Proton NMREdexcel A-Level Chemistry: Subtopic test

1. 1
   A student records the high-resolution 1H NMR spectrum of bromoethane, CH₃CH₂Br.

   (a)

   How many peaks are there in the high-resolution 1H NMR spectrum of bromoethane?

   [1 mark]

   • A1
   • B2
   • C3
   • D5

   (b)

   What is the splitting pattern of the peak caused by the CH₂ protons in bromoethane?

   [1 mark]

   • ASinglet
   • BDoublet
   • CTriplet
   • DQuartet

   (c)

   Explain why the peak caused by the CH₃ protons is a triplet.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A chemist records the high-resolution 1H NMR spectrum of ethanal, CH₃CHO. It shows two peaks, at chemical shifts (δ) of 2.2 ppm and 9.8 ppm.

   (a)

   What is the ratio of the area under the peak at δ 2.2 ppm to the area under the peak at δ 9.8 ppm?

   [1 mark]

   • A3 : 1
   • B1 : 3
   • C1 : 1
   • D2 : 1

   (b)

   Which proton is responsible for the peak at δ 9.8 ppm?

   [1 mark]

   • AThe three CH₃ protons
   • BA proton of an O–H group
   • CThe proton of the CHO group
   • DA proton attached to a benzene ring

   (c)

   Explain the splitting patterns of the two peaks in the spectrum of ethanal.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A chemist has an unlabelled ester which is either ethyl ethanoate, CH₃COOCH₂CH₃, or methyl propanoate, CH₃CH₂COOCH₃. Its high-resolution 1H NMR spectrum has three peaks: δ 1.2 (triplet, relative area 3), δ 2.0 (singlet, relative area 3) and δ 4.1 (quartet, relative area 2). Typical shift ranges (ppm): CH₃ in an alkyl chain 0.7–1.3; CH₃ or CH₂ next to C=O 2.0–2.7; CH₃ or CH₂ bonded to the oxygen of an ester 3.6–4.3.

   (a)

   Explain how the relative areas and the splitting patterns show that the ester contains a CH₃CH₂ group.

   [3 marks]

   (b)

   Deduce which ester it is. Justify your answer using chemical shift values and explain what the spectrum of the other ester would show.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A chemist analyses organic liquids using high-resolution 1H NMR spectroscopy. Typical shift ranges (ppm): CH₃ bonded to an alkyl carbon 0.7–1.2; CH₃ or CH₂ next to C=O 2.0–2.7; aldehyde CHO 9.4–10.0; carboxylic acid O–H 10.0–12.0.

   (a)

   Predict the high-resolution 1H NMR spectrum of propanoic acid, CH₃CH₂COOH, giving the chemical shift range, relative area and splitting pattern of each peak.

   [6 marks]

   (b)

   Compound B has the molecular formula C₄H₈O. Its spectrum has three peaks: δ 1.0 (triplet, relative area 3), δ 2.1 (singlet, relative area 3) and δ 2.4 (quartet, relative area 2). There is no peak between 9.4 and 10.0 ppm. Deduce the structure of B, justifying your answer.

   [6 marks]

   Total for question 4: 12 marks

End of questions