Interpreting the infrared spectrum of butanoic acid (butyric acid)

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

Interpretation of the infrared spectrum of butanoic acid

The most prominent infrared absorption lines of butanoic acid

The hydroxyl O-H stretching vibration absorptions occur at wavenumbers 3300 to 2500 cm^-1.

This is a broad series of bands due to interference from intermolecular hydrogen bonding between butanoic acid molecules - characteristic behaviour of carboxylic acids and any other molecule with a hydroxyl group.

One example of a hydrogen bond O^δ--H^δ+ǁǁǁ:O^δ- between butanoic acid molecules.

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  C=O stretching vibration absorptions occur at wavenumbers 1725 to 1700 cm^-1, characteristic of any molecule with a carbonyl group.

Although not unique, detection of these two groups in an infrared spectrum is indicative, but not proof, the molecule is a carboxylic acid e.g. the molecule could be a hydroxy-ketone!

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