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Advanced Organic Chemistry: Carbon-13 NMR spectrum of butane

Interpreting the Carbon-13 NMR 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 C-13 NMR spectra of butane

13C nmr spectrum of butane C4H10 CH3CH2CH2CH3 analysis of chemical shifts ppm interpretation of C-13 chemical shifts ppm of n-butane C13 13-C nmr doc brown's advanced organic chemistry revision notes 

TMS is the acronym for tetramethylsilane, formula Si(CH3)4, whose 13C atoms are arbitrarily given a chemical shift of 0.0 ppm. This is the 'standard' in 13C NMR spectroscopy and all other 13C resonances, called chemical shifts, are measured with respect to the TMS, and depend on the individual (electronic) chemical environment of the 13C atoms in an organic molecule - butane here.

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

Interpreting the C-13 NMR spectrum of butane

As you can see from the diagram above there are 2 different chemical shift lines in the C-13 NMR spectrum of butane indicating 2 different chemical environments of the 4 carbon atoms of butane.

CH3CH2CH2CH3

(Note the 2 different colours indicating the 2 different chemical environments of the 2 carbon atoms in butane).

13C chemical shifts (a) to (b) on the C-13 NMR spectrum diagram for butane.

There are only two chemical environments for the carbon atoms because of the symmetry of the butane molecule.

Two pairs of carbon atoms have the same chemical shift.

The carbon-13 NMR spectra provides direct evidence of 2 different carbon atom environments for the 4 carbon atoms in the butane molecule, deduced from the presence of 2 different 13C NMR chemical shifts (ppm).

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.

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. The two 13C chemical shifts for butane are reliable, not sure on the other pair.


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