Interpreting the infrared spectrum of 2-methylpropanoic acid (isobutyric 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 2-methylpropanoic acid

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Spectra obtained from a liquid film of 2-methylpropanoic acid. The right-hand part of the of the infrared spectrum of 2-methylpropanoic acid, wavenumbers ~1500 to 400 cm^-1 is considered the fingerprint region for the identification of 2-methylpropanoic acid and most organic compounds. It is due to a unique set of complex overlapping vibrations of the atoms of the molecule of 2-methylpropanoic acid.

Interpretation of the infrared spectrum of 2-methylpropanoic acid

The most prominent infrared absorption lines of 2-methylpropanoic 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 2-mrthyulpropanoic 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 2-methylpropanoic 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 2-methylpropanoic acid molecular structure.