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Advanced Organic Chemistry: Mass spectrum of 2-methylpropane

Interpreting the mass spectrum of 2-methylpropane (isobutane)

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 mass spectra of 2-methylpropane

mass spectrum of 2-methylpropane C4H10 (CH3)3CH fragmentation pattern of m/z m/e ions for analysis and identification of isobutane image diagram doc brown's advanced organic chemistry revision notes 

2-methylpropane  C4H10  alkanes structure and naming (c) doc b   alkanes structure and naming (c) doc b   alkanes structure and naming (c) doc b 

Interpreting the fragmentation pattern of the mass spectrum of 2-methylpropane

[M]+ is the molecular ion peak (M) with an m/z of 58 corresponding to [C4H10]+, the original 2-methylpropane molecule minus an electron, [(CH3)3CH]+.

You might see a tiny M+1 peak at m/z 59, corresponds to an ionised methylpropane molecule with one 13C atom in it i.e. an ionised butane molecule of formula [13C12C3H10]+

Carbon-13 only accounts for ~1% of all carbon atoms (12C ~99%), but the more carbon atoms in the molecule, the greater the probability of observing this 13C M+1 peak.

Methylpropane has 4 carbon atoms, so on average, ~1 in 25 molecules will contain a 13C atom.

The most abundant ion of the molecule under mass spectrometry investigation (2-methylpropane) is usually given an arbitrary abundance value of 100, called the base ion peak, and all other abundances ('intensities') are measured against it.

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of 2-methylpropane.

Unless otherwise indicated, assume the carbon atoms in 2-methylpropane are the 12C isotope.

Some of the possible positive ions, [molecular fragment]+, formed in the mass spectrometry of 2-methylpropane.

m/z value of [fragment]+ 57 43 42 41 39 29 28 27 15
[molecular fragment]+ [C4H9]+ [C3H7]+ [C3H6]+ [C3H5]+ [C3H3]+ [C2H5]+ [C2H4]+ [C2H3]+ [CH3]+

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of 2-methylpropane

Atomic masses: H = 1; C = 12;

Bond enthalpies kJ/mol: C-C = 348;  C-H = 412

Possible equations to explain the most abundant ion peaks of 2-methylpropane (tabulated above)

For branched alkanes, C-C bond scission of the parent molecule tends to occur at the point of branching (automatically the case with methylpropane) and note that the C-C bond is weaker than the C-H bond.

Formation of m/z 43 ion:

[(CH3)3CH]+  ===>  [(CH3)2CH]+  +  CH3

C-C bond scission, loss of methyl group, mass change 58 - 15 = 43.

This is a secondary carbocation, stabilised by the +I effect of the methyl groups.

The m/z 43 ion is the base peak ion, the most abundant and 'stable' ion fragment.

Formation of m/z 29 ion:

[(CH3)3CH]+  ===>  [C2H5]+  +  C2H5

C-C bond scission, mass change 58 - 29 = 29.

Formation of m/z 27 ion:

[C2H5]+  ===>  [C2H3]+  +  H2

Elimination of a hydrogen molecule from the m/z 29 ion, mass change 29 - 2 = 27.

Formation of m/z 15 ion:

[(CH3)3CH]+  ===>  [CH3]+  +  CH3CH2CH2

C-C bond scission, mass change 58 - 43 = 15, but this time the methyl group carries the positive charge.

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


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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 mass spectrum of 2-methylpropane has much lower relative abundances of the m/z 28 and 29 ions.

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.

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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