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Advanced Organic Chemistry: Carbon-13 NMR spectrum of 2-methylpropane

Interpreting the Carbon-13 NMR 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 C-13 NMR spectra of 2-methylpropane

13C nmr spectrum of 2-methylpropane C4H10 (CH3)3CH analysis of chemical shifts ppm interpretation of C-13 chemical shifts ppm of isobutane 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 - 2-methylpropane here.

2-methylpropane  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 2-methylpropane

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

CH3CH(CH3)CH3  or  (CH3)3CH

(Note the 2 different colours indicating the 2 different chemical environments of the 4 carbon atoms in 2-methylpropane).

13C chemical shifts (a) to (b) on the C-13 NMR spectrum diagram for 2-methylpropane.

There are only two chemical environments of the carbon atoms because of the symmetry of the 2-methylpropane molecule.

Three of the carbon atoms (of the methyl groups) 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 2-methylpropane 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. I'm not sure how reliable the pair of 13C chemical shifts for 2-methylpropane are, which seem surprisingly close together? The other pair are reliable.


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