Interpreting the infrared spectrum of 2-methylpropene (2-methylprop-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 2-methylpropene

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Spectra obtained from a liquid film of 2-methylpropene. The right-hand part of the of the infrared spectrum of 2-methylpropene, wavenumbers ~1500 to 400 cm^-1 is considered the fingerprint region for the identification of 2-methylpropene and most organic compounds. It is due to a unique set of complex overlapping vibrations of the atoms of the molecule of 2-methylpropene.

Interpretation of the infrared spectrum of 2-methylpropene

The most prominent infrared absorption lines of 2-methylpropene

There are prominent bands of C-H stretching/deformation vibration absorptions peaking at wavenumbers ~3095 to 3075 cm^-1, and also at wavenumbers 895 to 885 cm^-1, associated with the C-H bonds in a R₁R₂C=CH₂ grouping e.g. as in 2-methylpropene.

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 characteristic absorption for C=C stretching vibrations, peaking at wavenumbers 1680 to 1620 cm^-1, typical of an alkene like 2-methylpropene.

The absence of other specific functional group bands will show that a particular functional group is absent from the 2-methylpropene molecular structure.