mass spectrum of butane C4H10 CH3CH2CH2CH3 fragmentation pattern of m/z m/e ions for analysis and identification of n-butane image diagram doc brown's advanced organic chemistry revision notes

Advanced Organic Chemistry: Mass spectrum of butane CH₃CH₂CH₂CH₃

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Interpreting and explaining the mass spectrum of butane CH₃CH₂CH₂CH₃

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

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Introductory note on the mass spectrum of butane

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

If M represents the butane 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 butane and only the formation of singly charged positive are considered for the mass spectrum of butane.

I've included a stick diagram and table of m/z ions for the mass spectrum of butane 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 butane.

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 butane. 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 butane, but the mass spectrometer software does!

mass spectrum of butane C4H10 CH3CH2CH2CH3 fragmentation pattern of m/z m/e ions for analysis and identification of n-butane image diagram doc brown's advanced organic chemistry revision notes

Butane C₄H₁₀

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

Interpreting the fragmentation pattern of the mass spectrum of butane

[M]⁺ is the molecular ion peak (M) with an m/z of 58 corresponding to [C₄H₁₀]⁺, the original butane molecule minus an electron, [CH₃CH₂CH₂CH₃]⁺.

You might see a tiny M+1 peak at m/z 59, corresponds to an ionised butane 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.

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

Not clear with the molecular ion peak for butane, 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 (butane) 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 butane.

Unless otherwise indicated, assume the carbon atoms in butane are the ¹²C isotope.

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

The molecular ion peak is from the m/z 58 ion: [CH₃CH₂CH₂CH₃]⁺.

m/z value of [fragment]⁺	57	44	43	42	41	39	29	28	27	15
[molecular fragment]⁺	[C₄H₉]⁺	[¹³C¹²C₂H₇]⁺	[C₃H₇]⁺	[C₃H₆]⁺	[C₃H₅]⁺	[C₃H₃]⁺	[C₂H₅]⁺	[C₂H₄]⁺	[C₂H₃]⁺	[CH₃]⁺

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of butane

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

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 butane (tabulated above)

For linear alkanes, most of the most abundant ions correspond to ionised alkyl groups formed by bond C-C bond scission of the parent molecule e.g. propyl, ethyl and methyl, which subsequently lose hydrogen atoms to give slightly lighter ions. Note that the C-C bond is weaker than the C-H bond.

Formation of m/z 57 ion:

[CH₃CH₂CH₂CH₃]⁺ ===> [C₄H₉]⁺ + H

_{C-H bond scission, loss of proton, mass change
58-1 = 57, (M-1 ion peak)}

Formation of m/z 43 ion:

[CH₃CH₂CH₂CH₃]⁺ ===> [CH₃CH₂CH₂]⁺ + CH₃

C-C bond scission, loss of methyl group, mass change 58 - 15 = 43 (M-15 ion peak)

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

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

The m/z 39, 41 and 42 ions can be formed by hydrogen loss from the m/z 43 ion.

Formation of m/z 29 ion:

[CH₃CH₂CH₂CH₃]⁺ ===> [CH₃CH₂]⁺ + CH₂CH₃

C-C bond scission of the parent molecular ion, loss ethyl group,

mass change 58 - 29 = 29 (M-29 ion peak)

The butane molecule is effectively cleaved in half.

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

[CH₃CH₂CH₂]⁺ ===> [ 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.

Can also be formed by loss of H atom from the m/z 29 ion (above)

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:

[CH₃CH₂CH₂CH₃]⁺ ===> [CH₃]⁺ + CH₃CH₂CH₂

C-C bond scission, loss of propyl group.

mass change 58 - 43 = 15, 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 butane has much higher 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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