OCR A-Level Chemistry AS Breadth in chemistry (01), November 2021: Question 26

Mark scheme

Show the mark scheme

AO

Question Answer Marks Guidance

element

26 5

Mass spectrum:

M = 88 AO3.1

×3

IR:

Peak at 1630-1820 (cm–1) is C=O ALLOW stated values within stated ranges

Peak at 2500–3500 (cm–1) is O–H

AND carboxylic acid ALLOW ‘acid O–H

Structures IGNORE references to C–O peaks

ALLOW any combination of skeletal OR

structural OR displayed formula as long as

AO3.2 unambiguous

×2

Total 13

OCR (Oxford Cambridge and RSA Examinations)

The Triangle Building

Shaftesbury Road

Cambridge

CB2 8EA

OCR Customer Contact Centre

Education and Learning

Telephone: 01223 553998

Facsimile: 01223 552627

Email: general.qualifications@ocr.org.uk

www.ocr.org.uk

For staff training purposes and as part of our quality assurance programme your call may be

recorded or monitored

How to answer it

Structure Elucidation Using Mass Spectrometry and Infrared Spectroscopy

What this question tests

This question assesses your ability to combine analytical data from two key analytical techniques—Mass Spectrometry (MS) and Infrared (IR) Spectroscopy—to deduce functional groups, relative molecular mass, and draw valid structural isomers for an organic compound containing carbon, hydrogen, and oxygen only.

Question Analysis (Part 26)

Deducing Compound E from Mass and IR Spectra

💡 Key Knowledge: Spectral Analysis

Mass Spectrum: The peak furthest to the right with the highest m/z value corresponds to the molecular ion peak ( M ), giving the relative molecular mass ( Mᵣ = 88).
IR Spectrum (C=O): A sharp, deep trough between 1630–1820 cm⁻¹ indicates a carbonyl group ( C=O ).
IR Spectrum (O-H): A very broad absorption trough spanning 2500–3500 cm⁻¹ indicates a carboxylic acid O-H stretch (overlapping with C-H stretches).

✅ Correct Answer Breakdown

Molecular Mass: M = 88 identified from the mass spectrum.
Functional Group 1: Peak at 1630–1820 cm⁻¹ confirms C=O .
Functional Group 2: Peak at 2500–3500 cm⁻¹ confirms carboxylic acid ( O-H ).
Possible Isomers (C₄H₈O₂): Butanoic acid ( CH₃CH₂CH₂COOH ) AND 2-methylpropanoic acid ( (CH₃)₂CHCOOH ).

🧠 Exam Technique

Work methodically: Find the Mᵣ first, use the IR data to identify the homologous series (e.g., carboxylic acid), then construct structural isomers matching the molecular formula ( C₄H₈O₂ has an Mᵣ of (4×12) + (8×1) + (2×16) = 88 ).
Ensure structural or displayed formulas show all bonds and atoms clearly without ambiguity to secure structural mark points.

❌ Common Errors

Confusing the broad carboxylic acid O-H stretch ( 2500–3500 cm⁻¹ ) with a standard alcohol O-H stretch ( 3230–3550 cm⁻¹ which is smoother and less overlapping).
Misreading the molecular ion peak in the mass spectrum by picking a fragment instead of the highest m/z value.
Drawing carbon chain isomers that do not match the required molecular formula or molecular mass of 88.

Total Marks: 5
• 1 mark for M = 88
• 1 mark for identifying C=O peak range
• 1 mark for identifying carboxylic acid O-H peak range
• 2 marks for two correct, distinct structural/displayed formulas for butanoic acid and 2-methylpropanoic acid.

Topics

Module 6: Organic chemistry and analysis · 6.3 Analysis