Interpreting the infrared spectrum of butane

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 butane

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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     

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!

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.