Advanced Organic Chemistry: Infrared spectrum of 2-methylpropanoic acid (CH3)2CHCOOH

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Interpreting and explaining the infrared spectrum of 2-methylpropanoic acid (isobutyric acid)

[Author © Dr Phil Brown PhD: Doc Brown's advanced level organic chemistry exam revision notes suitable for students of UK A level chemistry courses & US K12 grade 11, grade 12 and AP honors chemistry courses: Molecular spectroscopy analysis of 2-methylpropanoic acid (IR spectra) [spectra page updated April 3rd 2026 *]

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Introductory note on the infrared spectrum of 2-methylpropanoic acid

Students and teachers please note my explanation of the infrared spectrum of 2-methylpropanoic acid is designed for advanced, but pre-university, chemistry courses.

Based in the infrared spectrum diagram for 2-methylpropanoic acid, only some of the most prominent peaks for particular bond vibrations are discussed, particularly if 2-methylpropanoic acid has a functional group with a particular characteristic wavenumber peak.

The infrared spectrum of 2-methylpropanoic acid is unique and the whole, or selected wavenumbers, can be used to fingerprint its identity, sometimes analysing a mixture containing 2-methylpropanoic acid or following its change of concentration in a reaction.

infrared spectrum of 2-methylpropanoic acid C4H8O2 (CH3)2CHCOOH wavenumbers cm-1 functional group detection fingerprint pattern identification of isobutyric acid doc brown's advanced organic chemistry revision notes 

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.

2-methylpropanoic acid (isobutyric acid), C4H8O2  (c) doc b   (c) doc b   (c) doc b

The molecular structure and naming of carboxylic acids and derivatives

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 Oδ--Hδ+ǁǁǁ:Oδ- 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.

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The mass spectrum of 2-methylpropanic acid (isobutyric acid)

The H-1 NMR spectrum of 2-methylpropanic acid (isobutyric acid)

The C-13 NMR spectrum of 2-methylpropanic acid (isobutyric acid)

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