Advanced Organic Chemistry: Infrared spectrum of octane CH3(CH2)6CH3

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Interpreting and explaining the infrared spectrum of octane

[Author © Dr Phil Brown PhD: Doc Brown's advanced level organic chemistry exam revision notes suitable for students of UK A level chemistry courses & US K12 grade 11, grade 12 and AP honors chemistry courses: Molecular spectroscopy analysis of octane [spectra page updated Mar 25th 2026 *]

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 Infrared spectroscopy - spectra index

Introductory note on the infrared spectrum of octane

Students and teachers please note my explanation of the infrared spectrum of octane is designed for advanced, but pre-university, chemistry courses.

Based in the infrared spectrum diagram for octane, only some of the most prominent peaks for particular bond vibrations are discussed, particularly if octane has a functional group with a particular characteristic wavenumber peak.

The infrared spectrum of octane is unique and the whole, or selected wavenumbers, can be used to fingerprint its identity, sometimes analysing a mixture containing octane or following its change of concentration in a reaction.

C8H18 infrared spectrum of octane wavenumbers cm-1 functional group detection fingerprint pattern identification of n-octane doc brown's advanced organic chemistry revision notes 

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.

Octane, C8H18, CH3(CH2)6CH3, CH3CH2CH2CH2CH2CH2CH2CH3, alkane  

an alkane  The molecular structure and naming of alkanes

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.

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What next? links associated with octane

The mass spectrum of octane

The H-1 NMR spectrum of octane

The C-13 NMR spectrum of octane

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