OCR A-Level Chemistry AS Breadth in chemistry (01), June 2023: Question 20
1 mark · Medium difficulty · Multiple Choice
Identify which fragment ion with a specific m/z value would be present in the mass spectrum of only one of pentan-2-ol and pentan-3-ol.
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Question text
20 Pentan-2-ol and pentan-3-ol are structural isomers with the molecular formula C5H12O and
Mr = 88.
The isomers can be distinguished from the fragment ions in their mass spectra.
Which fragment ion would you expect to be present in only one of these isomers?
A m/z = 29
B m/z = 45
C m/z = 59
D m/z = 73
Your answer [1]
Mark scheme
Show the mark scheme
20 B 1 AO2.6
Total 20
SECTION B
How to answer it
Distinguishing Isomers Using Mass Spectrometry Fragment Ions
This question assesses your understanding of molecular fragmentation in mass spectrometry (AO2.6). You must apply knowledge of molecular structures (pentan-2-ol vs. pentan-3-ol), cleavage positions along the carbon chain, and calculate mass-to-charge ( m/z ) ratios of specific fragment ions to determine which species is unique to only one isomer.
Question 20 (Multiple Choice)
Identifying Unique Fragment Ions in Pentan-2-ol and Pentan-3-ol
✅ Correct Answer
B: m/z = 45
Mark allocated: 1 mark for selecting B.
💡 Key Knowledge
- Pentan-2-ol structure: CH₃–CH(OH)–CH₂–CH₂–CH₃
- Pentan-3-ol structure: CH₃–CH₂–CH(OH)–CH₂–CH₃
- Fragmentation involves homolytic or heterolytic cleavage of C–C bonds adjacent to the functional group (alpha-cleavage).
🧠 Exam Technique
Systematically work through each option ( m/z value) by deducing possible fragment formulas for both isomers. Eliminate options that can be formed by both molecules.
❌ Common Errors
Guessing randomly or calculating the m/z of whole molecules rather than looking at specific alkyl and oxygenated fragments created during alpha-cleavage.
📐 Step-by-Step Analysis of Fragment Ions
- Option A ( m/z = 29 ): Represents an ethyl group cation ( C₂H₅⁺ , 2×12 + 5×1 = 29). Both pentan-2-ol and pentan-3-ol contain ethyl groups and can produce this fragment. (Present in both)
- Option B ( m/z = 45 ): Represents [CH(OH)CH₂CH₃]⁺ or similar oxygen-containing fragments. Specifically, pentan-3-ol can split to give a [CH(OH)CH₂CH₃]⁺ fragment or [CH(OH)CH₃]⁺ depending on cleavage. Let's check exact cleavage: Pentan-3-ol split adjacent to C-3 gives CH(OH)CH₂CH₃⁺ = 13 (CH) + 17 (OH) + 29 (C₂H₅) = 59, OR splitting to give CH(OH)CH₂CH₃ part. Wait, look closely at pentan-2-ol: cleavage next to C-2 gives [CH(OH)CH₃]⁺ = 13 + 17 + 15 = 45! Let's check pentan-3-ol: Can pentan-3-ol form m/z = 45 ( [CH(OH)CH₃]⁺ )? Pentan-3-ol has the structure CH₃CH₂CH(OH)CH₂CH₃ . It cannot form a CH(OH)CH₃ group directly by simple single-bond C-C cleavage because the carbon attached to OH is bonded to two ethyl groups, not a methyl and an ethyl. Therefore, m/z = 45 is unique to pentan-2-ol.
- Option C ( m/z = 59 ): Formed by pentan-2-ol via cleavage giving [CH(OH)CH₂CH₂CH₃]⁺ (13+17+43 = 73? No: C₃H₇O⁺ = 3×12 + 7 + 16 = 59). Both isomers can generate propyl/oxygen fragments around this region.
- Option D ( m/z = 73 ): Loss of a methyl group ( M⁺ - 15 ). 88 - 15 = 73 . Both secondary alcohols can lose a terminal methyl group to form a C₄H₉O⁺ fragment ion. (Present in both)
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 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.