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Advanced Organic Chemistry: H-1 NMR spectrum of hexane

The H-1 hydrogen-1 (proton) NMR spectrum of hexane

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 of hexane

low and high resolution H-1 proton nmr spectrum of hexane analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 1-H nmr for hexane 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 - hexane here.

The chemical shifts quoted in ppm on the diagram of the H-1 NMR spectrum of hexane represent the peaks of the intensity of the chemical shifts of (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 hexane molecule.

Hexane C6H14, alkanes structure and naming (c) doc b , alkanes structure and naming (c) doc b , alkanes structure and naming (c) doc b

Interpreting the H-1 NMR spectrum of hexane

For relatively simple molecules, the low resolution H-1 NMR spectrum of hexane is a good starting point (low resolution diagram above).

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

CH3CH2CH2CH2CH2CH3

Note the ratio 3:2:2 of the three colours of the protons in the three chemically different environments

Although there are 14 hydrogen atoms in the molecule, there only 3 possible chemical environments for the hydrogen atoms in hexane molecule.

The integrated signal proton ratio 6:4:4 = 3:2:2 observed, corresponds with the structural formula of hexane shown above.

The high resolution H-1 NMR spectrum of hexane

All low and high resolution spectra of hexane show 3 groups of protons and in the ratio expected from the formula of hexane.

The ppm quoted on the diagram represent the peak of resonance intensity for a particular proton group in the molecule of hexane - since the peak' is at the apex of a band of H-1 NMR resonances due to spin - spin coupling field splitting effects - see high resolution notes on hexane below.

So, using the chemical shifts and applying the n+1 rule to hexane

1H Chemical shift 0.89 ppm for the 2 x CH3 protons  CH3CH2CH2CH2CH2CH3

The CH3 proton resonance is split into a 1:2:1 triplet by the adjacent CH2 protons, (n+1 = 3).

Evidence for the presence of a CH2 group in the molecule of hexane

1H Chemical shift 1.27 ppm for the 2 x 'central' CH2 protons  CH3CH2CH2CH2CH2CH3

This CH2 proton resonance is split into a 1:4:6:4:1 quintet by the CH2 and CH2 protons on either side (n+1 = 5)

Evidence for the presence of a CH2CH2CH2 group in the molecule of hexane

Very high resolution needed to sort the two CH2 resonances because the chemical shifts are so close together.

1H Chemical shift 1.29 ppm for the 2 x 'outer' CH2 protons  CH3CH2CH2CH2CH2CH3

This CH2 proton resonance is split into a 1:5:10:10:5:1 sextet by the CH3 and CH2 protons on either side (n+1 = 6)

Evidence for the presence of a CH3CH2CH2 group in the molecule of hexane


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

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