Interpreting the infrared spectrum of 1-methylethyl 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 1-methylethyl methanoate

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

Interpretation of the infrared spectrum of 1-methylethyl methanoate

The most prominent infrared absorption lines of 1-methylethyl 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, the characteristic 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 1-methylethyl methanoate, with a C=O carbonyl group, but other types of compounds too.

For methanoate esters, 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 1-methylethyl methanoate molecular structure e.g. no hydroxyl group vibration at ~3500 to 3200 cm^-1, obviously not expected for an ester like 1-methylethyl 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.