Interpreting the infrared spectrum of octane

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 octane

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

Interpretation of the infrared spectrum of octane

The most prominent infrared absorption lines of octane

From 2950 to 2845 cm^-1 there are multiple peaks amounting to strong absorption due to C-H stretching vibrations in the -CH₂- or -CH₃ groups in octane.

The are other C-H absorptions due to C-H deformation vibrations of the CH₂ groups at 1480 to 1440 cm^-1.

These overlap with C-H vibrations of the CH₃ groups at wavenumbers 1385 to 1370 cm^-1 and 1470 to 1435 cm^-1.

Skeletal C-C vibrations show an absorption ~750 to 720 cm^-1 for -(CH₂)_n-, where n is >3, which is obviously the case in linear octane.

There are no specific characteristic lines for alkanes like octane because C-C and C-H vibrations are common to so many organic molecules and alkanes like octane have no functional group that gives a characteristic vibration.

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