Interpreting the infrared spectrum of but-1-ene

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 but-1-ene

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

Interpretation of the infrared spectrum of but-1-ene

The most prominent infrared absorption lines of but-1-ene

There are prominent bands of C-H stretching vibration absorptions peaking at wavenumbers ~3095 to 3010 cm^-1, and also 995 to 905 cm^-1, associated with the C-H bond in a RCH=CH₂ grouping e.g. as in but-1-ene.

Overlapping with the above, are prominent bands due to C-H stretching vibration absorptions peak at wavenumbers ~2975 to 2860 cm^-1, typical of molecules with a methyl alkyl group.

There is a very strong characteristic absorption for C=C stretching vibrations, peaking at wavenumbers 1645 to 1640 cm^-1, typical of an alkene like but-1-ene.

Around wavenumbers ~1470 to 1370 cm^-1 are absorptions dues to C-H deformation vibrations of the C-CH₃ grouping in but-1-ene.

The absence of other specific functional group bands will show that a particular functional group is absent from the but-1-ene molecular structure.