OCR A-Level Chemistry AS Breadth in chemistry (01), June 2019: Question 18

1 mark · Medium difficulty · Multiple Choice

Identify which compound is not likely to have a fragment ion at m/z = 43 in its mass spectrum from four given organic structures.

Practise this question

Question

Multiple-choice question 18 asking which compound is not likely to have a fragment ion at m/z = 43 in its mass spectrum. Four options A, B, C, and D show displayed formulas of organic compounds: A is propanoic acid (CH3CH2COOH), B is ethanoic acid (CH3COOH), C is butan-2-ol (CH3CH2CH(OH)CH3), and D is propan-1-ol (CH3CH2CH2OH).
Question text

18 Which compound is not likely to have a fragment ion at m/z = 43 in its mass spectrum?

H H O

A H C C C

H H OH

H O

B H C C

H OH

H CH3 H

C H C C C OH

H H H

H H H

D H C C C OH

H H H

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme for question 18 showing the correct answer is A with 1 mark.

18 A 1 AO2.5

How to answer it

Identifying Fragment Ions in Mass Spectrometry

What this question tests

This question assesses your ability to interpret mass spectra data by linking fragment ion mass-to-charge ratios (m/z) to specific structural fragments within organic molecules (specifically carboxylic acids and alcohols). It tests your skill in breaking down molecular structures to deduce possible cationic fragments.

Question 18 • Multiple Choice

Analyzing Fragment Ions at m/z = 43

✅ Correct Answer: A

Compound A (propanoic acid, CH₃CH₂COOH) is not likely to produce a significant fragment ion at m/z = 43. Instead, its major fragmentation pathways typically yield ions at m/z = 29 (CH₃CH₂⁺ or CHO⁺) and m/z = 45 (COOH⁺).

💡 Key Knowledge

  • m/z = 43 fragments: Commonly corresponds to the propyl cation (C₃H₇⁺) or the acetyl/propanoyl-type fragments like CH₃CO⁺.
  • Compound B: Ethanoic acid (CH₃COOH) has an acetyl-type feature or can lose methyl groups, but let's look at the others... wait, compound B is methanoic/ethanoic derivative, but let's check structures C and D.
  • Compound C: 2-methylpropan-1-ol contains a branched chain (CH₃)₂CH- group, which readily cleaves to form a stable secondary carbocation or fragment with m/z = 43 ((CH₃)₂CH⁺).
  • Compound D: Propan-1-ol (CH₃CH₂CH₂OH) can rearrange or fragment to give propyl cations (C₃H₇⁺) at m/z = 43.

🧠 Exam Technique

When tackling fragment ion questions, calculate the formula mass of common alkyl groups:
• CH₃⁺ = 15
• C₂H₅⁺ = 29
• C₃H₇⁺ / CH₃CO⁺ = 43
Match these standard masses directly to the alkyl chain branches or functional groups present in each isomer.

❌ Common Errors

  • Assuming all carboxylic acids fragment in the exact same pattern regardless of their alkyl chain length.
  • Failing to account for structural rearrangement or stable secondary carbocation formation in branched alcohols like compound C.
Mark Scheme Allocation: 1 mark awarded for selecting A (AO2.5: Applying chemical knowledge to mass spectrometry fragmentation patterns).

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 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.