad 'broad beard' absorption spanning 2500–3300 cm⁻¹ (absent here).

🧠 Exam Technique

1. Scan the high wavenumber region ( 3200–4000 cm⁻¹ ) first. Look for a broad OH peak. There is none, so instantly rule out alcohols and carboxylic acids (compounds B, C, and D).
2. Check the region around 1700 cm⁻¹ . Notice the very intense, sharp trough characteristic of a carbonyl group ( C=O ).
3. Match remaining options against your findings. Both A and C contain carbonyl groups, but option C also contains an alcohol group ( -OH ) which is missing from the spectrum. Thus, A is the only possible correct structure.

❌ Common Errors

• Failing to check the 3000+ region: Students often rush straight to the fingerprint region or look only at the carbonyl trough without confirming the absence of O-H groups.
• Confusing compound types: Confusing the sharp ketone C=O stretch with the very broad carboxylic acid O-H stretch overlapping the C-H region.