OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2023: Question 10

1 mark · Medium difficulty · Multiple Choice

Identify which ester is most likely to produce a mass spectrum with a fragment ion at m/z = 43 from four given structural formulae.

Practise this question

Question

Multiple choice question 10 asking which ester is most likely to produce a mass spectrum with a fragment ion at m/z = 43. Four options A, B, C, and D show different hydroxy-substituted esters with various alkyl chains and functional groups. Option A has a methyl ester of 2-hydroxy-2-methylpropanoic acid. Option B has a propyl ester of hydroxyethanoic acid (HO-CH2-CO-O-CH2CH2CH3). Option C has a methyl ester of 3-hydroxybutanoic acid. Option D has a 2-hydroxyethyl ester of 2-hydroxy-2-methylbutanoic acid. An answer box and mark allocation of [1] are shown at the bottom.
Question text

10 Which ester is most likely to produce a mass spectrum with a fragment ion at m/z = 43?

OH O

A H3C C C O CH3

CH3

O

B

HO CH2 C O CH2CH2CH3

OH O

C H C CH CH C O CH

32 3

OH O

D

CH3CH2 C C O CH2CH2OH

CH3

Your answer

[1]

Mark scheme

Show the mark scheme The mark scheme shows the correct answer for question 10 is B, worth 1 mark.

10 B 1 AO2.1

How to answer it

Mass Spectrometry Fragment Identification in Esters

📌 What this question tests

This question assesses your understanding of mass spectrometry fragmentation patterns in organic molecules (specifically esters), your ability to calculate formula masses of alkyl/acyl fragments from m/z values, and your skill in analysing structural isomer formulas.

Question Analysis & Answer Breakdown

Multiple Choice Question 10

✅ Correct Answer: B

Ester B contains a -CH₂CH₂CH₃ (propyl) group attached to the oxygen atom. Fragmentation of the alkyl-oxygen bond yields a propyl cation [CH₂CH₂CH₃]⁺ or isopropyl cation [C₃H₇]⁺ , which has an m/z value of 43 (3 × 12 + 7 = 43).

💡 Key Knowledge

  • Common m/z fragments: 15 [CH₃]⁺, 29 [C₂H₅]⁺ or [CHO]⁺, 43 [C₃H₇]⁺ or [CH₃CO]⁺, 57 [C₄H₉]⁺ or [C₂H₅CO]⁺.
  • Esters readily undergo cleavage adjacent to the carbonyl group or the ether oxygen.

🧠 Exam Technique

Systematically check the alkyl chains attached to each ester in the options. Calculate the mass of possible fragments formed by cleavage before guessing. Look out for standard stable carbocations.

❌ Common Errors

Students often waste time calculating the entire Mr of the ester molecule instead of focusing strictly on the specific fragment mass requested ( m/z = 43 ).

Mark Scheme Allocation: 1 mark available for selecting option B. (AO2.1 - Application of chemical knowledge to identify fragmentation patterns).

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), June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.