OCR A-Level Chemistry AS Breadth in chemistry (01), June 2018: Question 20

1 mark · Medium difficulty · Multiple Choice

Identify which alcohol is likely to have a fragment ion at m/z = 31 in its mass spectrum from a given list of four structural formulas.

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Question

Multiple choice question 20 asks which alcohol is likely to have a fragment ion at m/z = 31 in its mass spectrum. Four options are provided: A is CH3CH2CH(OH)CH3, B is CH3CH2CH2C(OH)(CH3)2, C is CH3CH(OH)CH2CH2CH3, and D is (CH3)2CHCH2OH. An empty answer box and the mark [1] are at the bottom.
Question text

20 Which alcohol is likely to have a fragment ion at m/z = 31 in its mass spectrum?

A CH3CH2CH(OH)CH3

B CH3CH2CH2C(OH)(CH3)2

C CH3CH(OH)CH2CH2CH3

D (CH3)2CHCH2OH

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme table shows question number 20 with the correct answer D and 1 mark allocated.

20 D 1

Total 20

SECTION B

How to answer it

Identifying Alcohols via Mass Spectrometry Fragmentation

What this question tests

This question assesses your understanding of mass spectrometry fragmentation patterns in organic molecules, specifically focusing on characteristic fragment ions produced by primary, secondary, and tertiary alcohols (AS Level Organic Chemistry / Analytical Techniques).

Question 20 Analysis

Identifying the m/z = 31 peak in alcohol mass spectra

✅ Correct Answer: D

Alcohol D ( (CH₃)₂CHCH₂OH ) is a primary alcohol with a terminal -CH₂OH group, which readily cleaves to form the [CH₂OH]⁺ fragment ion responsible for the peak at m/z = 31 .

💡 Key Knowledge

  • An m/z value of 31 corresponds to the fragment [CH₂OH]⁺ (Carbon = 12, Hydrogen = 1×2, Oxygen = 16; total = 31).
  • Primary alcohols containing an unsubstituted -CH₂OH group typically show a strong characteristic peak at m/z = 31 due to alpha-cleavage.

🧠 Exam Technique

  • Quickly calculate the molar mass of common fragments: [CH₂OH]⁺ = 31, [CH₃CHOH]⁺ = 45.
  • Draw out or mentally check the structural formula of each option to find where the hydroxyl group ( -OH ) is attached.

❌ Common Errors

  • Confusing primary alcohols with secondary or tertiary alcohols, which undergo different cleavage patterns.
  • Guessing based on chain length rather than analyzing the stable fragments that can be generated upon electron bombardment.
Mark Scheme Allocation: 1 mark awarded for selecting option D.

Topics

Module 4: Core organic chemistry · Module 6: Organic chemistry and analysis · 4.2 Alcohols, haloalkanes and analysis · 6.3 Analysis

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.