AQA A-Level Chemistry Paper 3, 2021: Question 11

1 mark · Medium difficulty · Multiple Choice

Spectroscopic and Structural Evidence for an Unknown Ester This multiple-choice question tests understanding of spectroscopic techniques and elemental analysis. Students must interpret ^1H NMR, ^13C NMR, and infrared data, along with percentage composition by mass, to assess whether the evidence supports a proposed ester structure. It challenges the ability to cross-reference empirical data with molecular structure.

Practise this question

Question

AQA A-Level Chemistry Paper 3, 2021: Question 11
Question text

11 Which statement does not support the suggestion that an unknown organic

compound is

[1 mark]

A Its 1H NMR spectrum has 3 peaks with an integration ratio of 2:3:3

B Its 13C NMR spectrum has 3 peaks.

C Its infrared spectrum has an absorption at 1735 cm−1

D It has 36.36% by mass of oxygen and 9.09% by mass of hydrogen.

Mark scheme

Show the mark scheme Mark scheme for AQA A-Level Chemistry Paper 3, 2021: Question 11

11 B 1 Its 13C NMR spectrum has 3 peaks.

How to answer it

MCQ Review: NMR and Spectral Evidence

Correct Answer: B

The correct answer is B – "Its 13C NMR spectrum has 3 peaks."
The correct structure (CH3COOCH2CH3) has 4 carbon environments:
  • A carbonyl carbon (C=O)
  • A methyl group next to the carbonyl (CH3-C=O)
  • An ethyl group's CH2 carbon (–CH2–)
  • An ethyl group's CH3 carbon (–CH3)
So it should produce 4 peaks in the 13C NMR, not 3.

Why the Other Options Are Incorrect:

A: The integration ratio of 2:3:3 in the 1H NMR matches:
  • 2H from CH2 (ethyl group)
  • 3H from CH3 on ethyl group
  • 3H from CH3 adjacent to the C=O group
This matches the structure, so A supports the identity.
C: The absorption at 1735 cm−1 is characteristic of a C=O stretch in an ester, so this supports the identity.
D: The mass percentages of oxygen and hydrogen are consistent with the molecular formula of the compound, so this also supports the structure.

Topics

Organic Chemistry · 3.3.15 Nuclear Magnetic Resonance Spectroscopy

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.