Advanced Organic Chemistry: The 13C NMR spectrum of ethanoic acid CH3COOH

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Interpreting and explaining the 13C NMR spectrum of ethanoic acid (acetic acid) CH3COOH

[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 ethanoic acid [spectra page updated RE-EDIT]

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 C-13 NMR spectroscopy - spectra index

See also comparing the infrared, mass, 1H and 13C NMR spectra of two isomers of C2H4O2

Exam practise questions based on the 13C NMR spectrum of ethanoic acid

Introductory note on the 13C NMR spectrum of ethanoic acid

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

The description does not involve the chemical shift δ spin-spin coupling effects for ethanoic acid 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 ethanoic acid molecule.

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

C-13 nmr spectrum of ethanoic acid analysis of chemical shifts ppm interpretation of C-13 chemical shifts ppm of ethanoic acid 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 - ethanoic acid here.

Ethanoic acid, C2H4O2, (c) doc b , (c) doc b , (c) doc b , (c) doc b aliphatic carboxylic acid

For more see Structure and naming of CARBOXYLIC ACIDS and DERIVATIVES, including isomers

Interpreting the C-13 NMR spectrum of ethanoic acid

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

CH3COOH

(note the colours indicating the 2 different chemical environment of the carbon atoms, the ratio is 1:1).

The two 13C chemical environments of the ethanoic acid molecule are the carbon atom of the methyl group and the carbon atom of the carboxylic acid group.

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

The carbon-13 NMR spectra provides direct evidence of 2 different chemical environments of the carbon atoms in the ethanoic acid molecule from 2 different chemical shifts (ppm).

Exam practise questions based on the 13C NMR spectrum of ethanoic acid

Key points about the 13C NMR spectrum  of ethanoic acid and exam practise questions

Key 13C NMR Features of Ethanoic Acid

Chemical shift (δ, ppm) Carbon type Origin Notes
~20 ppm, 20.8 CH3 carbon Methyl group adjacent to C=O Deshielded compared to alkane CH3 (~10–15 ppm)
~170–180 ppm, 170 C=O carbon Carboxyl carbon Strongly deshielded due to double bond and electronegative O atoms

Ethanoic acid has two distinct carbon environments → two signals in ¹³C NMR.

Spectra data source https://sdbs.db.aist.go.jp/Disclaimer.aspx for 13C δ ppm

Common Misconceptions

  • Expecting more than two signals → Students sometimes think OH contributes a carbon signal (it doesn’t).
  • Confusing acid C=O with ester C=O.
  • Assuming methyl carbon appears at ~10 ppm → In acids, it's shifted downfield (~20 ppm) due to deshielding by C=O.
  • Forgetting integration doesn’t apply in ¹³C NMR → Unlike ¹H NMR, peak areas don’t directly give proton counts.

Exam Revision Tips

  • Quote approximate ranges (20 ppm for CH3, 170–180 ppm for C=O).
  • Remember only two signals for ethanoic acid.
  • Compare isomers: methyl methanoate (ester) has two signals (OCH3 ~51 ppm, C=O ~162 ppm).
  • Distinguish acid versus aldehyde: aldehyde C=O ~190–200 ppm, acid ~170–180 ppm.
  • Be ready for exam trick questions about integration in ¹³C NMR (not used).

Practise Multiple Choice Questions based on the 13C NMR spectrum of ethanoic acid

Each question has A–D options, model answer, and feedback explaining distractors.

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Jot down your responses and check out the ANSWERS!

Give reasons for your answers

Q1. How many signals appear in the 13C NMR spectrum of ethanoic acid?

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

Q2. Which isomer of ethanoic acid shows two signals in 13C NMR?

  1. Ethanol
  2. Methyl methanoate
  3. Propanoic acid
  4. Methanal

Q3. Why does OH not give a signal in 13C NMR?

  1. It has no carbon atom
  2. It exchanges with solvent
  3. It is too broad
  4. It overlaps with CH₃

Q4. Which statement about 13C NMR integration is correct?

  1. It gives proton ratios
  2. It gives carbon ratios
  3. It is not used for quantitative analysis in routine spectra
  4. It always equals 1:1

Q5. Which feature confirms ethanoic acid in 13C NMR?

  1. Two signals: ~20 ppm and ~170 ppm
  2. Three signals including ~50 ppm
  3. One signal at ~170 ppm
  4. Four signals including ~200 ppm

If you think there are any errors email doc b asap

Jot down your responses and check out the ANSWERS!
Comparing the infrared, mass, 1H NMR and 13C NMR spectra of the 2 isomers of C2H4O2

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 ethanoic acid (acetic acid) and methyl methanoate (methyl formate) image sizes. SEE also More on the isomers of C2H4O2

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 ~3200 cm-1, found in the infrared spectrum of carboxylic acids, but absent in the infrared spectrum of esters.

MASS SPECTRA: Apart from the m/z of 60 for the parent molecular ion and m/z 15 ion [CH3]+, both ethanoic acid and methyl methanoate show few similarities in their mass spectra. Their base ion peaks are quite different - for ethanoic acid it is m/z 43 and for methyl methanoate it is m/z 31.

1H NMR SPECTRA: The 1H NMR spectra of ethanoic acid and methyl methanoate are similar with two 'wide-apart' chemical shift singlet peaks in the integrated proton ratio of 3:1 (meaning 2 different 1H chemical environments).

