OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2024: Question 11

1 mark · Medium difficulty · Multiple Choice

Identify which given compound could have produced the provided infrared spectrum.

Practise this question

Question

Question 11 asks to identify which compound could have produced the shown IR spectrum, which displays transmittance percentage on the y-axis from 0 to 100 and wavenumber on the x-axis from 4000 to 400 cm⁻¹, featuring a broad O-H absorption band around 3400 cm⁻¹ and a sharp C=O absorption band around 1700 cm⁻¹. Four multiple choice options are provided: A, HOCH2CHO; B, CH3CH2COOH; C, CH3CH2COOCH3; D, (CH3)2C(OH)COOH.
Question text

11 Which compound could have produced the IR spectrum shown below?

Transmittance

(%) 50

4000 3000 2000 1500 1000 500

Wavenumber / cm–1

A HOCH2CHO

B CH3CH2COOH

C CH3CH2COOCH3

D (CH3)2C(OH)COOH

Your answer

[1]

Mark scheme

Show the mark scheme The mark scheme indicates that the correct answer for question 11 is D, worth 1 mark.

11 D 1

How to answer it

Analysing an Infrared (IR) Spectrum

What this question tests

This question assesses your ability to interpret an infrared spectrum by identifying characteristic absorption peaks (wavenumbers) and matching them to specific functional groups found in organic molecules. You must systematically rule out incorrect options by spotting missing or extra peaks.

Question 11 - Multiple Choice

Identifying the Compound from its IR Spectrum

✅ Correct Answer: D

(CH₃)₂C(OH)COOH

This molecule contains both an alcohol group ( O-H ) and a carboxylic acid group ( C=O and O-H ), which perfectly account for all the major absorption features seen on the spectrum.

💡 Key Knowledge (Data Booklet Peaks)

  • O-H (alcohol, broad): 3200 – 3600 cm⁻¹
  • O-H (carboxylic acid, very broad): 2500 – 3300 cm⁻¹ overlapping with C-H stretches
  • C-H (alkane): 2850 – 3100 cm⁻¹
  • C=O (carbonyl): 1680 – 1750 cm⁻¹

🧠 Exam Technique

  • Scan for major peaks first: Look at the very broad trough spanning ~2500 to 3300 cm⁻¹, characteristic of a carboxylic acid O-H.
  • Check for the carbonyl peak: Locate the sharp, deep trough around 1700 cm⁻¹ indicating a C=O double bond.
  • Process of elimination: Cross-reference options to eliminate compounds lacking these key diagnostic bonds.

❌ Common Errors & Misconceptions

  • Confusing O-H types: Mistaking the very broad carboxylic acid O-H region for a simple alcohol or amine peak.
  • Ignoring overlapping regions: Forgetting that carboxylic acid O-H stretches are extremely broad and absorb across a wide wavenumber range, swallowing up adjacent C-H stretches.

🔍 Detailed Spectrum Breakdown for Option D

Looking closely at the provided spectrum:

  • There is a very broad absorption spanning 2500 cm⁻¹ to 3300 cm⁻¹, which is the trademark sign of a carboxylic acid O-H stretch.
  • There is a sharp, intense peak around 1710 cm⁻¹ corresponding to the C=O stretch in the acid group.
  • Sharp peaks just below 3000 cm⁻¹ confirm aliphatic C-H stretching from the methyl groups (CH₃)₂ .
Mark Allocation: 1 mark for selecting D.

Topics

Module 6: Organic chemistry and analysis · Module 4: Core organic chemistry · 6.3 Analysis · 4.2 Alcohols, haloalkanes and analysis

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.