Advanced Organic Chemistry: Infrared spectrum of octane

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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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Infrared spectroscopy - spectra index

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, CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH3, alkane

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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