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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

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.

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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 ratios are 6 : 4 : 2, so the low resolution spectrum should show 3 peaks in the ratio 3 : 2 : 1.

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

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


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