Advanced Organic Chemistry: The 1H NMR spectrum of ethanoic acid CH3COOH

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Interpreting and explaining the 1H 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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 H-1 proton 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 1H NMR spectrum of ethanoic acid

Introductory note on the 1H NMR spectra of ethanoic acid

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

The chemical shift δ splitting pattern effects for ethanoic acid are confined to a proton spin-spin coupling effects analysed using the n+1 rule for adjacent non-equivalent proton fields (n is the number of neighbouring protons in a non-equivalent different chemical environment for the ethanoic acid molecule).

It is assumed that the integrated intensities of the 1H NMR δ chemical shifts give the ratio of the protons in the different non-equivalent chemical environments of the ethanoic acid molecule.

The most common solvent used for investigating the 1H NMR spectrum of compounds like ethanoic acid, is CDCl3 and other deuterated solvents to avoid confusion with a 1H NMR signal, 2D (2H) has a different NMR chemical shift.

low and high resolution H-1 proton nmr spectrum of ethanoic acid analysis interpretation of chemical shifts ppm spin spin line splitting diagram doc brown's advanced organic chemistry revision notes

TMS is the acronym for tetramethylsilane, formula Si(CH3)4, whose protons are arbitrarily given a chemical shift of 0.0 ppm. This is the 'standard' in 1H NMR spectroscopy and all other proton shifts, called chemical shifts, depend on the individual (electronic) chemical environment of the hydrogen atoms in an organic molecule - ethanoic acid here.

The chemical shifts quoted in ppm on the diagram of the H-1 NMR spectrum of ethanoic acid represent the peaks of the intensity of the chemical shifts of (which are often groups of split lines at high resolution) AND the relative integrated areas under the peaks gives you the ratio of protons in the different chemical environments of the ethanoic acid molecule.

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 H-1 NMR spectrum of ethanoic acid

For relatively simple molecules, the low resolution H-1 NMR spectrum of ethanoic acid is a good starting point.

The hydrogen atoms (protons) of ethanoic acid occupy 2 different chemical environments so that the H-1 proton low resolution NMR spectra should show principal two 1H peaks in the ratio 3 : 1 (diagram above).

CH3COOH

(note the 3: 1 ratio of the 2 colours of the protons indicating the 2 chemically different environments)

The integrated proton ratio observed of 3:1, corresponds with the structural formula of ethanoic acid.

The two 1H chemical environments in the ethanoic acid molecule.

δ (i) The methyl group protons (CH3, 2.1 ppm chemical shift)

δ (ii) The hydroxy proton (OH, 11.4 ppm chemical shift).

The latter is quite large due to the combined field effect of the two highly electronegative oxygen atoms.

Both chemical shifts are observed as singlets.

There is no need to consider the very high resolution spectrum of ethanoic acid, as there are no other immediate protons adjacent to the H of the hydroxyl OH group, or the H's of the CH3 group so the spin-spin field splitting lines would be to weak to be observed.

Exam practise questions based on the 1H NMR spectrum of ethanoic acid

Key points about the 1H NMR spectrum of ethanoic acid and exam practise questions

Ethanoic acid's ¹H NMR spectrum shows two main signals: a singlet at ~2.1 ppm (CH3 group, 3H) and a broad singlet at ~11–12 ppm (acidic OH, 1H).

The integration ratio is 3:1, confirming CH3COOH.

The Key ¹H NMR Features of Ethanoic Acid

Chemical shift (δ, ppm) Proton type Origin Integration ratio Notes
~2.1 ppm, 2.1 CH3 (methyl) Protons adjacent to C=O 3H Appears as singlet (no splitting, adjacent to C=O not H)
~11–12 ppm, 11.4 OH (acidic) Carboxylic acid proton 1H Broad singlet, exchangeable, strongly deshielded

https://sdbs.db.aist.go.jp/Disclaimer.aspx for 1H δ ppm

Common Misconceptions

  • Expecting splitting of CH3 → It appears as a singlet because the adjacent carbonyl carbon has no hydrogens.
  • Assuming OH always appears clearly → Acid OH is broad, sometimes weak or exchangeable with solvent.
  • Confusing acid OH with aldehyde CH → Aldehyde proton appears ~9–10 ppm, not as broad as acid OH.
  • Forgetting integration ratios → Integration confirms 3:1 ratio, essential for identifying acid vs. ester.

Exam Revision Tips

  • Quote approximate chemical shifts (2 ppm for CH3, 11–12 ppm for OH).
  • Use integration ratios to confirm number of protons.
  • Distinguish acid OH vs. alcohol OH (acid OH much more downfield).
  • Compare isomers: methyl methanoate (ester) shows different shifts (~3.7 ppm for OCH3, ~8 ppm for aldehyde-like).
  • Remember exchangeable protons (OH/NH) may disappear with D2O shake.

