Interpreting the infrared spectrum of propanal (propionaldehyde)

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 propanal

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

Interpretation of the infrared spectrum of propanal  (propionaldehyde)

The most prominent infrared absorption lines of propanal

The stretching vibrations of the C-H bond of the alkyl grouping occur at wavenumbers 2975 to 2845 cm^-1.

Overlapping are the stretching vibrations of the C-H bond of the aldehyde group (CHO) occur at wavenumbers 2880 to 2650 cm^-1.

The strongest and most characteristic absorption band of propanal is at wavenumbers ~1740 to 1720 cm^-1 for the stretching vibrations of the carbonyl group bond C=O.

There are other multiple aliphatic aldehyde absorptions at wavenumbers ~1215 to 1435 cm^-1 but not particularly characteristic.

The absence of other specific functional group bands will show that particular functional group is absent from the propanal molecular structure.