low/high resolution 1H proton nmr spectrum of 2-methylpropane C4H10 (CH3)3CH analysis interpretation of chemical shifts ppm spin spin line splitting H-1 isobutane 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 2-methylpropane (isobutane)

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Interpreting the H-1 hydrogen-1 (proton) NMR spectrum of 2-methylpropane

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 2-methylpropane

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H-1 proton NMR spectroscopy - spectra index

low/high resolution 1H proton nmr spectrum of 2-methylpropane C4H10 (CH3)3CH analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 H-1 nmr for isobutane 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 - 2-methylpropane here.

The chemical shifts quoted in ppm on the diagram of the H-1 NMR spectrum of 2-methylpropane 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 2-methylpropane molecule.

2-methylpropane C₄H₁₀ 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 2-methylpropane

In terms of spin-spin coupling from the possible proton magnetic orientations, for 2-methylpropane I have only considered the interactions of non-equivalent protons on adjacent carbon atoms e.g. -CH-CH₃, protons etc.

For relatively simple molecules, the low resolution H-1 NMR spectrum of 2-methylpropane is a good starting point (low resolution diagram above, proton ratio 9:1).

The hydrogen atoms (protons) of 2-methylpropane 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 2-methylpropane).

CH₃CH(CH₃)CH₃ or (CH₃)₃CH

Note the proton ratio 9:1 of the 2 colours of the protons in the 2 chemically different environments

Chemical shifts (a) to (b) on the H-1 NMR spectrum diagram for 2-methylpropane.

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

The integrated signal proton ratio 9:1 observed in the high resolution H-1 NMR spectrum, corresponds with the structural formula of 2-methylpropane.

The high resolution 1H NMR spectrum of 2-methylpropane

In terms of spin-spin coupling from the possible proton magnetic orientations, for 2-methylpropane I have only considered the interactions of non-equivalent protons on adjacent carbon atoms e.g. -CH-CH₃, protons etc.

All low and high resolution spectra of 2-methylpropane show 2 groups of proton resonances and in the 9:1 ratio expected from the formula of 2-methylpropane.

The ppm quoted on the diagram represent the peak of resonance intensity for a particular proton group in the molecule of 2-methylpropane - 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 2-methylpropane below.

So, using the chemical shifts and applying the n+1 rule to 2-methylpropane and make some predictions using some colour coding! (In problem solving you work the other way round!)

(a) ¹H Chemical shift 0.8 ppm, methyl group protons: (CH₃)₃CH

This 1H resonance is split by the adjacent CH group proton into a 1:1 doublet (n+1 = 2)

Evidence for the presence of a CH group in the molecule of 2-methylpropane

(b) ¹H Chemical shift 2.0 ppm, CH proton: (CH₃)₃CH

This 1H resonance is split by the adjacent 3 x CH₃ proton into a dectet (n+9 = 10)

Evidence for the presence of a (CH₃)₃ group in the molecule of 2-methylpropane

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. 2-methylpropane has 3 'tertiary' methyl groups equating to 9 equivalent protons, hence the proton ratio of 9:1.

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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Links associated with 2-methylpropane

The chemistry of ALKANES revision notes INDEX

The infrared spectrum of 2-methylpropane (methylpropane, isobutane)

The mass spectrum of 2-methylpropane (methylpropane, isobutane)

The C-13 NMR spectrum of 2-methylpropane (methylpropane, isobutane)

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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