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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

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

For more see The molecular structure, classification and naming of alkanes

Interpreting the H-1 NMR spectrum of butane

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

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  protons.

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 adjacent 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

You might think this 1H NMR resonance is split into a sextet by the adjacent CH2 and CH3 protons (n+1 = 6), but this is not the case for the symmetrical butane molecule.

The two groups of CH2 protons are equivalent to each other and since they have the same chemical environment, giving the same chemical shift, their 1H fields cannot split each other's 1H resonance.

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

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

However, these interpretations are apparently an oversimplification !!!

This interpretation assumes that the two methyl groups (2 x CH3) are equivalent in every respect (coupling = zero), and it also assumes the methylene groups (2 x CH2) are also equivalent in every respect (again coupling is zero).

However, the real spectrum is more complicated because although both the methyl groups and both the methylene groups are chemically equivalent to each other, they are apparently not magnetically equivalent.

As a result the splitting pattern is more complex with other multiple resonance lines in the same region as the quartet for the CH2 groups and the triplet for the CH3 groups.

BUT don't worry, this is advanced university level NMR spectroscopy, and at pre-university level, for butane, the proton ratio 3:2 is the most important aspect of its 1H NMR spectrum indicating two chemical environments of the protons.

For more advanced stuff see http://u-of-o-nmr-facility.blogspot.com/2009/08/500-mhz-1-h-nmr-spectrum-of-butane.html

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
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.


Key words & phrases: C4H10 CH3CH2CH2CH3 Interpreting the proton H-1 NMR spectra of butane, low resolution & high resolution proton nmr spectra of butane, H-1 nmr spectrum of butane, understanding the hydrogen-1 nmr spectrum of butane, explaining the line splitting patterns from spin-spin coupling  in the high resolution H-1 nmr spectra of butane, revising the H-1 nmr spectrum of butane, proton nmr of butane, ppm chemical shifts of the H-1 nmr spectrum of butane, 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 butane, how to work out the number of chemically different protons in the structure of the butane organic molecule, how to analyse the chemical shifts in the hydrogen-1 H-1 proton NMR spectrum of butane using the n+1 rule to explain the spin - spin coupling ine splitting in the proton nmr spectrum of butane deducing the nature of the protons from the chemical shifts ppm in the H-1 nmr spectrum of butane examining the 1H nmr spectrum of  butane analysing the 1-H nmr spectrum of butane how do you sketch and interpret the H-1 NMR spectrum of butane interpreting interpretation of the 1H proton spin-spin coupling causing line splitting in the NMR spectrum of butane  assignment of chemical shifts in the proton 1H NMR spectrum of butane formula explaining spin-spin coupling for line splitting for n-butane

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


Links associated with butane

The chemistry of ALKANES revision notes INDEX

The infrared spectrum of butane

The mass spectrum of butane

The C-13 NMR spectrum of butane

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

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