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

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.

Not clear with the molecular ion peak for 2-methylpropane, but check out the ratio of the relative abundances of the m/z 43 and 44 fragment ions (see the mass spectrum diagram above and table of fragment ions below).

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.

The molecular ion peak is from the m/z 58 ion: [(CH₃)₃CH]⁺.

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 some of the most abundant ion peaks in the mass spectrum 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.

The m/z 44 ion peak for 2-methylpropane is probably [¹³C¹²C₂H₇]⁺ and should be about 1/25th of the m/z 43 ion peak (see note on isotopes in the introduction above).

Formation of m/z 29 ion:

[(CH₃)₃CH]⁺  ===>  [C₂H₅]⁺  +  C₂H₅

C-C bond scission in the parent molecular ion, proton rearrangement, mass change 58 - 29 = 29.

Or from C-C bond scission in the m/z 43 ion and proton rearrangement.

[(CH₃)₂CH]⁺   ===>  [ C₂H₅]⁺  +  CH₂

Formation of m/z 28 ion:

[C₃H₇]⁺  ===>  [C₂H₄]⁺  +  CH₃

C-C bond scission of the m/z 43 base peak ion, loss of methyl group.

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

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.

The chemistry of ALKANES revision notes INDEX

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

The H-1 NMR spectrum of 2-methylpropane (methylpropane, isobutane)

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

Mass spectroscopy index

ALL SPECTROSCOPY INDEXES

All Advanced Organic Chemistry Notes

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