OCR A-Level Chemistry AS Breadth in chemistry (01), June 2025: Question 20

1 mark · Easy difficulty · Multiple Choice

Identify which compound corresponds to the given infrared spectrum showing a strong absorption around 1720–1740 cm⁻¹ and no broad O–H peak.

Practise this question

Question

Question 20 showing an infrared spectrum with transmittance percentage plotted against wavenumber from 4000 to 400 cm⁻¹. There is a sharp C–H stretch just below 3000 cm⁻¹ and a sharp, intense carbonyl peak at approximately 1730 cm⁻¹, with no broad O–H absorption present. Four options are listed: A: CH3CH=CH2, B: CH3CH2CHO, C: CH3CH2CH2OH, and D: CH3CH2COOH.
Question text

20 The IR spectrum of a compound is shown below.

transmittance

(%)

4000 3000 2000 1500 1000 400

wavenumber / cm–1

Which compound could have produced the IR spectrum?

A CH3CH=CH2

B CH3CH2CHO

C CH3CH2CH2OH

D CH3CH2COOH

Your answer [1]

SECTION B

Mark scheme

Show the mark scheme The mark scheme snippet displays the bottom summary row indicating 'Section A Total 20' without specific marking points shown.

20 B 1

How to answer it

Identifying Carbonyl Compounds Using Infrared Spectroscopy

🔍 What this question tests

This question assesses your ability to deduce the functional group of an organic molecule from an infrared (IR) spectrum using characteristic absorption bands:

  • Recognising the sharp, prominent carbonyl ( C=O ) absorption stretch at ~1630–1820 cm⁻¹.
  • Distinguishing carbonyl compounds (aldehydes/ketones) from alcohols and carboxylic acids by noting the complete absence of broad O-H absorption bands.
  • Eliminating alkenes based on peak intensity, shape, and characteristic wavenumber values.
Question 20 • Multiple Choice • [1 Mark]

IR Spectrum Analysis & Identification

Deducing the unknown organic structure from characteristic wavenumber peaks

✅ Correct Answer

B — CH₃CH₂CHO (Propanal)

Mark Scheme Breakdown:
• 1 mark for letter B in the answer box.

📐 Spectral Analysis (Step-by-Step)

  1. Strong, sharp peak at ~1730 cm⁻¹: Unambiguously indicates a carbonyl group, C=O (aldehyde or ketone). This immediately rules out A ( CH₃CH=CH₂ ) and C ( CH₃CH₂CH₂OH ).
  2. No broad peak between 2500–3300 cm⁻¹: Carboxylic acid O-H stretches form a distinctive, very broad "trough" that overlaps the C-H region. Its complete absence rules out D ( CH₃CH₂COOH ).
  3. Peaks around 2700–2900 cm⁻¹: Characteristic C-H stretching vibrations, including the aldehyde C-H doublet around 2720 cm⁻¹ and 2820 cm⁻¹.

💡 Key Knowledge: Data Sheet Values

Always reference standard OCR Data Sheet values when solving IR questions:

  • C=O (aldehydes, ketones, acids): 1630–1820 cm⁻¹ (very strong, sharp)
  • O-H (alcohols): 3200–3600 cm⁻¹ (broad, rounded)
  • O-H (carboxylic acids): 2500–3300 cm⁻¹ (very broad, jagged baseline)
  • C=C (alkenes): 1620–1680 cm⁻¹ (often weak or medium intensity)
  • C-H (alkyl): 2850–3100 cm⁻¹

🧠 Exam Technique: Elimination Strategy

In Section A multiple-choice IR questions, negative evidence is just as powerful as positive evidence:

  • Look for the presence of C=O: The deep "spike" at ~1730 cm⁻¹ dropping down to ~20% transmittance instantly confirms a carbonyl group is present.
  • Look for the absence of O-H: The baseline above 3100 cm⁻¹ is virtually flat (~95–100% transmittance). If an -OH group were present, transmittance would drop drastically in this region.

❌ Common Errors & Pitfalls

  • Confusing Propanal (B) with Propanoic Acid (D): Both contain a C=O peak at ~1700 cm⁻¹, but students often forget that propanoic acid must also feature a massive, broad carboxylic O-H absorption spanning 2500–3300 cm⁻¹.
  • Mistaking C-H peaks for O-H peaks: The sharp spikes just under 3000 cm⁻¹ are standard aliphatic C-H bonds, not an alcohol O-H (which is broad and located further left between 3200–3600 cm⁻¹).
  • Confusing C=C with C=O: Alkene C=C stretches appear at a slightly lower wavenumber (1620–1680 cm⁻¹) and are substantially weaker in transmittance than a C=O stretch.

Topics

Module 4: Core organic chemistry · 4.2 Alcohols, haloalkanes and analysis

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.