Interpreting the infrared spectrum of cyclohexene

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 cyclohexene

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

Interpretation of the infrared spectrum of cyclohexene

The most prominent infrared absorption lines of cyclohexene

Multiple C-H stretching vibration absorption lines peaking at ~3100 to 2950 cm^-1.

The very characteristic absorption peaking at ~1640 cm^-1 for C=C stretching vibrations, typical of an aliphatic alkene molecule.

(I have seen the 1440 cm^-1 absorption peak attributed to C=C vibrations, but this does not fit in with most spectral data of aliphatic alkenes that I've come across which attributes this to CH₂ bending vibrations).

Most of the peaks in the fingerprint region are due to C-H vibrations of some origin e.g. from alky CH₂, C-H and =C-H groupings.

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