13C NMR SPECTRA: The 13C NMR spectra of ethanoic acid and methyl methanoate are similar, both molecules give two C-13 NMR spectral lines (meaning 2 different 13C chemical environments). The two 13C chemical shifts of ethanoic acid are much closer together.

Key words & phrases: Interpreting the C-13 NMR spectra of ethanoic acid, C-13 nmr spectrum of ethanoic acid, understanding the carbon-13 nmr spectrum of ethanoic acid, explaining the line pattern in the high resolution C-13 nmr spectra of ethanoic acid, revising the C-13 nmr spectrum of ethanoic acid, ppm chemical shifts of the C-13 nmr spectrum of ethanoic acid, how to construct the diagram of the C-13 nmr spectrum of ethanoic acid, how to analyse the chemical shifts in the carbon-13 NMR spectrum of ethanoic acid Molecular structure diagram of the carbon-13 NMR diagram for the 13C NMR spectrum of ethanoic acid. Deducing the number of different chemical environments of the carbon atoms in the ethanoic acid molecule from the 13C chemical shifts in the carbon-13 NMR spectrum of ethanoic acid. Revision notes on the carbon-13 NMR spectrum of ethanoic acid. Matching and deducing the structure of the ethanoic acid molecule from its 13C NMR spectrum. Carbon-13 NMR spectroscopy of carboxylic acids, 13C NMR spectra of ethanoic acid, an isomer of molecular formula C2H4O2 How do you interpret the C-13 NMR spectrum of ethanoic acid How to interpret the C-13 NMR spectrum of ethanoic acid Explanatory diagram of the 13C C-13 carbon-13 NMR spectrum of the ethanoic acid molecule in terms of its molecular structure. Listing data of all the chemical shift peaks in ppm in the carbon-13 NMR spectrum of ethanoic acid. How to explain the C-13 NMR spectrum of ethanoic acid. How to deduce the number of different carbon atom environments in the ethanoic acid molecule from its carbon-13 NMR spectrum to help work out the molecular structure of the ethanoic acid molecule. The uses and distinctive features of the carbon-13 NMR spectrum of the ethanoic acid molecule explained. What does the c-13 carbon-13 NMR spectrum tell us about the ethanoic acid molecule? How do you interpret the chemical shifts of the C-13 NMR spectrum of ethanoic acid CH3COOH How to interpret the C-13 NMR spectrum of ethanoic acid CH3COOH Explanatory diagram of the 13C C-13 carbon-13 NMR spectrum of the  number of different carbon atom environments in the ethanoic acid CH3COOH molecule from its carbon-13 NMR spectrum to help work out the molecular structure of the ethanoic acid CH3COOH molecule? The uses and distinctive features of the carbon-13 NMR spectrum of the ethanoic acid CH3COOH molecule explained. What do the number and values of the chemical shifts from the c-13 carbon-13 NMR spectrum tell us about the ethanoic acid CH3COOH molecule? explaining the decoupled carbon-13 NMR spectrum of ethanoic acid CH3COOH  with a detailed interpretation diagram of all the C-13 chemical shifts and intensities

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

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Q1. How many signals appear in the 13C NMR spectrum of ethanoic acid?

  1. 1

  2. 2

  3. 3

  4. 4

Answer: B.

  • Correct: two distinct carbons (CH₃ and C=O).

  • A = wrong, two environments exist.

  • C/D = wrong, OH doesn’t contribute carbon.

Q2. Which isomer of ethanoic acid shows two signals in 13C NMR?

  1. Ethanol

  2. Methyl methanoate

  3. Propanoic acid

  4. Methanal

Answer: B.

  • A, C and D are NOT isomers of ethanoic acid C2H4O2.

Q3. Why does OH not give a signal in 13C NMR?

  1. It has no carbon atom

  2. It exchanges with solvent

  3. It is too broad

  4. It overlaps with CH₃

Answer: A.

  • Correct: only carbons give signals.

Q4. Which statement about 13C NMR integration is correct?

  1. It gives proton ratios

  2. It gives carbon ratios

  3. It is not used for quantitative analysis in routine spectra

  4. It always equals 1:1

Answer: C.

  • Correct: integration not reliable in ¹³C NMR.

Q5. Which feature confirms ethanoic acid in 13C NMR?

  1. Two signals: ~20 ppm and ~170 ppm

  2. Three signals including ~50 ppm

  3. One signal at ~170 ppm

  4. Four signals including ~200 ppm

Answer: A.

Q6 Given our isomers of molecular formula C2H4O2 ...

A CH3COOH

B HCOOCH3

C HOCH=CHOH

D HOCH2CHO

There may be more than one answer.

(a) Which molecule will give only one significant 13C NMR peak in its spectrum?

ANSWER: C, symmetrical molecule, two equivalent carbon atom environments, same chemical shift.

(b) Which molecule gives three principal 13C chemical shifts in its 13C NMR spectrum?

ANSWER: D, the only molecule with three different carbon atom chemical environments.

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Links associated with ethanoic acid

The infrared spectrum of Ethanoic acid (acetic acid)

The mass spectrum of Ethanoic acid (acetic acid)

The H-1 NMR spectrum of Ethanoic acid (acetic acid)

The chemistry of CARBOXYLIC ACIDS and DERIVATIVES revision notes INDEX

Isomers of molecular formula C2H4O2 (Mr = 60)

C-13 NMR spectroscopy index

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