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

Advanced Organic Chemistry: Mass spectrum of 2-methylpropane (CH₃)₃CH

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Interpreting and explaining the mass spectrum of 2-methylpropane (isobutane)

[Author © Dr Phil Brown PhD: Doc Brown's advanced level organic chemistry exam revision notes suitable for students of UK A level chemistry courses & US K12 grade 11, grade 12 and AP honors chemistry courses: Molecular spectroscopy analysis of 2-methylpropane (mass spectra) [spectra page updated April 3rd 2026 *]

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Introductory note on the mass spectrum of 2-methylpropane

Students and teachers please note my explanation of the mass spectrum of 2-methylpropane is designed for advanced, but pre-university, chemistry courses.

If M represents the 2-methylpropane molecule, the initial ionisation to give the molecular ion is:

M_(g) + high KE e^- ==> [M•]⁺_(g) + 2e^- and fragmentation equations assume [M]⁺ is the start of the processes and all species are in a gaseous state.

I've not usually shown an unpaired electron on e.g. an ion or a non-ionised alkyl radical R e.g.

[M•]⁺ ==> [X]⁺ + R•, but you should be aware this is a more accurate depiction of some processes.

I've used simplified equations to show how some of the ions that might be formed in the fragmentation pattern for the mass spectrum of 2-methylpropane.

I've included a stick diagram and table of m/z ions for the mass spectrum of 2-methylpropane and doing the mass spectrum analysis under standard conditions, databases can be compiled based on complex fingerprint patterns, often involving the relative intensities of many fragment ions, and used to identify compounds including 2-methylpropane.

In selected cases, where two different fragment ions have the same integer m/z value, I've pointed out that modern mass spectrometers can measure relative ion mass to four decimal places. So, using accurate isotopic masses, I've calculated and compared the accurate ion masses if appropriate for 2-methylpropane. BUT strictly speaking, 0.0005 should be deducted for singly charged ions to account for the loss of the electron in their formation. I have NOT done this for 2-methylpropane, but the mass spectrometer software does!

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

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

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.

The base peak ion for the mass spectrum of 2-methylpropane is the m/z 43 [C₃H₇]⁺

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]⁺.

m/z value of [fragment]⁺	57	44	43	42	41
[molecular fragment]⁺	[C₄H₉]⁺	[¹³C¹²C₂H₇]⁺	[C₃H₇]⁺	[C₃H₆]⁺	[C₃H₅]⁺

m/z value of [fragment]⁺	40	39	29	28	27	15
[molecular fragment]⁺	[C₃H₄]⁺	[C₃H₃]⁺	[C₂H₅]⁺	[C₂H₄]⁺	[C₂H₃]⁺	[CH₃]⁺

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

PLEASE NOTE I have found it difficult to find 'authentic' equations to explain mass spectra fragmentation patterns and it is complex chemistry! I've identified the formulae of the ionised fragments on the mass spectrum diagram, but the equations are from the internet or my conjecture as to how the ions might be formed - please take care in using the information, especially for assignments at university or pre-university level.

Atomic masses: H = 1; C = 12 (~1% 13)

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 57 ion:

[(CH₃)₃CH]⁺ ===> [(CH₃)₂CH]⁺ + CH₃

C-H bond scission, loss of proton,

mass change 58 - 1 = 57 (M-1 ion peak)

Formation of m/z 43 ion:

[(CH₃)₃CH]⁺ ===> [(CH₃)₂CH]⁺ + CH₃

C-C bond scission, loss of methyl group,

mass change 58 - 15 = 43 (M-15 ion peak)

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

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

The m/z 43 ion can lose hydrogen atoms/molecule to give the m/z ions 42 down to 39.

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 (M-29 ion peak)

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

[(CH₃)₂CH]⁺ ===> [ C₂H₅]⁺ + CH₂

The m/z 29 ion can lose protons to give the m/z 28 and 27 uons.

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]⁺ + C₃H₇

C-C bond scission,

mass change 58 - 43 = 15 (M-43 ion peak)

but this time the methyl group carries the positive charge.

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 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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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)

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