Interpreting the infrared spectrum of propyl methanoate

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 propyl methanoate

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

Interpretation of the infrared spectrum of propyl methanoate

The most prominent infrared absorption lines of propyl methanoate

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 like propyl methanoate, with a C=O carbonyl group.

For many alkyl esters (e.g. ethyl or propyl etc.), C-O stretching vibration absorptions occur in the wavenumber region 1200 to 1180 cm^-1.

The absence of other specific functional group bands will show that a particular functional group is absent from the propyl methanoate molecular structure e.g. no hydroxyl group vibration at ~3500 to 3200 cm^-1., obviously not expected for an ester like propyl methanoate, 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 propyl methanoate molecular structure.