AQA A-Level Chemistry Paper 3, June 2022: Question 25

1 mark · Easy difficulty · Multiple Choice

Identify which compound is an isomer of ethyl ethanoate.

Practise this question

Question

Question 25 asks 'Which compound is an isomer of ethyl ethanoate?' with four multiple-choice options: A butyl methanoate, B methyl propanoate, C methyl butanoate, and D propanoic acid, each with an oval selection box next to it. Worth 1 mark.
Question text

25 Which compound is an isomer of ethyl ethanoate?

[1 mark]

A butyl methanoate

B methyl propanoate

C methyl butanoate

D propanoic acid

Mark scheme

Show the mark scheme Mark scheme table row for question 25: shows the correct answer is B (AO1), worth 1 mark, with the compound name given as methyl propanoate.

25 B (AO1) 1 methyl propanoate

How to answer it

Isomers of Esters and Carboxylic Acids

📋 What this question tests

This question assesses your understanding of structural isomerism and IUPAC nomenclature of organic compounds (specifically esters and carboxylic acids). You need to be able to:

  • Determine the molecular formula of an ester from its IUPAC name.
  • Understand that structural isomers must share the exact same molecular formula.
  • Rapidly count total carbon atoms across alkyl and carboxylate chains.
Question 25 • Multiple Choice

Identifying the Isomer of Ethyl Ethanoate

AQA Chemistry • Organic Chemistry • 1 Mark (AO1)

✅ Correct Answer

B — methyl propanoate

Mark Scheme: 1 mark for option B.

💡 Key Knowledge

  • Isomers: Molecules with the same molecular formula but different structural formulas.
  • Ester General Formula: Saturated acyclic mono-esters and carboxylic acids share the general formula CnH2nO₂ .
  • Ethyl ethanoate:
    • "Ethyl" group = 2 carbons ( −CH₂CH₃ )
    • "Ethanoate" chain = 2 carbons ( CH₃COO− )
    • Total = 4 carbons → Formula: C₄H₈O₂

📐 Step-by-Step Analysis of All Options

Because all options are either saturated aliphatic esters or carboxylic acids, all match the general formula CnH2nO₂ . Therefore, an isomer must have exactly 4 carbon atoms:

Option Carbon Breakdown Molecular Formula Isomer?
A butyl methanoate butyl (4 C) + methanoate (1 C) = 5 C C₅H₁₀O₂ ❌ No (5 carbons)
B methyl propanoate methyl (1 C) + propanoate (3 C) = 4 C C₄H₈O₂ ✅ Yes (Matches ethyl ethanoate)
C methyl butanoate methyl (1 C) + butanoate (4 C) = 5 C C₅H₁₀O₂ ❌ No (5 carbons)
D propanoic acid propanoic acid (3 C) = 3 C C₃H₆O₂ ❌ No (3 carbons)

🧠 Exam Technique Shortcut

In multiple choice questions on esters:

  • Sum the prefix numbers:
    ethyl (2) + ethanoate (2) = 4.
  • Look directly for the option whose sum equals 4:
    methyl (1) + propanoate (3) = 4.
  • This takes less than 10 seconds and prevents unnecessary drawing of full displayed formulas under time pressure.

❌ Common Errors & Pitfalls

  • Mixing up functional group isomers: Forgetting that carboxylic acids are functional group isomers of esters. Propanoic acid ( C₃H₆O₂ ) is indeed a carboxylic acid, but students carelessly pick it thinking of "prop/eth" without counting the total carbons (it would need to be butanoic acid).
  • Miscounting the carboxyl carbon: Thinking "propanoate" means a 3-carbon chain attached to a carbonyl carbon (which would be 4 carbons total). Remember, the carbonyl carbon is included in the IUPAC root name.

Topics

Organic Chemistry · 3.3.1 Introduction to Organic Chemistry · 3.3.9 Carboxylic Acids and Derivatives

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.