Interpreting and explaining the infrared spectrum of propanoic acid (propionic acid)

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

The molecular structure and naming of carboxylic acids and derivatives

Interpretation of the infrared spectrum of propanoic acid

The two most prominent and characteristic infrared absorption lines of propanoic 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 propanoic 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 propanoic acid molecules (diagram above).

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).

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 not present in a molecular structure of propanoic acid.

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