Interpreting the infrared spectrum of butan-2-ol

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 butan-2-ol

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

Interpretation of the infrared spectrum of liquid film butan-2-ol

The most prominent infrared absorption lines of butan-2-ol

The most characteristic absorption bands are the broad O-H stretching vibration bands at wavenumbers ~3550 to 3230 cm^-1, the breadth is caused by interference of hydrogen bonding interactions, common to all hydrogen bonded molecules with a hydroxyl group.

R-O–H^δ+llll^δ–:O-R ... etc.

C-H stretching vibration absorption occurs ~2900 cm^-1, wavenumbers common to any molecule with alkyl groups.

There are C-O stretching vibration absorption bands at wavenumbers ~1350 to 1030 cm^-1.

C-H stretching vibration absorptions appear at wavenumbers 1470 to 1370 cm^-1.

The absence of other specific functional group bands will show that a particular functional group is absent from the butan-2-ol molecular structure.

In the vapour state there is no hydrogen bonding between the butan-2-ol molecules so the broad band at wavenumbers ~3550 to 3230 cm^-1 (for H bonded molecules) doesn't show up.

Instead, you get a sharper narrower band at wavenumbers 3670 to 3580 cm^-1, characteristic of a non-hydrogen bonded hydroxyl molecule.