Interpreting the infrared spectrum of methyl propanoate (methyl propionate)

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 methyl propanoate

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

Interpretation of the infrared spectrum of methyl propanoate

The most prominent infrared absorption lines of methyl propanoate

C-H stretching vibration absorptions occur in the wavenumber region 2975 to 2860 cm^-1.

Not very important, common to most organic molecules.

For saturated aliphatic esters, C=O stretching vibration absorptions occur in the wavenumber region 1750 to 1735 cm^-1.

This is a very characteristic absorption for any molecule e.g. an ester, with a carbonyl group.

For methyl propanoate, C-O stretching vibration absorptions occur in the wavenumber region 1200 to 1170 cm^-1.

The absence of other specific functional group bands will show that a particular functional group is absent from the methyl ethanoate molecular structure e.g. no hydroxyl group vibration, a wide band at ~3500 to 2500 cm^-1, obviously not expected for an ester like methyl propanoate, but expected for a carboxylic acid like the isomeric butanoic acid.

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