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Interpreting the fragmentation pattern of the mass spectrum of butanoic acid

[M]⁺ is the molecular ion peak (M) with an m/z of 88 corresponding to [C₄H₈O₂]⁺, the original butanoic acid molecule minus an electron, [CH₃CH₂CH₂COOH]⁺.

It is a very small peak, suggesting the parent molecular ion is not very stable relative to other fragmentation ions.

You might see a tiny M+1 peak at m/z 89, corresponds to an ionised butanoic acid molecule with one ¹³C atom in it i.e. an ionised butanoic acid molecule of formula [¹³C¹²C₃H₈O₂]⁺

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.

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

However, it doesn't look in right proportion to the m/z 88 ion, so perhaps there is another M+1 molecular ion.

The most abundant ion of the molecule under mass spectrometry investigation (butanoic acid) 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 butanoic acid.

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

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

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

Atomic masses: H = 1;  C = 12;  O = 16

Bond enthalpies kJ/mol: C-C = 348;  C-H = 412;  C-O = 360;  C=O

Possible equations to explain the most abundant ion peaks of butanoic acid (tabulated above)

Formation of m/z 87 ion:

[CH₃CH₂CH₂COOH]⁺  ===>  [C₄H₈O₂]⁺  +  H

C-H bond scission, hydrogen atom loss, mass change 88 - 1 = 87.

Formation of m/z 73 ion:

[CH₃CH₂CH₂COOH]⁺  ===>  [CH₂CH₂COOH]⁺  +  CH₃

C-C bond scission, loss of methyl group, mass change 88 - 15 = 73.

Formation of m/z 60 ion:

[CH₃CH₂CH₂COOH]⁺  ===>  [C₂H₄O₂]⁺  +  C₂H₄

Elimination of ethene, mass change 88 - 28 = 60.

This due to the McLafferty rearrangement, details not required, but is a characteristic peak in the mass spectroscopy of carboxylic acid molecules.

The m/z 60 ion is the base peak ion, the most abundant and 'stable' ion fragment.

Formation of m/z 45 ion:

[CH₃CH₂CH₂COOH]⁺  ===>  [COOH]⁺  +  CH₃CH₂CH₂

C-C bond scission of parent molecular ion, mass change 88 - 43 = 45.

Formation of m/z 43 ion:

[CH₃CH₂CH₂COOH]⁺  ===>  [CH₃CH₂CH₂]⁺  +  COOH

C-C bond scission of parent molecular ion, mass change 88 - 45 = 43.

Further hydrogen atom/molecule losses lead to m/z ions of mass 42, 41 and 39.

Formation of m/z 29 ion:

[CH₃CH₂CH₂COOH]⁺  ===>  [CH₃CH₂]⁺  +  CH₂COOH

C-C bond scission of parent molecular ion, mass change 88 - 59 = 29.

Further hydrogen atom/molecule losses lead to m/z ions of mass 28 and 27.

The chemistry of CARBOXYLIC ACIDS and DERIVATIVES revision notes INDEX

Mass spectroscopy index

ALL SPECTROSCOPY INDEXES

All Advanced Organic Chemistry Notes

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