Advanced Organic Chemistry: The carbon-13 NMR spectrum of ethanol

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Interpreting & explaining the 13C NMR spectrum of ethanol (ethyl alcohol)

[Author © Dr Phil Brown GRIC, PhD: Doc Brown's advanced level organic chemistry exam revision notes suitable for students of UK A level chemistry courses & US K12 grade 11, grade 12 and AP honors chemistry courses: Molecular spectroscopy analysis of ethanol [spectra page updated RE-EDIT]

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 Key points and practice questions

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 C-13 NMR spectroscopy - spectra index

Introductory note on the 13C NMR spectrum of ethanol

Students and teachers please note that my explanation of the carbon-13 NMR spectrum of ethanol is designed for advanced, but pre-university, chemistry courses.

The description does not involve the chemical shift δ spin-spin coupling effects for ethanol and the relative size of the carbon-13 NMR shifts does not give the ratio of the carbon atoms in the different non-equivalent chemical environments of the ethanol molecule.

The most common solvent used for investigating the C13 NMR spectrum of compounds like ethanol, is CDCl3 and other deuterated solvents.

C-13 nmr spectrum of ethanol analysis of chemical shifts ppm interpretation of C-13 chemical shifts ppm of ethyl alcohol doc brown's advanced organic chemistry revision notes 

TMS is the acronym for tetramethylsilane, formula Si(CH3)4, whose 13C atoms are arbitrarily given a chemical shift of 0.0 ppm. This is the 'standard' in 13C NMR spectroscopy and all other 13C shifts, called chemical shifts, depend on the individual (electronic) chemical environment of the 13C atoms in an organic molecule - ethanol here.

Ethanol C2H6O, alcohols and ether structure and naming (c) doc b , alcohols and ether structure and naming (c) doc b , alcohols and ether structure and naming (c) doc b , alcohols and ether structure and naming (c) doc b aliphatic alcohol

Revision notes on the structure and naming (nomenclature) of aliphatic ALCOHOLS and ETHERS

Interpreting the C-13 NMR spectrum of ethanol

As you can see from the diagram above there are only 2 chemical shift lines in the C-13 NMR spectrum of ethanol indicating 2 different chemical environments of carbon atoms.

CH3CH2OH

(Note the 2 colours indicating the different chemical environment of the carbon atoms in ethanol).

There are only two possible 13C NMR shifts in the ethanol molecule, 18.1 and 57.8 ppm.

The carbon attached to the very electronegative oxygen atom of the hydroxy group tends have the higher chemical shift than the carbon atom of an alkyl group.

See also comparing the IR, mass, 1H NMR and 13C NMR spectra of isomers of C2H6O below.

Key points and practice questions based on the 13C NMR spectrum of ethanol

Ethanol’s 13C NMR spectrum shows two distinct signals: one for the methyl carbon (~18 ppm) and one for the methylene carbon bonded to OH (~58 ppm). These shifts help distinguish ethanol from isomers like methoxymethane.

Practice multiple choice questions based on the 13C NMR spectrum of ethanol

Key 13C NMR Shifts of Ethanol

Chemical Shift (ppm) Carbon Type Environment
~18,  18.1 ppm CH3 Methyl carbon (CH3–CH2–OH)
~58,  57.8 ppm CH2 Methylene bonded to OH (CH3–CH3–OH)

Sources: Doc Brown’s 13C NMR notes, ChemicalBook spectrum data

Common Misconceptions

  • Expecting three peaks: Ethanol has only two unique carbon environments.
  • Misidentifying CH2 shift: Students may confuse ~58 ppm with ether or ester carbons.
  • Assuming OH gives a carbon signal: OH is a proton, not a carbon—no direct ¹³C signal.
  • Confusing ethanol with methoxymethane: Methoxymethane only shows one 13C signal, both carbon atoms are equivalent, not so in ethanol.

Exam Revision Tips

  • Count unique carbon environments: Ethanol has two; symmetry matters.
  • Use chemical shift ranges: CH3 ~10–30 ppm, CH2–OH ~50–65 ppm.
  • Compare with isomers: Methoxymethane has different shifts due to ether linkage.
  • Annotate spectra: Label each peak with environment and shift.
  • Link to structure: Use skeletal formulas to predict environments.

Practice Multiple Choice Questions based on the 13C NMR spectrum of ethanol

Each question includes feedback and distractor analysis.

Jot your responses down and check out your answers

ANSWERS to the Practice Multiple Choice Questions

Q1. How many distinct carbon signals appear in ethanol’s 13C NMR spectrum?

  1. 1
  2. 2
  3. 3
  4. 4

Q2. What is the likely chemical shift of the CH3 carbon in ethanol?

  1. ~10 ppm
  2. ~18 ppm
  3. ~58 ppm
  4. ~74 ppm

Q3. Which carbon gives the signal at ~58 ppm in ethanol?

  1. CH3
  2. CH2–OH
  3. OH
  4. C=O

Q4. Why does ethanol show only two 13C NMR signals?

  1. It has two carbon atoms
  2. It has symmetrical structure
  3. CH3 and CH2 are equivalent
  4. OH does not give a signal

Q5. Which molecule shows three different 13C NMR signals?

