Advanced Organic Chemistry: Infrared spectrum of butane CH3CH2CH2CH3

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Interpreting and explaining the infrared spectrum of butane CH3CH2CH2CH3

[Author © Dr Phil Brown GRIC, PhD: Doc Brown's advanced level organic chemistry exam revision notes suitable for students of UK A level chemistry courses, IB chemistry & US K12 grade 11, grade 12 and AP honors chemistry courses: Molecular spectroscopy - analysing the infrared spectra of butane [spectra page updated RE-EDIT]

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See also comparing the IR, mass, 1H NMR and 13C NMR spectra of 2-methylpropane and butane

Introductory note on the infrared spectrum of butane

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

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

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

infrared spectrum of butane C4H10 CH3CH2CH2CH3 wavenumbers cm-1 functional group detection fingerprint pattern identification of n-butane doc brown's advanced organic chemistry revision notes 

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

Butane   C4H10  alkanes structure and naming (c) doc b  alkanes structure and naming (c) doc b  alkanes structure and naming (c) doc b

For more see The molecular structure, classification and naming of alkanes

Interpretation of the infrared spectrum of butane

The most prominent infrared absorption lines of butane

For butane, the C-H stretching vibration absorptions occur in bands of wavenumbers 2975 to 2845 cm-1, principal 'peak' at 2970 cm-1.

C-H deformation-bending vibration absorptions occur in bands of wavenumbers 1470 to 1365 cm-1 (a significant double peak).

C-C-C/C-C skeletal vibrations absorptions occur in bands of wavenumbers 750 to 720 cm-1.

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

Apart from the fingerprint pattern for butane, these are all typical alkane/alkyl vibrations and offer little of value in investigating molecular structure.

There is no significant characteristic peak for a functional group infrared absorption band, because 2-methylpropane doesn't have one, i.e. absence of characteristic functional group frequencies in the infrared spectrum of butane, means absence of that functional group in the butane molecule, to state the obvious!

See also comparing the IR, mass, 1H NMR and 13C NMR spectra of 2-methylpropane and butane
Comparing the infrared, mass, 1H NMR and 13C NMR spectra of the 2 alkane isomers of C4H10

NOTE: The images are linked to their original detailed spectral analysis pages AND can be doubled in size with touch screens to increase the definition to the original butane and 2-methylpropane image sizes.

The infrared spectra of butane and 2-methyl propane are quite similar, mainly due to C-H stretching and deformation vibrations, but you can see significant differences in the fingerprint region at wavenumbers 1500 to 600 cm-1 e.g. butane has a more prominent absorption band at ~720 cm-1 due to C-C-C skeletal vibrations.

The mass spectra of butane and 2-methyl propane are quite similar and both have a base ion peak of m/z 43 [C3H7]+, but here significant differences in the ratios of the m/z ions 27 to 29 [C2H3,4,5]+.

The 1H NMR spectra of butane and 2-methyl propane are quite similar in that both show the 8 hydrogen atoms exist in only 2 different chemical environment. However, they can be distinguished from each other by the different integrated proton ratios. Butane gives a (2) : (3) proton ratio and 2-methylbutane a (1) : (9) proton ratio.

The 13C NMR spectra of butane and 2-methyl propane are quite similar in that both show the 4 carbon atoms exist in only 2 different chemical environments.

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What next? Links associated with butane

The chemistry of ALKANES revision notes INDEX

The mass spectrum of butane

The H-1 NMR spectrum of butane

The C-13 NMR spectrum of butane

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