OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2023: Question 1

1 mark · Easy difficulty · Multiple Choice

Identify which compound is used for proton exchange in NMR spectroscopy among the given multiple-choice options.

Practise this question

Question

Multiple choice question 1 asking which compound is used for proton exchange in NMR spectroscopy, with options A (CCl4), B (CDCl3), C (D2O), and D (Si(CH3)4), followed by a box for the answer and 1 mark allocation.
Question text

1 Which compound is used for proton exchange in NMR spectroscopy?

A CCl4

B CDCl3

C D2O

D Si(CH3)4

Your answer

[1]

Mark scheme

Show the mark scheme The mark scheme table shows question 1 with the correct answer as C, worth 1 mark, under AO element AO1.1.

AO

Question Answer Marks Guidance

element

1 C 1 AO1.1

How to answer it

Proton Exchange in NMR Spectroscopy

What this question tests

This question assesses your factual recall and understanding of specific chemical reagents used in ¹H (proton) NMR spectroscopy. Specifically, it tests your ability to distinguish between solvents used to dissolve samples (like CDCl₃ ) and reagents specifically added to cause proton exchange to identify labile protons (such as -OH and -NH groups).

Question 1

Identifying the Proton Exchange Reagent

✅ Correct Answer

C ( D₂O )

Awarded 1 mark for correct selection of option C. (AO1.1 - Demonstration of knowledge)

💡 Key Knowledge

  • Deuterated water ( D₂O ) is added to an NMR sample to identify labile protons (OH and NH groups).
  • Deuterium ( ²H or D) has an even mass number and an even atomic number, meaning its nuclear spin is zero. Therefore, it is NMR inactive in a proton spectrometer and does not produce a peak.
  • During proton exchange, the D from D₂O swaps with the H in -OH or -NH groups, causing the peak corresponding to that proton to disappear from the spectrum.

🧠 Exam Technique

Read the stem extremely carefully! Students often rush past key phrases like "proton exchange" and mistake the question for asking about NMR solvents. Always pause to ensure you are matching the reagent to its exact functional role in the protocol.

❌ Common Errors

  • Choosing B ( CDCl₃ ): This is the most common distractor. While CDCl₃ is the most frequently used solvent in NMR spectroscopy because it lacks protons, it is not used for proton exchange.
  • Choosing A ( CCl₄ ): Also an NMR solvent (and completely aprotic), but it does not undergo proton exchange and is far less common than deuterated solvents.
  • Choosing D ( Si(CH₃)₄ ): This is TMS (tetramethylsilane), used as the internal reference standard to set δ = 0 ppm , not a proton exchange reagent.

Topics

Module 6: Organic chemistry and analysis · 6.3 Analysis

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.