OCR A-Level Chemistry Synthesis and analytical techniques (02), November 2021: Question 12

1 mark · Medium difficulty · Multiple Choice

Identify which mass-to-charge ratio peak appears in the mass spectrum of propanal but not propanone

Practise this question

Question

Multiple choice question 12 asking which peak would be in the mass spectrum of propanal, CH3CH2CHO, but not propanone, CH3COCH3. Four options are given: A, m/z = 15; B, m/z = 29; C, m/z = 43; D, m/z = 58, with a box for the answer.
Question text

12 Which peak would be in the mass spectrum of propanal, CH3CH2CHO, but not in the mass

spectrum of propanone, CH3COCH3?

A m/z = 15

B m/z = 29

C m/z = 43

D m/z = 58

Your answer

[1]

Mark scheme

Show the mark scheme Mark scheme showing the correct answer for question 12 is B.

12 B 1 AO2.1

How to answer it

Identifying Carbonyl Compounds Using Mass Spectrometry

What this question tests

This question assesses your understanding of mass spectrometry fragmentation patterns and structural isomerism in organic chemistry (specifically aldehydes vs ketones). You need to deduce the m/z values of specific fragment ions formed when molecular ions break apart during electron impact ionization.

Question Analysis (Multiple Choice - Part 12)

Comparing Fragmentation in Propanal and Propanone

Propanal ( CH₃CH₂CHO ) and propanone ( CH₃COCH₃ ) are functional group isomers with the molecular formula C₃H₆O and a relative molecular mass of 58. This question requires you to identify a fragment unique to the aldehyde.

✅ Correct Answer: Option B (m/z = 29)

The peak at m/z = 29 corresponds to the [CH₃CH₂]+ or [CHO]+ fragment. Propanal features an ethyl group attached to a carbonyl carbon, allowing the loss of a [CHO] radical to form [CH₃CH₂]+ , or cleavage to form [CHO]+ . Propanone lacks this structural arrangement and cannot easily form a stable m/z = 29 fragment in significant abundance.

💡 Key Knowledge: Fragmentation Rules

  • Molecular Ion Peak ( m/z = 58 ): Both isomers share this peak because they have the same Mᵣ . (Rules out Option D).
  • Common Alkyl Fragments ( m/z = 15 and 43 ): Both molecules contain methyl groups ( [CH₃]+ = 15) and acyl/propyl cations ( [CH₃CO]+ or [C₃H₇]+ = 43), meaning these peaks appear in both spectra. (Rules out Options A and C).

🧠 Exam Technique

When tackling isomer mass spec questions, systematically write out the possible fragments and their m/z values for both molecules before choosing. Cross off options that appear in both spectra.

❌ Common Errors

Students frequently select Option D ( m/z = 58 ), forgetting that structural isomers share the exact same molecular mass and therefore both produce a molecular ion peak at the same m/z value.

Mark Scheme Allocation: 1 mark awarded for selecting B (AO2.1 - applying chemical knowledge of fragmentation to unfamiliar structural contexts).

Topics

Module 6: Organic chemistry and analysis · 6.1 Aromatic compounds, carbonyls and acids · 6.3 Analysis

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.