Practise Multiple Choice Questions based on the 1H NMR spectrum of ethanoic acid

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

If you think there are any errors email doc b asap

Jot down your responses and check out the ANSWERS!

Give reasons for your answers

Q1. What is the integration ratio of signals in ethanoic acid?

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

Q2. Why does the CH3 signal appear as a singlet?

  1. Coupling with OH proton
  2. No adjacent hydrogens on carbonyl carbon
  3. Solvent effect
  4. Instrument resolution

Q3. Which is an isomer of ethanoic acid also has Mᵣ = 60 but a different 1H NMR spectrum?

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

Q4. Which proton is exchangeable with D2O?

  1. CH3
  2. CH
  3. OH
  4. Aromatic H

Q5. Which signal would be absent in methyl methanoate compared to ethanoic acid?

  1. ~2 ppm CH3
  2. ~11 ppm OH
  3. ~3.7 ppm OCH3
  4. ~9 ppm aldehyde-like H

Q6. Which signal distinguishes aldehydes from acids?

  1. ~11 ppm OH
  2. ~9–10 ppm CH (aldehyde)
  3. ~2 ppm CH₃
  4. ~7 ppm aromatic

Q7. Why is the OH signal broad?

  1. Rapid proton exchange and hydrogen bonding
  2. Instrument error
  3. Solvent impurity
  4. Coupling with CH3

Q9. Which feature distinguishes acids from esters in NMR?

  1. Acid OH at ~11 ppm
  2. Ester OCH3 at ~3.7 ppm
  3. CH3 at ~2 ppm
  4. Integration ratio

    Q10 Given our isomers of molecular formula C2H4O2 ...

    A CH3COOH

    B HCOOCH3

    C HOCH=CHOH

    D HOCH2CHO

    There may be more than one answer.

If you think there are any errors email doc b asap

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). However, although no splitting is observed, the 1H chemical shifts for the two molecules are different, those of ethanoic acid are much further apart.

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 splitting pattern from proton spin-spin coupling effects is analysed using the n+1 rule for adjacent non-equivalent proton fields (n is the number of neighbouring protons in a non-equivalent different chemical environment).

Number of protons 1H causing splitting Splitting pattern produced from the n+1 rule and the theoretical ratio of line intensities
0 means no splitting             1            
1 creates a doublet           1   1          
2 creates a triplet         1   2   1        
3 creates a quartet       1   3   3   1      
4 creates a quintet     1   4   6   4   1    
5 creates a sextet   1   5   10   10   5   1  
6 creates a septet 1   6   15   20   15   6   1

Key words & phrases: Interpreting the proton H-1 NMR spectra of ethanoic acid, low resolution & high resolution proton nmr spectra of ethanoic acid, H-1 nmr spectrum of ethanoic acid, understanding the hydrogen-1 nmr spectrum of ethanoic acid, explaining the line splitting patterns in the high resolution H-1 nmr spectra of ethanoic acid, revising the H-1 nmr spectrum of ethanoic acid, proton nmr of ethanoic acid, ppm chemical shifts of the H-1 nmr spectrum of ethanoic acid, explaining and analyzing spin spin line splitting in the H-1 nmr spectrum, how to construct the diagram of the H-1 nmr spectrum of ethanoic acid, how to work out the number of chemically different protons in the structure of the ethanoic acid organic molecule, how to analyse the chemical shifts in the hydrogen-1 H-1 proton NMR spectrum of ethanoic acid Molecular structure diagram of the proton NMR diagram for the 1H NMR spectrum of ethanoic acid. The proton ratio in the 1H NMR spectrum of ethanoic acid. Deducing the number of different chemical environments of the protons in the ethanoic acid molecule from the 1H chemical shifts in the hydrogen-1 NMR spectrum of ethanoic acid. Analysing the high resolution 1H NMR spectrum of ethanoic acid. Analysing the low resolution 1H NMR spectrum of ethanoic acid. You may need to know the relative molecular mass of ethanoic acid to deduce the molecular formula from the proton ratio of the 1H NMR spectrum of ethanoic acid. Revision notes on the proton NMR spectrum of ethanoic acid. Matching and deducing the structure of the ethanoic acid molecule from its hydrogen-1 NMR spectrum. Proton NMR spectroscopy of carboxylic acids, 1H NMR spectra of ethanoic acid, an isomer of molecular formula C2H4O2 How do you interpret the H-1 NMR spectrum of ethanoic acid How to interpret the H-1 NMR spectrum of ethanoic acid Explanatory diagram of the 1H H-1 proton 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 proton NMR spectrum of ethanoic acid. How to explain the H-1 NMR spectrum of ethanoic acid. The values of the integrated proton ratios in the 1-H NMR spectrum of the ethanoic acid molecule. How to work out the molecular structure of the ethanoic acid molecule from its proton NMR spectrum. The uses and distinctive features of the proton NMR spectrum of the ethanoic acid molecule explained. What does the H-1 proton NMR spectrum tell us about the structure and properties of the ethanoic acid molecule? How do you interpret the H-1 NMR spectrum of ethanoic acid CH3COOH How to interpret the H-1 NMR spectrum of ethanoic acid CH3COOH Explanatory diagram of the chemical shifts of the 1H H-1 proton NMR spectrum of the ethanoic acid CH3COOH molecule in terms of its molecular structure. Listing data of all the chemical shift peaks in ppm in the proton NMR spectrum of ethanoic acid CH3COOH. How to explain the H-1 NMR spectrum of ethanoic acid CH3COOH. The chemical shifts and  integrated values of the proton ratios in the 1-H NMR spectrum of the ethanoic acid CH3COOH molecule. How to work out the molecular structure of the ethanoic acid CH3COOH molecule from its proton NMR spectrum. The uses and distinctive features of the proton NMR spectrum of the ethanoic acid CH3COOH molecule explained. What does the H-1 proton NMR spectrum chemical shifts tell us about the structure and properties of the ethanoic acid CH3COOH molecule? explaining the spin-spin proton coupling effects in the 1H NMR spectrum of ethanoic acid CH3COOH. interpretation diagram explaining the proton splitting pattern produced from the n+1 rule and the theoretical ratio of chemical shift and values of intensities for the proton NMR spectrum lines of ethanoic acid CH3COOH

