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6.9.4Organic Chemistry and Analysis

Carbon-13 NMR — OCR A Level Chemistry A H432 Revision Notes & Topical Questions

5 min read 5 parts Lesson 31 of 33

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Part 1 of 5Principles of Carbon-13 NMR

6.9.4 Carbon-13 NMR

Carbon-13 nuclei behave like tiny magnets and absorb radio waves in a strong magnetic field.

Environments
  • Each peak in the spectrum corresponds to one distinct carbon environment, so symmetrical molecules show fewer peaks.
Standard
  • Chemical shifts are measured in ppm relative to tetramethylsilane (TMS), whose carbon peak is set at 0.
Solvents
  • The sample is dissolved in a deuterated solvent such as CDCl₃, in which hydrogen has been replaced by deuterium.
Information
  • The spectrum gives the number of environments and their chemical shifts, but peak heights are not simply proportional to atom numbers.
Splitting
  • In standard carbon-13 spectra each environment appears as a single line, so the pattern of splitting is not used for interpretation.

Part 2 of 5Chemical Shifts

Chemical Shifts
Alkyl carbons
  • Carbons bonded only to C and H appear at the lowest shifts, roughly 5–40 ppm.
Carbons with N, O or halogen
  • Electronegative atoms increase the shift, with C–O around 50–90 ppm.
Alkene and aromatic carbons
  • Carbons in C=C and in a benzene ring appear at about 90–160 ppm.
Carbonyl carbons
  • Carbons in C=O appear at the highest shifts, about 160–220 ppm, with ketones and aldehydes above esters and acids.
Type of carbonApproximate shift / ppm
C–C (alkyl)5–40
C–O50–90
C=C and aromatic90–160
C=O ester or acid160–185
C=O aldehyde or ketone190–220

Part 3 of 5Counting Environments

1
Draw the structure in full and look for planes or axes of symmetry.
2
Carbons related by symmetry share one environment and produce one peak.
3
Carbons with different neighbours or different groups produce separate peaks.
4
Count the unique environments to predict the number of peaks in the spectrum.
5
Match the shifts to the likely type of carbon to confirm the structure.

Part 4 of 5Worked Example

Question State the number of peaks in the carbon-13 NMR spectra of propanone, pentan-3-one and 2-methylpropan-2-ol.

  1. Propanone CH₃COCH₃the two CH₃ carbons are equivalent, plus the C=O carbon, giving 2 peaks.
  2. Pentan-3-one CH₃CH₂COCH₂CH₃: the two CH₃ are equivalent, the two CH₂ are equivalent and C=O is unique, giving 3 peaks.
  3. 2-methylpropan-2-ol (CH₃)₃COH: the three CH₃ carbons are equivalent and the C–OH carbon is different, giving 2 peaks.

Answer Propanone has 2 peaks, pentan-3-one has 3 peaks and 2-methylpropan-2-ol has 2 peaks.

Part 5 of 5Exam Checklist & Tips

6.9.4 – Key Points for the Exam

  • Chemical shift The position of a peak relative to TMS, measured in ppm.
  • TMS Tetramethylsilane, the reference compound set at 0 ppm.
  • Carbon environment A set of carbon atoms in an identical bonding and symmetry situation.
  • Deuterated solvent A solvent in which hydrogen atoms are replaced by deuterium.

Common Mistakes

  • Counting every carbon atom as a separate peak instead of every environment.
  • Using peak height to determine the number of carbons in carbon-13 NMR.
  • Placing a C=O carbon at 50–90 ppm instead of above 160 ppm.
  • Forgetting that TMS is the reference at 0 ppm.

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