OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2022: Question 12

✅ Correct Answer: A

Compound CH₃CH₂COOCH₂CH₃ (ethyl propanoate) is the correct option.

Why? It contains two separate ethyl groups ( -CH₂CH₃ ). Each ethyl group consists of a triplet (split by the adjacent 2 protons) and a quartet (split by the adjacent 3 protons), perfectly yielding two distinct triplets in its spectrum.

💡 Key Knowledge

• The n + 1 Rule: Splitting of a peak is given by n + 1 , where n is the number of equivalent protons on adjacent carbon atom(s).
• A -CH₂- group adjacent to a -CH₃ group splits into a quartet ( 3 + 1 = 4 ).
• A -CH₃ group adjacent to a -CH₂- group splits into a triplet ( 2 + 1 = 3 ).

🧠 Exam Technique

Instead of drawing out every single spectrum, systematically analyze the adjacent proton environments for each candidate:

• Look for isolated -CH₂- groups positioned between two different functional groups (which often produce triplets or pentets).
• Check symmetry: symmetrical molecules like HOOCCH₂CH₂COOH reduce the number of unique environments.

❌ Common Errors

• Confusing splitting with integration: Mixing up the number of peaks in a multiplet (splitting) with the relative peak areas (integration ratio).
• Ignoring symmetry: Assuming every carbon chain has unique hydrogen environments without checking for planes of symmetry.
• Forgetting exchangeable protons: Carboxylic acid protons ( -COOH ) typically appear as singlets and do not follow standard coupling in standard dry conditions, eliminating options C and D from producing multiple clean triplets.