AQA A-Level Chemistry Paper 3, 2017: Question 12

✅ Correct Answer

C — butanal

The spectrum features a very prominent, sharp absorption peak at approximately 1725 cm⁻¹, which confirms a C=O carbonyl group. The lack of any broad O–H absorption band rules out carboxylic acids and alcohols.

🧠 Exam Technique: 2-Step Elimination

1. Step 1: Look for C=O (1680–1750 cm⁻¹)
   A very deep, narrow trough is present at ~1725 cm⁻¹. This confirms a carbonyl group.
   Eliminates A (1-bromobutane) and B (butan-1-ol).
2. Step 2: Check for O–H bands
   Look for a broad trough at 3230–3550 cm⁻¹ (alcohol) or an extremely wide trough at 2500–3000 cm⁻¹ (carboxylic acid). Neither is present.
   Eliminates D (butanoic acid).

💡 Key Knowledge (AQA Data Sheet Values)

• C=O (aldehydes, ketones, carboxylic acids, esters): 1680–1750 cm⁻¹ (sharp, strong).
• O–H (alcohols): 3230–3550 cm⁻¹ (smooth, broad trough).
• O–H (carboxylic acids): 2500–3000 cm⁻¹ (very broad, jagged trough overlapping C–H stretches).
• C–H (alkanes/alkyl chains): 2850–2960 cm⁻¹ (present in almost all organic molecules).
• Aldehyde C–H stretch: Twin peaks at ~2720 cm⁻¹ and ~2820 cm⁻¹ (just to the right of standard C–H).

❌ Common Errors & Misconceptions

• Confusing butanal with butanoic acid: Students see a C=O peak and rush to choose butanoic acid without checking whether the characteristic broad carboxylic acid O–H band (2500–3000 cm⁻¹) is present.
• Misidentifying C–H as O–H: The sharp, spiked peaks just below 3000 cm⁻¹ are standard alkyl C–H bonds, NOT alcohol or acid O–H bonds.
• Wasting time in the fingerprint region: Complex peaks below 1500 cm⁻¹ are unique to the compound but should not be used for basic functional group identification in multiple-choice questions.