Infrared spectroscopyAQA A-Level Chemistry: Subtopic test

1. 1
   A student records the infrared spectrum of an organic liquid with the molecular formula C₃H₆O. The spectrum has a strong absorption at 1715 cm⁻¹ and no broad absorption anywhere between 2500 and 3600 cm⁻¹. Data: C=O 1680–1750 cm⁻¹; O–H (alcohols) 3230–3550 cm⁻¹; O–H (carboxylic acids) 2500–3000 cm⁻¹; C=C 1620–1680 cm⁻¹.

   (a)

   Which bond is responsible for the absorption at 1715 cm⁻¹?

   [1 mark]

   • AC–H
   • BO–H
   • CC=O
   • DC–O

   (b)

   Which of these compounds is consistent with the spectrum?

   [1 mark]

   • APropanone
   • BProp-2-en-1-ol
   • CPropanoic acid
   • DCyclopropanol

   (c)

   Explain how the spectrum shows that the compound is neither an alcohol nor a carboxylic acid.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A pharmaceutical chemist checks the purity of a solid drug product that was made in a reaction using ethanol as the solvent. The drug molecule contains no O–H bond. The chemist compares the infrared spectrum of each batch with the spectrum of a pure reference sample. One batch has an extra broad absorption at 3350 cm⁻¹. Data: C–H 2850–3100 cm⁻¹; O–H (alcohols) 3230–3550 cm⁻¹; O–H (carboxylic acids) 2500–3000 cm⁻¹; C=O 1680–1750 cm⁻¹.

   (a)

   What is the purpose of comparing the region below about 1500 cm⁻¹ with the reference spectrum?

   [1 mark]

   • AIt shows which functional groups are present, one by one
   • BIt acts as a fingerprint, so an exact match identifies the compound
   • CIt gives the relative molecular mass
   • DIt shows the number of carbon atoms

   (b)

   Which impurity is most likely to give the extra absorption at 3350 cm⁻¹?

   [1 mark]

   • AHexane
   • BEthanoic acid
   • CPropanone
   • DEthanol

   (c)

   Explain how the spectrum shows that this batch is impure and what the impurity is likely to be.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Two isomers, P and Q, both have the molecular formula C₃H₆O₂. The infrared spectrum of P has a very broad absorption from 2500 to 3000 cm⁻¹ and a strong absorption at 1710 cm⁻¹. The spectrum of Q has a strong absorption at 1740 cm⁻¹ and a strong absorption at 1200 cm⁻¹, but no broad absorption above 2500 cm⁻¹. Data: C=O 1680–1750 cm⁻¹; C–O 1000–1300 cm⁻¹; O–H (carboxylic acids) 2500–3000 cm⁻¹; O–H (alcohols) 3230–3550 cm⁻¹.

   (a)

   Deduce the functional group present in P and in Q, referring to the bonds responsible for the absorptions.

   [3 marks]

   (b)

   Give a possible structural formula for P and for Q. Explain how the fingerprint region of the spectrum of Q could be used to confirm its identity.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Infrared radiation is absorbed by the bonds in molecules, and each type of bond absorbs at a characteristic wavenumber. This is used both in chemical analysis and to explain the behaviour of gases in the atmosphere.

   (a)

   Explain how the absorption of infrared radiation by carbon dioxide, methane and water vapour contributes to global warming, and why increasing their concentrations makes the effect greater.

   [6 marks]

   (b)

   A student records the infrared spectrum of a liquid with the molecular formula C₄H₈O₂. It shows a strong absorption at 1710 cm⁻¹ and a broad absorption centred at 3300 cm⁻¹. The student claims that the liquid is a carboxylic acid. Evaluate this claim, using the data: C=O 1680–1750 cm⁻¹; O–H (alcohols) 3230–3550 cm⁻¹; O–H (carboxylic acids) 2500–3000 cm⁻¹.

   [6 marks]

   Total for question 4: 12 marks

End of questions