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Advanced Organic Chemistry: Infrared spectrum of butane

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

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

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.

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!

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


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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.


Key words & phrases: C4H10 CH3CH2CH2CH3 image and diagram explaining the infrared spectrum of butane, complete infrared absorption spectrum of butane, comparative spectra of butane, prominent peaks/troughs for identifying functional groups in the infrared spectrum of butane, important wavenumber values in cm-1 for peaks/troughs in the infrared spectrum of butane, revision of infrared spectroscopy of butane, fingerprint region analysis of butane, how to identify butane from its infrared spectrum, identifying organic compounds like butane from their infrared spectrum, how to analyse the absorption bands in the infrared spectrum of butane detection of functional groups in the butane molecule example of the infrared spectrum of a molecule like butane with a functional group  interpreting interpretation of the infrared spectrum of butane shows presence of no functional group in n-butane

Diagram of absorption of wavenumber peaks in the infrared spectrum of butane. Characteristic peak wavenumbers in the infrared spectrum of butane. Finger print identification pattern using the infrared spectrum of butane. Revision notes on the infrared spectrum of butane. Matching and deducing the structure of the butane molecule from  its infrared spectrum.


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

Infrared spectroscopy index

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