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Advanced Organic Chemistry: 1H NMR spectrum of butane

Interpreting the H-1 hydrogen-1 (proton) NMR spectrum of butane

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 butane

low/high resolution 1H proton nmr spectrum of butane C4H10 CH3CH2CH2CH3 analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 H-1 nmr for n-butane explaining spin-spin coupling for line splitting 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 resonances, called chemical shifts, are measured with respect to the TMS, and depend on the individual (electronic) chemical environment of the hydrogen atoms in an organic molecule - butane here.

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

Butane   C4H10  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 butane

In terms of spin-spin coupling from the possible proton magnetic orientations, for butane I have only considered the interactions of non-equivalent protons on adjacent carbon atoms e.g. -CH2-CH3, -CH2-CH2- protons.

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

The hydrogen atoms (protons) of butane occupy 2 different chemical environments so that the low resolution NMR spectra should show 2 principal peaks of different H-1 NMR chemical shifts (diagram above for butane).

CH3CH2CH2CH3

Note the proton ratio 6:4 (observed as 3:2) of the 2 colours of the 10 protons in the 2 chemically different environments

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

Although there are 10 hydrogen atoms in the molecule, there are only 2 possible different chemical environments for the hydrogen atoms in butane molecule because of its symmetry.

The integrated signal proton ratio 3:2 observed in the high resolution H-1 NMR spectrum, corresponds with the structural formula of butane.

The high resolution 1H NMR spectrum of butane

All low and high resolution spectra of butane show 2 groups of proton resonances and in the 3:2 ratio expected from the formula of butane.

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

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

(a) 1H Chemical shift 0.87 ppm, methyl group protons: CH3CH2CH2CH3

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

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

(b) 1H Chemical shift 1.35 ppm, CH2 group protons: CH3CH2CH2CH3

The two groups of CH2 protons are equivalent to each other and since they have the same chemical shift, they cannot split each other.

Therefore the CH2 proton resonance is just split into a 1:3:3:1 quartet by the methyl protons (n+3 = 4).

Evidence for the presence of a CH3 group in the molecule of butane

See also comparing the IR, mass, 1H NMR and 13C NMR spectra of 2-methylpropane and butane


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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Comparing the infrared, mass, 1H NMR and 13C NMR spectra of the 2 alkane isomers of C4H10

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 butane and 2-methylpropane image sizes.

The infrared spectra of butane and 2-methyl propane are quite similar, mainly due to C-H stretching and deformation vibrations, but you can see significant differences in the fingerprint region at wavenumbers 1500 to 600 cm-1.

The mass spectra of butane and 2-methyl propane are quite similar and both have a base ion peak of m/z 43 [C3H7]+, but here significant differences in the ratios of the m/z ions 27 to 29 [C2H3,4,5]+.

The 1H NMR spectra of butane and 2-methyl propane are quite similar in that both show the 8 hydrogen atoms exist in only 2 different chemical environment. However, they can be distinguished from each other by the different integrated proton ratios. Butane gives a (2) : (3) proton ratio and 2-methylbutane a (1) : (9) proton ratio. Butane has a pair of equivalent methyl groups of protons and a pair of equivalent CH2 proton groups, hence the proton ratio of 3:2.

The 13C NMR spectra of butane and 2-methyl propane are quite similar in that both show the 4 carbon atoms exist in only 2 different chemical environments.


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