ANSWERS to the Practice Multiple Choice Questions based on the 1H NMR spectrum of ethanoic acid

If you think there are any errors email doc b asap

Q1. What is the integration ratio of signals in ethanoic acid?
  1. 1:1
  2. 3:1
  3. 2:1
  4. 4:1

Answer: B.
  • Correct: 3H methyl, 1H OH.

Q2. Why does the CH3 signal appear as a singlet?
  1. Coupling with OH proton
  2. No adjacent hydrogens on carbonyl carbon
  3. Solvent effect
  4. Instrument resolution

Answer: B.
  • Correct: carbonyl carbon has no H, so no splitting.

Q3. Which is an isomer of ethanoic acid also has Mᵣ = 60 but a different 1H NMR spectrum?
  1. Ethanol
  2. Ethanal
  3. Propanoic acid
  4. Methyl methanoate

Answer: D.
  • Ester shows OCH3 (~3.7 ppm), no acid OH.

Q4. Which proton is exchangeable with D2O?
  1. CH3
  2. CH
  3. OH
  4. Aromatic H

Answer: C.
  • Acid OH disappears with D2O shake.

Q5. Which signal would be absent in methyl methanoate compared to ethanoic acid?
  1. ~2 ppm CH3
  2. ~11 ppm OH
  3. ~3.7 ppm OCH3
  4. ~9 ppm aldehyde-like H

Answer: B.
  • Ester lacks acid OH.

Q6. Which signal distinguishes aldehydes from acids?
  1. ~11 ppm OH
  2. ~9–10 ppm CH (aldehyde)
  3. ~2 ppm CH₃
  4. ~7 ppm aromatic

Answer: A.
  • Aldehydes lack acid OH.

Q7. Why is the OH signal broad?
  1. Rapid proton exchange and hydrogen bonding
  2. Instrument error
  3. Solvent impurity
  4. Coupling with CH3

Answer: A.
  • Correct: exchange broadens OH.

Q9. Which feature distinguishes acids from esters in NMR?
  1. Acid OH at ~11 ppm
  2. Ester OCH3 at ~3.7 ppm
  3. CH3 at ~2 ppm
  4. Integration ratio

Answer: A.
  • Acid OH is diagnostic.

Q10 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 1H NMR peak in its spectrum?

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

(b) Which molecule gives three principal 1H chemical shifts in its 1H NMR spectrum and in what integrated ratio?

ANSWER: D, the only molecule with three different proton chemical environments, proton ratio 1 : 2 : 1

(c) Which molecule's 1H NMR spectrum will 'lose' a peak if D2O is added to the sample under investigation?

ANSWERS: A, C and D, because you get proton exchange via the labile OH protons in alcohols and carboxylic acids via the reaction R-OH + D2O R-OD  + HOD, which suppresses the 1H signal, the 1D (2H) NMR signal has a significantly different chemical shift resonance.

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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 C-13 NMR spectrum of Ethanoic acid (acetic acid)

Isomers of molecular formula C2H4O2 (Mr = 60)

The chemistry of CARBOXYLIC ACIDS and DERIVATIVES revision notes INDEX

H-1 proton NMR spectroscopy index

(Please read 8 points at the top of the 1H NMR index page)

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 Index of sets of isomers for a given molecular formula, some include IR and NMR spectroscopy data

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