37.3 Carbon-13 NMR spectroscopy
- Syllabus
- 9701–2028–2029
- Topic
- 37.3
- Level
- A2
Learning objectives
37.3.1Carbon-13 NMR interpretation• Analyse and interpret a carbon-13 NMR spectrum of a simple molecule to deduce:: (a) the different environments of the carbon atoms present; (b) the possible structures for the molecule37.3.2Carbon-13 NMR peak count• Predict or explain the number of peaks in a carbon-13 NMR spectrum for a given molecule
Use carbon-13 peak count and shifts together to constrain a structure
To interpret a simple 13C NMR spectrum: 1) count sample peaks to find the number of chemically distinct carbon environments; 2) assign each chemical-shift region to plausible carbon environments; 3) combine those constraints with the molecular formula and other data; 4) draw candidate structures; 5) reject any candidate whose environment count or required shifts disagree.
| Approximate delta / ppm | Carbon environment indicated |
|---|---|
| 0-50 | saturated alkyl carbon |
| 30-70 | saturated carbon near C=O, halogen or O (use the narrower supplied data ranges) |
| 100-125 | nitrile carbon |
| 110-160 | alkene or arene carbon |
| 160-185 | carboxylic acid or ester carbonyl carbon |
| 190-220 | aldehyde or ketone carbonyl carbon |
Propanone gives two sample signals: one in the ketone C=O region for the carbonyl carbon and one in the saturated-carbon region for the two equivalent CH3 carbons. Two peaks therefore represent three carbon atoms in two environments.
Treat TMS at 0 ppm as the reference and ignore a labelled solvent peak such as CDCl3 near 77 ppm. Routine 13C peak heights are not directly proportional to the number of carbons in an environment.
Peak count alone rarely proves one structure, and a shift region alone does not determine connectivity. A proposed structure must satisfy both the number of distinct environments and every chemically plausible peak position.
Predict one carbon-13 peak for each chemically distinct carbon environment
Label every carbon, compare the atoms/groups reached in each direction, and merge only carbons related by genuine molecular symmetry or chemical equivalence. The number of remaining classes is the predicted number of 13C signals.
| Molecule | Carbon atoms | Distinct environments / predicted peaks | Reason |
|---|---|---|---|
| propane | 3 | 2 | the two terminal CH3 carbons are equivalent |
| propanone | 3 | 2 | the two CH3 carbons are equivalent; C=O is distinct |
| propan-2-ol | 3 | 2 | the two CH3 carbons are equivalent; central C-O carbon is distinct |
| butan-2-one | 4 | 4 | carbonyl C, adjacent CH3, CH2 and terminal CH3 all have different surroundings |
| benzene | 6 | 1 | all six ring carbons are symmetry-equivalent |
To explain a reduced peak count, state which carbons are equivalent and identify the symmetry that exchanges them without changing the molecule. Merely saying 'it is symmetrical' is incomplete if the equivalent positions are not identified.
Do not count carbon atoms, hydrogens or peak height. Butan-2-one is not a three-environment molecule: its two terminal CH3 groups are not equivalent, and its CH2 and carbonyl carbon add two further environments, giving four.