Carbon-13 NMREdexcel A-Level Chemistry: Subtopic test

1. 1
   A technician in a teaching laboratory has two unlabelled bottles of propanol isomers, one containing propan-1-ol and the other propan-2-ol. Both have the molecular formula C₃H₈O. She plans to tell them apart by recording the 13C NMR spectrum of each.

   (a)

   How many peaks are there in the 13C NMR spectrum of propan-1-ol, CH₃CH₂CH₂OH?

   [1 mark]

   • A1
   • B2
   • C3
   • D4

   (b)

   Which carbon atom in propan-1-ol is responsible for the peak at the highest chemical shift?

   [1 mark]

   • AThe carbon of the CH₂ group bonded to the OH group
   • BThe carbon of the CH₃ group
   • CThe carbon of the middle CH₂ group
   • DNone, because all three carbons give a peak at the same shift

   (c)

   Propan-2-ol, CH₃CH(OH)CH₃, gives only two peaks in its 13C NMR spectrum. Explain why.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A pharmaceutical chemist records the 13C NMR spectrum of the solvent ethyl ethanoate, CH₃COOCH₂CH₃. It shows four peaks, at chemical shifts (δ) of 14, 21, 60 and 171 ppm.

   (a)

   How many different carbon environments are there in ethyl ethanoate, CH₃COOCH₂CH₃?

   [1 mark]

   • A1
   • B2
   • C3
   • D4

   (b)

   Which carbon atom gives the peak at δ 60 ppm?

   [1 mark]

   • AThe carbon of the C=O group
   • BThe carbon of the CH₂ group bonded to oxygen
   • CThe carbon of the CH₃ group bonded to the C=O group
   • DThe carbon of the CH₃ group of the ethyl group

   (c)

   Explain why the peak at δ 171 ppm is at a much higher chemical shift than the peak at δ 14 ppm.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A laboratory holds four isomeric alcohols of molecular formula C₄H₁₀O: butan-1-ol, butan-2-ol, 2-methylpropan-1-ol and 2-methylpropan-2-ol. A technician uses 13C NMR spectroscopy to tell them apart. Typical chemical shift ranges (ppm): C–C 5–40; C–O 50–90.

   (a)

   Deduce the number of peaks in the 13C NMR spectra of 2-methylpropan-2-ol and of 2-methylpropan-1-ol, and justify your answers.

   [3 marks]

   (b)

   A sample from the laboratory gives a 13C NMR spectrum with three peaks, at δ 19, 31 and 69 ppm. Deduce which isomer the sample is. Justify your answer and explain the peak at δ 69 ppm.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   An analytical chemist uses 13C NMR spectroscopy to identify isomers in samples from a chemical plant. She counts the peaks in each spectrum and uses these typical chemical shift ranges (ppm): carbon in alkyl groups (C–C) 5–40; ring carbons in benzene derivatives 110–160; C=O in ketones and aldehydes 190–220.

   (a)

   Explain how 13C NMR spectroscopy can be used to distinguish between 1,2-, 1,3- and 1,4-dimethylbenzene.

   [6 marks]

   (b)

   A student claims that compound Z, a ketone of formula C₅H₁₀O, is pentan-2-one. The 13C NMR spectrum of Z has three peaks, at δ 8, 35 and 211 ppm. Evaluate the student's claim and deduce the structure of Z.

   [6 marks]

   Total for question 4: 12 marks

End of questions