Interpreting and explaining the infrared spectrum of ethanoic acid (acetic acid) CH₃COOH

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

Exam practise questions based on the infrared spectrum of ethanoic acid

Introductory note on the infrared spectrum of ethanoic acid

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

Based in the infrared spectrum diagram for ethanoic acid, only some of the most prominent peaks for particular bond vibrations are discussed, particularly if ethanoic acid has a functional group with a particular characteristic wavenumber peak.

The infrared spectrum of ethanoic acid is unique and the whole, or selected wavenumbers, can be used to fingerprint its identity, sometimes analysing a mixture containing ethanoic acid or following its change of concentration in a reaction.

Spectra obtained from a liquid film of ethanoic acid. The right-hand part of the of the infrared spectrum of ethanoic acid, wavenumbers ~1500 to 400 cm^-1 is considered the fingerprint region for the identification of ethanoic acid and most organic compounds. It is due to a unique set of complex overlapping vibrations of the atoms of the molecule of ethanoic acid.

Interpretation of the infrared spectrum of ethanoic acid (an aliphatic carboxylic acid)

The most prominent infrared absorption lines of ethanoic acid

A very broad peak at wavenumbers 2800 to 3550 cm^-1 for O-H stretching vibrations - the breadth is due to the complexity of interference vibrations between hydrogen bonded ethanoic acid molecules.

One example of a hydrogen bond O^δ--H^δ+ǁǁǁ:O^δ- between ethanoic acid molecules due to the big difference in electronegativity between hydrogen and oxygen.

For more on hydrogen bonding see Physical properties of alcohols (boiling points, solubility) and intermolecular forces

and The physical properties of carboxylic acids - trends discussed and explained, mpts, bpts, solubility, odours

The O-H stretching vibrations overlap with C-H stretching vibrations, typically for alkyl groups at wavenumbers 2975 to 2845 cm^-1 (so take care when looking for the presence of a hydroxyl group in a molecule).

The relatively sharp and strong absorption band at wavenumber ~1700 cm^-1 is due to the stretching vibration of the C=O (carbonyl group of the carboxylic acid group).

Peaking at wavenumber ~1300 cm^-1 are the C-O stretching vibrations of the C-O bond.

Around wavenumber 1400 cm^-1 are complex bands due to the interaction of C=O and O-H vibrations.

There is also an O-H vibration band at wavenumber ~900 cm^-1.

The presence of strong absorption at wavenumbers for C=O and O-H stretching vibrations are very indicative of a carboxylic acid group in the molecule, but not proof or exclusive e.g. you can have a hydroxy-ketone!

The absence of other specific functional group bands will show that particular functional group is absent from the ethanoic acid molecular structure.

Exam practise questions based on the infrared spectrum of ethanoic acid

Key points about the infrared spectrum of ethanoic acid and exam practise questions

Ethanoic acid’s IR spectrum shows a broad O–H stretch (2500–3300 cm⁻¹), a sharp C=O stretch (~1700 cm⁻¹), and C–O stretches (1200–1300 cm⁻¹).

These are diagnostic of carboxylic acids. Students often confuse the broad acid O–H with alcohol O–H, and examiners test this distinction frequently.

Key IR Spectrum Features of Ethanoic Acid (CH₃COOH)

Common Misconceptions

• Confusing O–H stretches: Alcohol O–H (~3200–3600 cm⁻¹, narrower) versus acid O–H (broader, lower range).
• Overlooking overlap: Acid O–H overlaps with C–H stretches, making interpretation tricky.
• Assuming all carbonyls are identical: Aldehydes, ketones, esters, and acids all have C=O peaks but differ slightly in wavenumber and accompanying features.
• Ignoring fingerprint region: Students often skip this, but examiners may ask about its importance in confirming identity.

Exam Revision Tips

• Always quote approximate wavenumbers (e.g., “~1700 cm⁻¹ for C=O”).
• Distinguish acid O–H vs. alcohol O–H in written answers.
• Use multiple peaks together (O–H + C=O + C–O) to confirm carboxylic acid.
• Be prepared to compare isomers (e.g., ethanoic acid vs. methyl methanoate).
• Practice interpreting spectra with both functional group regions and fingerprint region.

Practise Multiple Choice Questions based on the infrared 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. Which peak confirms the presence of an OH group of a carboxylic acid in ethanoic acid?

1. Sharp peak at 1700 cm⁻¹
2. Broad band 2500–3300 cm⁻¹
3. Sharp peak at 2900 cm⁻¹
4. Strong band at 1200 cm⁻¹

Q2. Ethanoic acid versus ethanol: which feature distinguishes them?

1. Broad O–H 2500–3300 cm⁻¹
2. Sharp O–H 3200–3600 cm⁻¹
3. C=O at 1700 cm⁻¹
4. C–H at 2900 cm⁻¹

Q3. Which wavenumber is most diagnostic of a carbonyl group?

1. 1700 cm⁻¹
2. 2500–3300 cm⁻¹
3. 2900 cm⁻¹
4. 1200 cm⁻¹

Q4. Which isomer of ethanoic acid lacks the broad O–H stretch?

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

Q5. Which molecule shows both C=O and broad O–H?

1. Ethanoic acid
2. Ethanol
3. Methyl methanoate
4. Propanone

Q6. Why is acid O–H broader than alcohol O–H?

1. Stronger hydrogen bonding
2. Weaker hydrogen bonding
3. Presence of C=O
4. Fingerprint region overlap

Q7. Which peak would be absent in methyl methanoate compared to ethanoic acid?

1. 1700 cm⁻¹
2. 2500–3300 cm⁻¹
3. 1200 cm⁻¹
4. 2900 cm⁻¹

Q8. Which feature distinguishes aldehydes from acids?

1. C=O at 1700 cm⁻¹
2. Fingerprint region
3. C–H stretch at 2900 cm⁻¹
4. Acid O–H broad band

Q9. Which molecule shows IR wavenumbers for C=O but no O–H?

1. Ethanoic acid
2. Ethanol
3. Propanone
4. Propanoic acid

Q10. Which combination confirms carboxylic acid?

1. C=O + broad O–H + C–O
2. C=O + sharp O–H
3. C–H + C–O
4. Fingerprint region only

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. You see this distinguishing broad OH stretching vibration band in the infrared spectrum of ethanoic acid.
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).

Website content © Dr Phil Brown 2000+. All copyrights reserved on revision notes, images, quizzes, worksheets etc. Copying of Doc Brown's pre-university/college advanced level chemistry website material is NOT permitted. Exam revision summaries & references to chemistry course specifications are unofficial. These organic chemistry revision notes on the spectroscopy of ethanoic acid (acetic acid) - its infrared spectrum of ethanoic acid is analysed and explained and suitable for use of pre-university students studying AQA advanced level chemistry, Edexcel advanced level chemistry, OCR advanced level chemistry, IB advanced level chemistry, WJEC (Eduqas) advanced level chemistry, CIE Cambridge advanced level chemistry, US grade 11-12 AP honors chemistry courses and they will also prove useful to 1st year undergraduate students of chemistry.