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 butane

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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 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]⁺.

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

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 43 ion:

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

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

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).

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.

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.

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, 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 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.