low/high resolution 1H proton nmr spectrum of butane C4H10 CH3CH2CH2CH3 analysis interpretation of chemical shifts ppm spin spin line splitting H-1 n-butane 1-H nmr explaining spin-spin coupling for line splitting doc brown's advanced organic chemistry revision notes

Advanced Organic Chemistry: 1H NMR spectrum of butane CH₃CH₂CH₂CH₃

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Interpreting the H-1 hydrogen-1 (proton) NMR spectrum of butane CH₃CH₂CH₂CH₃

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

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H-1 proton NMR spectroscopy - spectra index

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

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 adjacent and 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

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 [C₂H_3,4,5]⁺.

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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What next? 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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