Interpreting the infrared spectrum of ethanoic acid (acetic acid)

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK IB KS5 A/AS GCE advanced A level organic chemistry students US K12 grade 11 grade 12 organic chemistry courses involving molecular spectroscopy analysing infrared spectra of ethanoic acid

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

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

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).

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.

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.
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).