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Advanced Organic Chemistry: Carbon-13 NMR spectrum of Pentane

Carbon-13 NMR spectrum of Pentane

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

See also comparing the infrared, mass, 1H NMR and 13C NMR spectra of the 3 alkane isomers of C5H12

C-13 nmr spectrum of pentane analysis of chemical shifts ppm interpretation of C-13 chemical shifts ppm of pentane diagram 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 shifts, called chemical shifts, depend on the individual (electronic) chemical environment of the 13C atoms in an organic molecule - pentane here.

Pentane  C5H12, 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 and naming of alkanes

Interpreting the C-13 NMR spectrum of Pentane

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

CH3CH2CH2CH2CH3  

(Note the 3 different colours of the carbon atoms in pentane, indicating the three different 13C chemical environments)

Although there are five carbon atoms in the molecule, pentane is a symmetrical molecule with only three possible 13C chemical environments for the 5 carbon atoms, so three chemical shifts are observed in C-13 NMR spectrum.

Comparing the infrared, mass, 1H NMR and 13C NMR spectra of the 3 alkane isomers of C5H12

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 pentane, 2-methylbutane and 2,2-dimethylpropane image sizes.

Comparing the infrared spectra of pentane, 2-methylbutane and 2,2-dimethylpropane

Pentane, 2-methylbutane and 2,2-dimethylpropane are structural isomers of molecular formula C5H12

Pentane, 2-methylbutane and 2,2-dimethylpropane exemplify infrared spectra of  the alkane homologous series CnH2n+2  hydrocarbon molecules, where n = 5

INFRARED SPECTRA (above): There are, as expected, differences in the fingerprint region at wavenumbers 1500 to 400 cm-1, but there is no specific infrared absorption band for a functional group. The infrared spectra of pentane and 2-methylbutane seem very similar, but that of 2,2-dimethylpropane seems much simpler.

Comparing the mass spectra of pentane, 2-methylbutane and 2,2-dimethylpropane

Pentane, 2-methylbutane and 2,2-dimethylpropane are structural isomers of molecular formula C5H12

Pentane, 2-methylbutane and 2,2-dimethylpropane exemplify the mass spectra of  the alkane series CnH2n+2  hydrocarbon molecules, where n = 5

MASS SPECTRA (above): All three hydrocarbons show some similarities in their mass spectra e.g. m/z ions 27 to 29 for [C2Hx]+ (x = 2 and 4). The molecular ion peaks will be the same for all three isomers (m/z 72), but it is very tiny for 2,2-dimethypropane. The pattern ratios for m/z 39 to 43 are similar for pentane and 2-methylbutane, but m/z 42 and 43 ions are almost absent from the 2,2-dimethylpropane spectrum. The base peak ion for pentane is m/z 43, but for 2-methylbutane and 2,2-dimethylpropane it is m/z 57.

Comparing the 1H proton NMR spectra of pentane, 2-methylbutane and 2,2-dimethylpropane

Pentane, 2-methylbutane and 2,2-dimethylpropane are structural isomers of molecular formula C5H12

Pentane, 2-methylbutane and 2,2-dimethylpropane exemplify the 1H proton NMR spectra of the alkane homologous series CnH2n+2  hydrocarbon molecules where, n = 5

1H NMR SPECTRA (above): The 1H NMR spectra of all three molecules give different proton ratios for the different 1H chemical environments i.e. pentane's proton ratio is 3:2:1 (from 6:4:2 H's in the molecule). 2-methylbutane's proton ratio is 6:1:2:3 and 2,2-dimethylpropane's doesn't have a proton ratio, all hydrogen atoms are equivalent. This means all three isomeric C5H12 hydrocarbons can be distinguished from their 1H NMR spectra.

Comparing the carbon-13 NMR spectra of pentane, 2-methylbutane and 2,2-dimethylpropane

Pentane, 2-methylbutane and 2,2-dimethylpropane are structural isomers of molecular formula C5H12

Pentane, 2-methylbutane and 2,2-dimethylpropane exemplify the carbon-13 NMR spectra of members of  the alkane homologous series CnH2n+2  hydrocarbon molecules, where n = 5

13C NMR SPECTRA (above): The 13C NMR spectra of the three molecules show different numbers of carbon-13 chemical environments i.e different numbers of 13C NMR resonance lines. So, pentane gives three 13C chemical shifts, 2-methylbutane four and 2,2-dimethylpropane two. This means all three isomeric C5H12 hydrocarbons can be distinguished from their 13C NMR spectra.

Key words & phrases: Interpreting the C-13 NMR spectra of pentane, C-13 nmr spectrum of pentane, understanding the carbon-13 nmr spectrum of pentane, explaining the line pattern in the high resolution C-13 nmr spectra of pentane, revising the C-13 nmr spectrum of pentane, ppm chemical shifts of the C-13 nmr spectrum of pentane, how to construct the diagram of the C-13 nmr spectrum of pentane

Molecular structure diagram of the carbon-13 NMR diagram for the 13C NMR spectrum of pentane. Deducing the number of different chemical environments of the carbon atoms in the pentane molecule from the 13C chemical shifts in the carbon-13 NMR spectrum of pentane. Revision notes on the carbon-13 NMR spectrum of pentane. Matching and deducing the structure of the pentane molecule from its 13C NMR spectrum. Carbon-13 NMR spectroscopy of alkane, 13C NMR spectra of pentane, an isomer of molecular formula C5H12


Associated links

The chemistry of ALKANES revision notes INDEX

The infrared spectrum of pentane

The mass spectrum of Pentane

The H-1 NMR spectrum of pentane

C-13 NMR spectroscopy index

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