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Advanced Organic Chemistry: Carbon-13 NMR spectrum of propane

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Interpreting the Carbon-13 NMR spectrum of propane

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 propane

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C-13 NMR spectroscopy - spectra index

See also comparing the infrared, mass, 1H NMR and 13C NMR spectra of propane, cyclopropane and propene

13C nmr spectrum of propane C3H8 analysis of chemical shifts ppm interpretation of C-13 chemical shifts ppm of alkane 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 - propane here.

Propane C3H8, alkanes structure and naming (c) doc b , 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 propane

As you can see from the diagram above there are 2 different chemical shift lines in the C-13 NMR spectrum of propane indicating 2 different chemical environments of the 3 carbon atoms of propane.

CH3CH2CH3 

(Note the 2 different colours indicating the 2 different chemical environments of the 3 carbon atoms in propane).

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

The carbon atoms of the two 'end' methyl groups in propane are equivalent to each other i.e. identical chemical environments.

The carbon-13 NMR spectra provides direct evidence of 2 different carbon atom environments for the 3 carbon atoms in the propane molecule, deduced from the presence of 3 different 13C NMR chemical shifts (ppm).

Comparing the infrared, mass, 1H NMR and 13C NMR spectra of propane, cyclopropane and propene

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 ethane and ethene image sizes.

Comparing the infrared spectra of propane, propene and cyclopropane.

Cyclopropane and propene are structural isomers of molecular formula C3H6.

Propane and propene exemplify the infrared spectra of lower members of  the alkane and alkene homologous series of CnH2n+2 and CnH2n hydrocarbon molecules where n = 3.

INFRARED SPECTRA (above): Apart from the significant differences in the fingerprint region at wavenumbers 1500 to 400 cm-1, the most striking differences are: (i) propene shows the characteristic absorption at ~1700 cm-1 for the C=C stretching vibrations, absent in the other two spectra, (ii) cyclopropane shows an absorption band at 2200 cm-1, absent in the other two spectra, (iii) propane has an absorption band at ~750 cm-1, absent in the other two spectra.

Comparing the mass spectra of propane, propene and cyclopropane.

Cyclopropane and propene are structural isomers of molecular formula C3H6.

Propane and propene exemplify the mass spectra of lower members of  the alkane and alkene homologous series of CnH2n+2 and CnH2n hydrocarbon molecules where n = 3.

MASS SPECTRA (above): All three hydrocarbons show some similarities in their mass spectra e.g. m/z ions 26 to 28 for [C2Hx]+ (x = 2 to 4) and m/z 14 and 15 ions - but these are found in most aliphatic hydrocarbon spectra. The molecular ion peaks will be the same for the isomeric propene and cyclopropane (m/z 42) but that of propane will be 2 mass units higher at m/z 44. The base ion peak m/z values are all different, propane 29, propene 41 and cyclopropane 42.

Comparing the 1H proton NMR spectra of propane, propene and cyclopropane.

Cyclopropane and propene are structural isomers of molecular formula C3H6.

Propane and propene exemplify the 1H proton NMR spectra of lower members of  the alkane and alkene homologous series of CnH2n+2 and CnH2n hydrocarbon molecules where n = 3.

1H NMR SPECTRA (above): The 1H NMR spectra of all three molecules give different proton ratios i.e. propane 3:1 (actually 6:2 in the molecule), propene 2:1:3 (spectrum and molecule) and cyclopropane just a singlet for the six protons, so all three can be distinguished from each other by their 1H NMR spectra..

Comparing the carbon-13 NMR spectra of propane, propene and cyclopropane.

Cyclopropane and propene are structural isomers of molecular formula C3H6.

Propane and propene exemplify the carbon-13 NMR spectra of lower members of  the alkane and alkene homologous series of CnH2n+2 and CnH2n hydrocarbon molecules where n = 3.

13C NMR SPECTRA (above): The 13C NMR spectra of the three molecules show different numbers of carbon-13 chemical environments i.e propane 2, propene 3 and cyclopropane only 1, so all three could be distinguished from each other.

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Links associated with propane

The chemistry of ALKANES revision notes INDEX

The infrared spectrum of propane

The mass spectrum of propane

The H-1 NMR spectrum of propane

C-13 NMR spectroscopy index

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