Carbon-13 NMREdexcel A-Level Chemistry: Revision notes
Section 1
What 13C NMR shows
13C NMR spectroscopy detects the carbon-13 isotope, which has nuclear spin (carbon-12 does not). Only about 1% of carbon atoms are 13C, but this is enough to give a spectrum.
The sample is placed in a strong magnetic field and absorbs radio-frequency radiation. The spectrum shows one peak for each carbon environment in the molecule, so it tells you the positions of the carbon atoms in the structure.
The position of a peak is its chemical shift, δ, measured in ppm relative to a reference compound, tetramethylsilane (TMS), which is set at 0 ppm.
Section 2
Carbon environments and the number of peaks
Carbon atoms are in the same environment if they are bonded to the same atoms and groups in the same way, usually because of symmetry. Equivalent carbons give one peak.
To predict the number of peaks:
- Draw the full structure and look for symmetry
- Group equivalent carbons
- Count the groups
Examples: propan-2-ol, CH₃CH(OH)CH₃, has 2 peaks (two equivalent CH₃ carbons, one CH carbon). 2-Methylpropan-2-ol has 2 peaks (three equivalent CH₃ carbons, one C–OH carbon). 1,4-dimethylbenzene has 3 peaks (CH₃, two methyl-bearing ring carbons, four ring CH carbons).
Do not count the carbon atoms in the formula. Count the carbon environments: C₆H₁₄ can give far fewer than six peaks.
Section 3
Chemical shift and electronegative atoms
Electronegative atoms (O, N, Cl, Br) withdraw electron density from a neighbouring carbon, which deshields it. A more deshielded carbon has a higher chemical shift.
Typical ranges (ppm):
- C–C (alkyl): 5–40
- C–N: 35–60
- C–O: 50–90
- C=C: 90–150
- Aromatic ring carbons: 110–160
- C=O in acids and esters: 160–185
- C=O in aldehydes and ketones: 190–220
The more electronegative atoms bonded to a carbon, the higher its shift, which is why a C=O carbon appears far downfield of an alkyl carbon.
Always use the data booklet table of shift ranges. Quote the range when you assign a peak, e.g. 'δ 69 lies in the C–O range, 50–90 ppm'.
Section 4
Worked example: choosing between isomers
A ketone C₅H₁₀O gives three peaks, at δ 8, 35 and 211.
- Three peaks means three carbon environments, so the molecule is symmetrical.
- The peak at 211 lies in the 190–220 range, so it is a ketone C=O carbon.
- Pentan-2-one would give 5 peaks and 3-methylbutan-2-one would give 4.
- Pentan-3-one, CH₃CH₂COCH₂CH₃, has equivalent CH₃ carbons, equivalent CH₂ carbons and one C=O carbon: 3 peaks.
The compound is pentan-3-one.
Section 5
Justifying a structure from 13C data
To justify a structure, follow this order:
- State the number of peaks, and so the number of carbon environments
- Use symmetry to explain which carbons are equivalent
- Assign peaks using chemical shift ranges and the groups present
- Check that the structure fits the formula and the data, and that other isomers do not
The number of peaks can be fewer than the number of carbon atoms but never more.
Writing 'the peak is at 211 so it is a ketone' is not enough. Name the carbon (the C=O carbon) and give the shift range.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Carbon-13 NMR
- 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.Propan-2-ol, CH₃CH(OH)CH₃, gives only two peaks in its 13C NMR spectrum. Explain why.2 marks
- 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.Explain why the peak at δ 171 ppm is at a much higher chemical shift than the peak at δ 14 ppm.2 marks
- 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.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
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).