Advanced Organic Chemistry: Infrared spectrum of butanoic acid

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Interpreting the infrared spectrum of butanoic acid (butyric 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 butanoic acid

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Infrared spectroscopy - spectra index

infrared spectrum of butanoic acid C4H8O2 CH3CH2CH2COOH wavenumbers cm-1 functional group detection fingerprint pattern identification of butyric acid doc brown's advanced organic chemistry revision notes

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.

Butanoic acid (butyric acid(c) doc b  (c) doc b  (c) doc b a carboxylic 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 Oδ--Hδ+ǁǁǁ:Oδ- 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.

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