  1. Propan-1-ol
  2. Methanol
  3. Methoxymethane
  4. Ethanoic acid

Q6. What causes the downfield shift (~58 ppm) in ethanol’s spectrum?

  1. CH3 group
  2. CH2 bonded to OH
  3. CH2 bonded to CH3
  4. Aromatic ring

Q7. Which feature best distinguishes methoxyethane from ethanol in 13C NMR?

  1. Number of signals
  2. Presence of OH
  3. Pattern of chemical shifts
  4. Integration of signal strength

Q8. Why is OH not seen in 13C NMR?

  1. It’s a weak signal
  2. It’s a proton
  3. It’s shielded
  4. It’s symmetrical

ANSWERS to the Practice Multiple Choice Questions

 
Comparing the infrared, mass, 1H NMR and 13C NMR spectra of the 2 isomers of C2H6O

NOTE: The images are linked to their original detailed spectral analysis pages AND can be doubled in size with touch screens to increase the definition to the original ethanol (ethyl alcohol) and methoxymethane (dimethyl ether) image sizes.

INFRARED SPECTRA: Apart from the significant differences in the fingerprint region at wavenumbers 1500 to 400 cm-1, the most striking difference is the broad O-H stretching band ~3400 cm-1, found in the infrared spectrum of alcohols, but absent in the infrared spectrum of ethers.

MASS SPECTRA: Both ethanol and methoxymethane show some similarities in their mass spectra, but their base ion peaks are quite different - for ethanol it is m/z 31 and for methoxymethane it is m/z 45.

1H NMR SPECTRA: The 1H NMR spectra of ethanol and methoxymethane are quite significantly different. Ethanol gives 3 peaks in the proton ratio 3:2:1 (3 different chemical environments), whereas methoxymethane only gives one 1H chemical shift peak (all 6 protons in the same chemical environment).

13C NMR SPECTRA: The 13C NMR spectra of ethanol and methoxymethane are different. Ethanol gives two 13C resonances, but methoxymethane only one (2 different 13C chemical environments and a 13C single chemical environment).

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ANSWERS to the Practice Multiple Choice Questions based on the 13C NMR spectrum of ethanol

Each question includes feedback and distractor analysis.

Q1. How many distinct carbon signals appear in ethanol’s 13C NMR spectrum?

  1. 1
  2. 2
  3. 3
  4. 4

Answer: B

Feedback: Ethanol has two unique carbon environments: CH3 and CH2–OH.

Distractors:

  • A: Misses one carbon
  • C/D: Overcounts environments

Q2. What is the likely chemical shift of the CH3 carbon in ethanol?

  1. ~10 ppm
  2. ~18 ppm
  3. ~58 ppm
  4. ~74 ppm

Answer: B

Feedback: Methyl carbon appears around 18 ppm.

Distractors:

  • A: Too low
  • C: CH2–OH
  • D: Methoxymethane O–CH3

Q3. Which carbon gives the signal at ~58 ppm in ethanol?

  1. CH3
  2. CH2–OH
  3. OH
  4. C=O

Answer: B

Feedback: CH2 bonded to OH appears around 58 ppm, electronegative atoms like oxygen increase the value of the chemical shift.

Distractors:

  • A: ~18 ppm
  • C: OH is not a carbon
  • D: Not present in ethanol

Q4. Why does ethanol show only two 13C NMR signals?

  1. It has two carbon atoms
  2. It has symmetrical structure
  3. CH3 and CH2 are equivalent
  4. OH does not give a signal

Answer: A

Feedback: Ethanol has two carbon atoms in different environments.

Distractors:

  • B: Ethanol is not symmetrical
  • C: CH3 and CH2 are not equivalent
  • D: OH is not a carbon

Q5. Which molecule shows three different 13C NMR signals?

  1. Propan-1-ol 3 13C δ
  2. Methanol 1 13C δ
  3. Methoxymethane 1 13C δ
  4. Ethanoic acid 2 13C δ

Answer: A

Feedback:

  • B 1, C 1, D 2

Q6. What causes the downfield shift (~58 ppm) in ethanol’s spectrum?

  1. CH3 group
  2. CH2 bonded to OH
  3. CH2 bonded to CH3
  4. Aromatic ring

Answer: B

Feedback: Electronegative OH deshields CH2, shifting it downfield, increase in chemical shift.

Distractors:

  • A: Upfield (~18 ppm)
  • C: Not present
  • D: Not in ethanol

Q7. Which feature best distinguishes methoxyethane from ethanol in 13C NMR?

  1. Number of signals
  2. Presence of OH
  3. Pattern of chemical shifts
  4. Integration of signal strength

Answer: A

Feedback: 1 and 2 13C chemical shifts respectively.

Distractors:

  • A: Both show two signals
  • B: OH not seen in 13C
  • D: Integration not used in 13C

Q8. Why is OH not seen in 13C NMR?

  1. It’s a weak signal
  2. It’s a proton
  3. It’s shielded
  4. It’s symmetrical

Answer: B

Feedback: OH is a hydrogen, not a carbon—no 13C signal.

Distractors:

  • A/C/D: Misconceptions about signal origin

Associated links with ethanol

The infrared spectrum of Ethanol (ethyl alcohol)

The mass spectrum of Ethanol (ethyl alcohol)

The H-1 NMR spectrum of Ethanol (ethyl alcohol)

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