HOME PAGE * KS3 SCIENCES * GCSE BIOLOGY CHEMISTRY PHYSICS * ADVANCED LEVEL CHEMISTRY

SPECTROSCOPY INDEXES  *  All Advanced Organic Chemistry Notes  *  [SEARCH BOX]

Advanced Organic Chemistry: H-1 hydrogen-1 (proton) NMR spectrum of Pentane

H-1 hydrogen-1 (proton) 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 H-1 NMR spectra

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

low and high resolution H-1 proton nmr spectrum of pentane analysis interpretation of chemical shifts ppm spin spin line splitting diagram doc brown's advanced organic chemistry revision notes

TMS is the acronym for tetramethylsilane, formula Si(CH3)4, whose protons are arbitrarily given a chemical shift of 0.0 ppm. This is the 'standard' in 1H NMR spectroscopy and all other proton shifts, called chemical shifts, depend on the individual (electronic) chemical environment of the hydrogen atoms in an organic molecule - pentane here.

The chemical shifts quoted in ppm on the diagram of the H-1 NMR spectrum of pentane represent the peaks of the intensity of the chemical shifts of pentane (which are often groups of split lines at high resolution) AND the relative integrated areas under the peaks gives you the ratio of protons in the different chemical environments of the pentane molecule.

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 H-1 NMR spectrum of pentane

For relatively simple molecules, the low resolution H-1 NMR spectrum of pentane is a good starting point.

The hydrogen atoms (protons) of pentane occupy 3 different chemical environments so that the H-1 proton low resolution NMR spectra should show 3 chemical shift peaks (diagram above).

CH3CH2CH2CH2CH3  (note the ratio of the three colours of the protons in pentane)

Although there are 12 hydrogen atoms in the molecule, pentane is a symmetrical molecule with only three possible chemical environments for the 12 hydrogen atoms.

The peak area ratio of different chemical environments is the same ratio as the number of protons that occupy these different situations.

The molecular proton ratios is 6 : 4 : 2, so the low resolution spectrum should show 3 peaks in the ratio 3 : 2 : 1,  from the integrated proton ratio from the spectrum.

The high resolution spectrum of pentane is even more complex than shown in the diagram.

CH3CH2CH2CH2CH3

From the n+1 rule the end CH3 group of protons (H3) resonance lines are split into a triplet by the neighbouring CH2 groups (H2), at a chemical shift of 0.884 ppm).

The chemical shift resonance of the central CH2 group of protons (H2) is also split into a quintet (1:4:6:4:1) by four protons from equivalent proton CH2 groups either side of it (H2) at a chemical shift of 1.26 ppm).

The two 'end' CH2 groups of protons (H2) chemical shift is split into a sextet (1:5:10:10:5:1) by CH2 (H2) and CH3 (H3) groups of protons (total of 5 protons), centred around a chemical shift ~1.30 ppm.

The last two sets of chemical shift lines are bunched together  - you need very high resolution to sort out a septet of chemical shifts, all very close together.

This sextet of resonances is characteristic of a propyl group (CH3CH2CH2).


Number of protons 1H causing splitting Splitting pattern produced from the n+1 rule and the theoretical ratio of line intensities
0 means no splitting             1            
1 creates a doublet           1   1          
2 creates a triplet         1   2   1        
3 creates a quartet       1   3   3   1      
4 creates a quintet     1   4   6   4   1    
5 creates a sextet   1   5   10   10   5   1  
6 creates a septet 1   6   15   20   15   6   1
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 proton H-1 NMR spectra of pentane, low resolution & high resolution proton nmr spectra of pentane, H-1 nmr spectrum of pentane, understanding the hydrogen-1 nmr spectrum of pentane, explaining the line splitting patterns in the high resolution H-1 nmr spectra of pentane, revising the H-1 nmr spectrum of pentane, proton nmr of pentane, ppm chemical shifts of the H-1 nmr spectrum of pentane, explaining and analyzing spin-spin line splitting in the H-1 nmr spectrum, how to construct the diagram of the H-1 nmr spectrum of pentane, how to work out the number of chemically different protons in the structure of the pentane organic molecule

Molecular structure diagram of the proton NMR diagram for the 1H NMR spectrum of pentane. The proton ratio in the 1H NMR spectrum of pentane. Deducing the number of different chemical environments of the protons in the pentane molecule from the 1H chemical shifts in the hydrogen-1 NMR spectrum of pentane. Analysing the high resolution 1H NMR spectrum of pentane. Analysing the low resolution 1H NMR spectrum of pentane. You may need to know the relative molecular mass of pentane to deduce the molecular formula from the proton ratio of the 1H NMR spectrum of pentane. Revision notes on the proton NMR spectrum of pentane. Matching and deducing the structure of the pentane molecule from its hydrogen-1 NMR spectrum. Proton NMR spectroscopy of alkane, 1H 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 C-13 NMR spectrum of Pentane

H-1 proton NMR spectroscopy index  (Please read 8 points at the top of the 1H NMR index page)

ALL SPECTROSCOPY INDEXES

All Advanced Organic Chemistry Notes

Use My Google search site box

Email doc b: chem55555@hotmail.com

TOP OF PAGE

 Doc Brown's Chemistry 

*

TOP OF PAGE