Interpreting and explaining the infrared spectrum of 3-hydroxybutanone  (3-hydroxybutan-2-one, acetoin)

Spectra obtained from a liquid film of 3-hydroxybutanone. The right-hand part of the of the infrared spectrum of 3-hydroxybutanone, wavenumbers ~1500 to 400 cm^-1 is considered the fingerprint region for the identification of 3-hydroxybutanone and most organic compounds. It is due to a unique set of complex overlapping vibrations of the atoms of the molecule of 3-hydroxybutanone.

The molecular structure and naming of aldehydes and ketones

Interpretation of the infrared spectrum of 3-hydroxybutanone

The most prominent infrared absorption lines of 3-hydroxybutanone.

3-hydroxybutanone has two functional groups, the carbonyl group C=O of a ketone and the hydroxyl group OH of an alcohol, so you expect both to feature in the infrared spectrum.

One of the most characteristic absorption bands is 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.

In this case the O-H and C-H vibrations are clearly distinguishable - not always so, you often get quite an overlap to give an even larger broad absorption band.

R-O–H^δ+llll^δ–:O-R ... etc.

C-H stretching vibration absorption occurs ~2900 to 3000 cm^-1, wavenumbers common to any molecule with alkyl groups.

The strongest and most characteristic absorption band of butanone is at wavenumbers ~1700 to 1725 cm^-1 for the stretching vibrations of the carbonyl group bond C=O.

There are multiple C-H absorptions at wavenumbers ~1215 to 1435 cm^-1, but non particularly characteristic of ketones or alcohols.

There are C-O stretching vibration absorption bands at wavenumbers ~1120 to 1030 cm^-1. and 1350 to 1260 cm^-1.

Other 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 3-hydroxybutanone molecular structure.

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