OCR A-Level Chemistry AS Breadth in chemistry (01), June 2024: Question 20

1 mark · Medium difficulty · Multiple Choice

Identify the possible identity of a fragment ion with m/z = 45 produced from the mass spectrum of the alcohol CH3CH2CH(CH3)OH.

Practise this question

Question

Multiple choice question 20 showing an alcohol molecule CH3CH2CH(CH3)OH and four options A, B, C, and D for a fragment ion with m/z = 45. Option A is CH2CHOH+, option B is CH2CH2OH+, option C is CH3CHOH+, and option D is CH3CH2CH+. An empty box labeled 'Your answer' is provided at the bottom.
Question text

20 An alcohol CH3CH2CH(CH3)OH produces a mass spectrum.

The mass spectrum contains a large fragment ion at m/z = 45.

What is the possible identity of this fragment ion?

A CH CHOH+

B CH CH OH+

C CH CHOH+

D CH CH CH+

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme indicating the correct answer for question 20 is C, worth 1 mark.

20 C 1

How to answer it

Mass Spectrometry Fragment Identification

What this question tests

This question assesses your ability to interpret fragmentation patterns in mass spectra for organic molecules (specifically alcohols), calculate relative molecular mass (Mr) of fragment ions, and link structural formulas to m/z values.

Question 20 (Multiple Choice)

Identifying an alcohol fragment ion at m/z = 45

✅ Correct Answer: C

CH₃CHOH⁺

Let's calculate the relative mass of option C: (2 × 12.0) + (5 × 1.0) + 16.0 = 24 + 5 + 16 = 45.

💡 Key Knowledge

  • Alcohols commonly undergo alpha-cleavage (cleavage of the carbon-carbon bond adjacent to the carbon bearing the OH group).
  • The stability of the resulting oxonium-type or secondary carbocation fragment explains why these peaks are prominent in the mass spectrum.

🧠 Exam Technique

Quickly calculate the Mr values of the options provided before guessing. Rule out options that do not sum to m/z = 45 first, then match the remaining choices to the structure of CH₃CH₂CH(CH₃)OH (butan-2-ol).

❌ Common Errors

Students often miscount hydrogen atoms when evaluating formulas or confuse structural isomers like CH₂CH₂OH⁺ (option B, Mr = 45) which doesn't match the alpha-cleavage fragmentation pattern of the specific branched reactant molecule.

Mark Allocation: 1 mark for selecting C.

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 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.