Carbon-13 NMR looks at the carbon skeleton directly. Each different carbon environment gives one line, and its chemical shift says what that carbon is bonded to. Put it together with infrared and mass spectra and an unknown compound has nowhere to hide.
Only about 1.1% of carbon atoms are 13C, but they absorb radio waves in a magnetic field just as protons do. The number of peaks equals the number of different carbon environments, so symmetry matters: propan-2-ol gives two peaks, propan-1-ol three. The chemical shift range is wide, about 0 to 220 ppm, and tells you the type of carbon: alkyl 5–55, C–O 50–90, alkene and aromatic 110–160, C=O 160–220. Routine spectra show no splitting, so every peak is a single line, and peak heights do not reliably count carbons. Combined with IR (functional groups) and mass spectra (Mr), it pins down a structure.
Draw the full structure, then look for symmetry. Carbons that can be swapped by a mirror plane or rotation are equivalent and share one peak.
| Compound | Structure | Carbon environments = peaks |
|---|---|---|
| propan-1-ol | CH3CH2CH2OH | 3 |
| propan-2-ol | CH3CH(OH)CH3 | 2 |
| propanone | CH3COCH3 | 2 |
| 2-methylpropan-2-ol | (CH3)3COH | 2 |
| benzene | C6H6 | 1 |
| methylbenzene | C6H5CH3 | 5 (four ring environments + CH3) |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
How many peaks are in the 13C NMR spectrum of propan-2-ol, CH3CH(OH)CH3?
In which range does the carbonyl carbon of propanone appear in its 13C NMR spectrum?
Why are peaks in a routine 13C NMR spectrum not split?
How many peaks does benzene give in its 13C NMR spectrum?
The more electronegative the atoms bonded to a carbon, and the more multiple bonds it has, the further left its peak appears.
| Type of carbon | δ / ppm | Example peak |
|---|---|---|
| alkyl, C–C and C–H only | 5–55 | CH3 in ethanol, 18 |
| carbon bonded to oxygen, C–O | 50–90 | CH2OH in ethanol, 58 |
| alkene C=C and aromatic ring carbons | 110–160 | benzene, 128 |
| carbonyl in acids and esters | 160–185 | ethyl ethanoate, 171 |
| carbonyl in aldehydes and ketones | 190–220 | propanone, 206 |
Each technique answers a different question. Work through the unknown in the simulator, unlocking the mass spectrum, infrared and NMR clues in turn, then combine the evidence below.
| Technique | What it tells you |
|---|---|
| mass spectrum | Mr from the M+ peak; number of carbons from the M+1 peak; fragments such as CH3CO+ at 43 |
| infrared | functional groups: C=O near 1700 cm−1, broad O–H near 3200–3600 cm−1 in alcohols |
| 13C NMR | number and type of carbon environments |
| 1H NMR | number, type and ratio of hydrogen environments, and neighbouring H from splitting |
Count environments, not carbon atoms: the two CH3 groups of propanone give one peak. 13C peaks are not split and their heights do not give the number of carbons, unlike proton peak areas. A carbon bonded to O sits at 50–90, while a C=O carbon is far further left at 160–220; do not confuse the two.
Unlocks once the four checks above are done. Worth more XP, written to A Level Paper 4 standard.
Which isomer of C4H10O gives only two peaks in its 13C NMR spectrum?
How many peaks are in the 13C NMR spectrum of methylbenzene, C6H5CH3?
Compound W, C3H6O, has an IR absorption at 1720 cm−1, three 13C NMR peaks and a 1H NMR peak at 9.8. What is W?
A 13C NMR spectrum has peaks at 180, 27 and 9. Which compound fits best?
This skill is now recorded on your profile. Keep the chain going.