Topic 15 · page 22 of 28

Carbon-13 NMR

Each carbon atom in a different environment gives one peak in a carbon-13 NMR spectrum. The chemical shift of the peak shows what the carbon is bonded to, and the number of peaks shows how many different environments there are.

15 min read15.1815.19(i)15.19(ii)

What carbon-13 NMR measures 15.18

Most carbon atoms are carbon-12, which does not show NMR. About 1% are carbon-13, whose nucleus behaves like a tiny magnet and does. Placed in a strong magnetic field, carbon-13 nuclei absorb radio waves of a frequency that depends on the atoms around them.

The spectrum is a plot of absorption against chemical shift, δ, in parts per million (ppm). δ is measured relative to tetramethylsilane (TMS), which is given the value 0. The axis runs from high δ on the left to 0 on the right, and a peak that is further left is called more downfield.

One peak per environment 15.19(ii)

Carbon atoms in the same environment give one peak. Two carbons are in the same environment when they are bonded to the same atoms in the same way, including when symmetry makes them equivalent.

Three schematic carbon-13 NMR spectra on a reversed axis from 220 to 0 ppm. Propanone has two peaks at 206 for C=O and 30 for CH3. Pentan-3-one has three peaks at 211 for C=O, 36 for CH2 and 8 for CH3. Butanone has four peaks at 209 for C=O, 37 for CH2, 29 for CH3 next to the C=O and 7 for the other CH3.
Each spectrum has one peak per carbon environment. The C=O peak is far downfield.
Compound Carbon atoms Carbon environments Peaks
propanone, CH₃COCH₃ 3 C=O, and 2 equivalent CH₃ 2
pentan-3-one, CH₃CH₂COCH₂CH₃ 5 C=O, 2 equivalent CH₂, 2 equivalent CH₃ 3
butanone, CH₃COCH₂CH₃ 4 C=O, CH₂, CH₃ next to C=O, other CH₃ 4

The number of peaks is less than the number of carbon atoms whenever some carbons are equivalent by symmetry. To justify a number of peaks, say: "there are n peaks because there are n carbon atoms in different environments", and name them.

Chemical shifts 15.19(i)

The shift tells you what kind of carbon each peak is, using a data table. Use the ranges given in your exam booklet, and compare with these typical values.

Bar chart of carbon-13 chemical shift ranges on a reversed axis from 220 to 0 ppm: C=O in aldehydes and ketones 190 to 220, C=O in acids and esters 160 to 185, C=C and aromatic carbon 90 to 150, C–O 50 to 90, C–C alkyl 5 to 55.
Typical carbon-13 shift ranges. The C=O carbon is the most downfield.
Carbon environment δ / ppm
C=O in aldehydes and ketones 190–220
C=O in carboxylic acids and esters 160–185
C=C 90–150
C–O in alcohols and esters 50–90
C–C (alkyl) 5–55

The more electronegative the atoms around a carbon, the further downfield its peak. A carbon bonded to oxygen is further left than a carbon bonded only to carbon and hydrogen.

Worked exampleEthyl ethanoate, CH₃COOCH₂CH₃

Predict the number of peaks and the region of each.

  1. C=O (ester): in the 160–185 range, about 171.
  2. –OCH₂–: carbon bonded to oxygen, in the 50–90 range, about 60.
  3. CH₃ next to the C=O: alkyl carbon, in the 5–55 range, about 21.
  4. CH₃ of the ethyl group: alkyl carbon, in the 5–55 range, about 14.

Four different environments give 4 peaks.

Check yourself

  1. Explain why a carbon-13 NMR spectrum of propanone has two peaks and not three.
  2. State the number of peaks in the carbon-13 NMR spectrum of each: (a) propan-2-ol, (b) propan-1-ol, (c) 2-methylpropan-2-ol, (d) pentan-2-one.
  3. A compound has a carbon-13 NMR peak at δ = 205 and another at δ = 30. Suggest which carbon atoms they are.
  4. Which carbon-13 peak is most downfield in ethyl ethanoate? Explain why.
  5. Pentan-2-one and pentan-3-one are isomers. How could their carbon-13 spectra be used to tell them apart?

Answers cover these until you have tried the questions

  1. The two CH₃ carbons are in the same environment, because each is bonded to the same C=O carbon and three hydrogens. They are equivalent by symmetry and give one peak. The C=O carbon gives the other, so there are 2 peaks.
  2. (a) 2: two equivalent CH₃ carbons, and the CH(OH) carbon. (b) 3: CH₃, CH₂ and CH₂OH are all different. (c) 2: three equivalent CH₃ carbons, and the central C–OH carbon. (d) 5: all five carbon atoms are in different environments.
  3. δ = 205 is in the 190–220 range: the C=O carbon of an aldehyde or ketone. δ = 30 is in the 5–55 range: a C–C carbon (an alkyl carbon, here probably a CH₃ next to the C=O).
  4. The C=O carbon (about 171), which is bonded to two oxygen atoms, which are very electronegative and draw electron density away from it, so its peak is furthest downfield.
  5. Pentan-2-one has 5 carbon environments, so 5 peaks. Pentan-3-one is symmetrical and has only 3 peaks. Counting the peaks is enough to tell them apart.