NMR spectroscopyEdexcel International A Level Chemistry: Subtopic test

1. 1
   Three structural isomers of formula C₄H₁₀O are examined by ¹³C NMR spectroscopy: butan-1-ol, butan-2-ol and 2-methylpropan-2-ol.

   (a)

   How many peaks are seen in the ¹³C NMR spectrum of butan-2-ol, CH₃CH(OH)CH₂CH₃?

   [1 mark]

   • A1
   • B2
   • C3
   • D4

   (b)

   Which isomer gives a ¹³C NMR spectrum with exactly two peaks?

   [1 mark]

   • AButan-1-ol
   • B2-methylpropan-2-ol
   • CButan-2-ol
   • DAll three isomers

   (c)

   Explain why the ¹³C NMR spectrum of 2-methylpropan-2-ol, (CH₃)₃COH, has only two peaks even though the molecule has four carbon atoms.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   The ¹H NMR spectrum of ethyl ethanoate, CH₃COOCH₂CH₃, is recorded in a solvent that contains no hydrogen atoms, using tetramethylsilane (TMS) as a reference standard.

   (a)

   Which statement gives a reason for using TMS as the reference standard?

   [1 mark]

   • AAll 12 hydrogen atoms are in the same environment, giving one sharp peak at a lower chemical shift than most other protons
   • BIt is polar and dissolves in water
   • CIt has the highest chemical shift of any proton
   • DIt splits the peaks of the sample into multiplets

   (b)

   What is the ratio of the peak areas in the ¹H NMR spectrum of ethyl ethanoate?

   [1 mark]

   • A1 : 1 : 1
   • B3 : 5
   • C3 : 2 : 3
   • D2 : 2 : 3

   (c)

   Deduce the splitting patterns of the signals from the CH₂ group and from the CH₃ of the ethyl group in ethyl ethanoate, explaining your answers.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Compound Z has the molecular formula C₄H₈O₂ and is an ester. Its ¹H NMR spectrum has three peaks: δ 3.7 (singlet, relative peak area 3), δ 2.3 (quartet, relative peak area 2) and δ 1.1 (triplet, relative peak area 3). Its ¹³C NMR spectrum has four peaks.

   (a)

   Use the ¹H NMR data to deduce the structure of compound Z. Explain the singlet, the quartet and the triplet.

   [3 marks]

   (b)

   Another isomer of Z is propan-2-yl methanoate, HCOOCH(CH₃)₂. Explain how its ¹³C NMR and ¹H NMR spectra would differ from those of Z.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Two structural isomers of C₃H₇Br, 1-bromopropane (CH₃CH₂CH₂Br) and 2-bromopropane ((CH₃)₂CHBr), are in unlabelled bottles. A technician plans to identify them using NMR spectroscopy.

   (a)

   Explain how the ¹³C NMR spectra and the low-resolution ¹H NMR spectra would allow the technician to tell the two isomers apart.

   [6 marks]

   (b)

   Predict the splitting pattern and relative chemical shift of each signal in the high-resolution ¹H NMR spectrum of 1-bromopropane, and state how the CH and CH₃ signals of 2-bromopropane are split.

   [6 marks]

   Total for question 4: 12 marks

End of questions