OCR A-Level Chemistry AS Breadth in chemistry (01), June 2023: Question 19

1 mark · Medium difficulty · Multiple Choice

Identify the organic compound that produced the given infrared spectrum from a choice of four options.

Practise this question

Question

An infrared spectrum showing transmittance percentage on the vertical axis against wavenumber in cm^-1 on the horizontal axis (from 4000 to 400). A strong, sharp absorption peak is visible around 1700 cm^-1, along with C-H stretching peaks just below 3000 cm^-1. Below the spectrum, a multiple-choice question asks which compound could have produced it, with options A (CH3CH2CH=CH2), B (CH3CHOHCH2CH3), C (CH3CH2COCH3), and D ((CH3)2CHCOOH), alongside a box for the answer.
Question text

19 An organic compound produces the infrared spectrum below.

transmittance

(%)

4000 3000 2000 1500 1000 500

wavenumber / cm–1

Which compound could have produced this IR spectrum?

A CH3CH2CH=CH2

B CH3CHOHCH2CH3

C CH3CH2COCH3

D (CH3)2CHCOOH

Your answer [1]

Mark scheme

Show the mark scheme A mark scheme table showing that the correct answer for question 19 is C, worth 1 mark, assigned to assessment objective AO2.5.

19 C 1 AO2.5

How to answer it

Identifying Functional Groups Using Infrared Spectroscopy

What this question tests

This question assesses your ability to interpret infrared (IR) spectra by matching characteristic absorption wavenumber ranges to specific covalent bonds and functional groups, allowing you to deduce the correct organic molecule structure from a multiple-choice list.

Question 19 (Multiple Choice)

Analyzing the IR Spectrum

✅ Correct Answer

C ( CH₃CH₂COCH₃ )

Mark Awarded: 1 / 1 (AO2.5 - Analysing information given to make deductions)

💡 Key Knowledge

  • C=O bond (Carbonyl): Strong, sharp dip around 1680–1750 cm⁻³ . (Clearly visible as the deepest trough just below 1700 cm⁻³).
  • C-H (Alkyl) stretch: Strong absorption just below 3000 cm⁻³ ( 2850–2950 cm⁻³ ).
  • O-H (Alcohol) stretch: Broad absorption around 3200–3600 cm⁻³ (Absent here).
  • O-H (Carboxylic acid) stretch: Very broad absorption spanning 2500–3300 cm⁻³ (Absent here).
  • C=C (Alkene) stretch: Weak-to-medium peak around 1620–1680 cm⁻³ (Absent here).

🧠 Exam Technique

Eliminate options systematically using the Data Booklet absorption values:

  1. Scan the high wavenumber region above 3000 cm⁻³ . There is no broad O-H peak, ruling out B (alcohol) and D (carboxylic acid).
  2. Look for major peaks between 1600–1800 cm⁻³ . There is a very prominent, sharp trough at roughly 1715 cm⁻³ , indicating a C=O carbonyl group.
  3. Compare remaining options A and C. Compound A is an alkene (lacks C=O, has C=C). Compound C is a ketone ( CH₃CH₂COCH₃ ), perfectly matching the strong C=O stretch.

❌ Common Errors

  • Confusing O-H and C-H regions: Students frequently mistake saturated alkyl C-H stretches just under 3000 cm⁻³ for alcohol O-H peaks. Remember that alcohol O-H is much broader and appears at higher wavenumbers ( >3200 cm⁻³ ).
  • Ignoring the fingerprint region: Trying to interpret every tiny dip below 1500 cm⁻³ instead of focusing on the diagnostic functional group region ( >1500 cm⁻³ ).

Topics

Module 6: Organic chemistry and analysis · 6.3 Analysis

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.