Proton NMRAQA A-Level Chemistry: Subtopic test

1. 1
   A chemist records ¹H NMR spectra of organic compounds to find how many hydrogen atoms are in each environment. The sample for each spectrum is dissolved in a solvent, and tetramethylsilane (TMS) is added as the standard.

   (a)

   Which solvent is suitable for recording a ¹H NMR spectrum?

   [1 mark]

   • ACDCl₃
   • BCHCl₃
   • CCH₃CH₂OH
   • DH₂O

   (b)

   A compound contains 8 hydrogen atoms. Its ¹H NMR spectrum has three peaks, and the integration trace shows steps of 1.5 cm, 1.0 cm and 1.5 cm. How many hydrogen atoms give the peak with the 1.0 cm step?

   [1 mark]

   • A1
   • B2
   • C3
   • D4

   (c)

   The integration values for the three peaks in the spectrum of an ester, C₄H₈O₂, are 6.0, 4.0 and 6.0 (arbitrary units). Explain how these data are used to work out the relative numbers of equivalent protons.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   The ¹H NMR spectra of aliphatic compounds show splitting of peaks by hydrogen atoms on adjacent carbon atoms. A student is predicting the splitting patterns for 1,1-dichloroethane, CH₃CHCl₂, using the n + 1 rule.

   (a)

   What is the splitting pattern of the signal from the CH₃ protons in 1,1-dichloroethane?

   [1 mark]

   • ASinglet
   • BTriplet
   • CDoublet
   • DQuartet

   (b)

   Which compound gives a ¹H NMR spectrum consisting of a single peak (a singlet)?

   [1 mark]

   • ACH₃CH₂Br
   • BCH₃CH₂CH₃
   • CCH₃CH₂OH
   • D(CH₃)₄C

   (c)

   Use the n + 1 rule to explain why the signal from the CH₃ protons in 1,1-dichloroethane is split in the way you chose in part (a).

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Compound R has the molecular formula C₄H₈O₂. Its ¹H NMR spectrum has three peaks: δ = 1.2 (triplet, relative area 3), δ = 2.1 (singlet, relative area 3) and δ = 4.1 (quartet, relative area 2). Shift data (δ / ppm): R–CH₃ 0.7–1.2; CH₃ next to C=O 2.1–2.6; CH₂ bonded to the O of an ester 3.7–4.1.

   (a)

   Explain how the splitting patterns in the spectrum of R show that it contains an ethyl group, –CH₂CH₃, attached to an atom with no hydrogen atoms.

   [3 marks]

   (b)

   Use the chemical shift, integration and splitting data to deduce the structure of R, explaining the origin of each peak.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student studies the ¹H NMR spectra of carbonyl compounds and esters. Shift data (δ / ppm): R–CH₃ 0.7–1.2; CH₃ or CH₂ next to C=O 2.1–2.6; CH₃ or CH₂ bonded to the O of an ester 3.7–4.1.

   (a)

   Predict the ¹H NMR spectrum of butanone, CH₃COCH₂CH₃. Give the number of peaks, their relative areas, their splitting patterns and approximate chemical shifts, explaining each.

   [6 marks]

   (b)

   Compound T has the molecular formula C₄H₈O₂ and its ¹H NMR spectrum has three peaks: δ = 1.1 (triplet, relative area 3), δ = 2.3 (quartet, relative area 2) and δ = 3.7 (singlet, relative area 3). Deduce the structure of T and explain why ethyl ethanoate, CH₃COOCH₂CH₃, is not consistent with the spectrum.

   [6 marks]

   Total for question 4: 12 marks

End of questions