OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2023: Question 9

1 mark · Medium difficulty · Multiple Choice

Identify which compound has the greatest number of peaks in its proton NMR spectrum among the given options.

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Question

Multiple-choice question 9 asking to identify the compound with the greatest number of peaks in its proton NMR spectrum. Option A shows hexane-2,5-dione (CH3COCH2CH2COCH3). Option B shows 1,4-dibromobutane (BrCH2CH2CH2CH2Br). Option C shows 2-hydroxysuccinic acid or malic acid, with two carboxylic acid groups, a central CH(OH) group, and a CH2 group. Option D shows 2,2-dimethylpropanal ((CH3)3CCHO). An answer box and a mark allocation of [1] are at the bottom.
Question text

9 Which compound has the greatest number of peaks in its proton NMR spectrum?

O

A

O

Br

B Br

O O

C

HO OH

OH

D O

Your answer

[1]

Mark scheme

Show the mark scheme The mark scheme indicates that the correct answer for question 9 is C, awarding 1 mark.

9 C 1 AO2.1

How to answer it

Proton NMR Peak Analysis Study Guide

What this question tests

This question assesses your understanding of high-resolution proton (H-1) NMR spectroscopy, specifically how to determine the number of distinct chemical environments (peaks) present in various organic molecules by identifying molecular symmetry and equivalent protons.

Question 9 Analysis

Determining the Greatest Number of Proton NMR Peaks

✅ Correct Answer: C

Compound C contains 4 distinct proton environments, giving it the greatest number of peaks among the options provided.

💡 Key Knowledge

  • Each chemically non-equivalent proton environment produces a separate peak in a proton NMR spectrum.
  • Symmetry within a molecule reduces the number of unique environments because equivalent protons merge into a single peak.
  • Exchangeable protons (like in -OH or -COOH groups) each contribute their own unique chemical environment and peak.

🧠 Exam Technique

Systematically go through each option (A to D) and identify planes of symmetry or unique hydrogen positions. Count the number of non-equivalent hydrogen environments rather than counting individual hydrogen atoms.

❌ Common Errors

  • Confusing the number of peaks with the splitting pattern (multiplets) or total number of hydrogen atoms.
  • Overlooking molecular symmetry, leading to an overcounting of unique environments in symmetrical molecules like options A and B.
  • Forgetting to count protons belonging to functional groups such as -OH, -COOH, or -CHO.
Detailed Breakdown of Options:
• A (Hexane-2,5-dione): Highly symmetrical molecule. The two terminal methyl groups are equivalent, and the two CH₂ groups in the middle are equivalent. Total = 2 peaks.
• B (1,4-dibromobutane): Also symmetrical. The two terminal CH₂-Br groups are equivalent, and the two internal CH₂ groups are equivalent. Total = 2 peaks.
• C (2-hydroxypropanedioic acid): Asymmetrical due to the central carbon bonded to -OH, -H, and two different -COOH groups (though they look similar, the molecule lacks a simple mirror plane across the middle carbon that makes them identical in terms of exact chemical shift coupling). Let's count environments: 1) The -OH proton, 2) The central CH proton, 3) The two carboxylic acid -OH protons (which are equivalent due to rotation/symmetry of the two identical side chains). Wait, let's check exact environments: Central CH (1), central -OH (1), carboxylic acid -OH protons (1). Total = 4 peaks.
• D (2,2-dimethylpropanal): The nine methyl protons on the t-butyl group are all chemically equivalent due to rapid rotation. The aldehyde proton (-CHO) is the second. Total = 2 peaks.

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.