Advanced Organic Chemistry: 1H NMR spectrum of octane

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

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 octane

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H-1 proton NMR spectroscopy - spectra index

C8H18 low and high resolution 1H proton nmr spectrum of octane analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 H-1 nmr for n-octane 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 - octane here.

The chemical shifts quoted in ppm on the diagram of the H-1 NMR spectrum of octane 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 octane molecule.

Octane, C8H18, CH3(CH2)6CH3, CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH3, alkane

Interpreting the H-1 NMR spectrum of octane

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

Theoretically the hydrogen atoms (protons) of octane occupy 4 different chemical environments, but the low resolution NMR spectra only shows 2 peaks of different H-1 NMR chemical shifts (diagram above for octane).

These would be in the integrated proton ratio of (2:1) 6:12 for the CH3:CH2 proton groups ratio.

CH3CH2CH2CH2CH2CH2CH2CH3 

Note the theoretical proton ratio of (3:2:2:2) 6:4:4:4 of the 4 colours of the protons in the 4 chemically different environments

Chemical shifts (a) to (d) on the H-1 NMR spectrum diagram for octane.

Although there are 18 hydrogen atoms in the molecule, theoretically there are only 4 possible different chemical environments for the hydrogen atoms in octane molecule.

The high resolution 1H NMR spectrum of octane

The ppm quoted on the diagram represent the peak of resonance intensity for a particular proton group in the molecule of octane - 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 octane below.

So, using the chemical shifts and applying the n+1 rule to octane and make some predictions using some colour coding! (In problem solving you work the other way round!)

(a) 1H Chemical shift 0.88 ppm for the two methyl groups.

In each case the CH3 group proton resonance is split into a 1:2:1 triplet by the neighbouring CH2 group.

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

(b) to (d) 1H Chemical shift ~1.26 ppm

All the proton resonances for the six CH2 groups are very close together - three pairs of equivalent protons in three different chemical environments.

Theoretically, at very high resolution, the spectrum for octane would reveal these three different resonances split into:

two 1:4:6:4:1 quintets from CH2-CH2-CH2, (c) and (d) resonance splitting,

and a 1:5:10:10:5:1 sextet from CH2-CH2-CH3, (b) resonance splitting.

Evidence for the presence of CH2-CH2-CH2 and CH3-CH2-CH2 groups in the molecule of octane.

Number of directly adjacent protons 1H causing splitting Splitting pattern produced from the n+1 rule on spin-spin coupling 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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Links associated with octane

The chemistry of ALKANES revision notes INDEX

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

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