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Advanced Organic Chemistry: Infrared spectrum of 2-methylpropane

Interpreting the infrared spectrum of 2-methylpropane (isobutane)

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 2-methylpropane

infrared spectrum of 2-methylpropane C4H10 (CH3)3CH wavenumbers cm-1 functional group detection fingerprint pattern identification of isobutane doc brown's advanced organic chemistry revision notes 

Spectra obtained from a liquid film of 2-methylpropane. The right-hand part of the of the infrared spectrum of 2-methylpropane, wavenumbers ~1500 to 400 cm-1 is considered the fingerprint region for the identification of 2-methylpropane and most organic compounds. It is due to a unique set of complex overlapping vibrations of the atoms of the molecule of 2-methylpropane.

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Interpretation of the infrared spectrum of 2-methylpropane

The most prominent infrared absorption lines of 2-methylpropane

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.

C-C-C/C-C skeletal vibrations absorptions occur in bands of wavenumbers 1250 to 1170 cm-1.

The absence of other specific functional group bands will show that a particular functional group is absent from the 2-methylpropane 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.

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. 2-methylpropane has a prominent absorption at ~1200 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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