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Interpreting the fragmentation pattern of the mass spectrum of propanoic acid

[M]⁺ is the molecular ion peak (M) with an m/z of 74 corresponding to [C₃H₆O₂]⁺, the original propanoic acid molecule minus an electron, [CH₃CH₂COOH]⁺.

This is a relatively large peak, suggesting the parent molecular ion is relatively stable.

The small M+1 peak at m/z 75, corresponds to an ionised propanoic acid molecule with one ¹³C atom in it i.e. an ionised propanoic acid molecule of formula [¹³C¹²C₂H₆O₂]⁺

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.

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

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

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

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

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

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

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

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

Formation of m/z 73 ion:

[CH₃CH₂COOH]⁺  ===>  [C₃H₅O₂]⁺  +  H

C-H bond scission, loss of hydrogen atom, mass change 74 - 1 = 73.

Formation of m/z 57 ion:

[CH₃CH₂COOH]⁺  ===>  [CH₃CH₂C=O]⁺  +  OH

Scission of C-O bond, loss of hydroxyl group, mass change 74 - 17 = 57.

Loss of protons leads to the m/z 56 and 55 ions.

Formation of m/z 45 ion:

[CH₃CH₂COOH]⁺  ===>  [COOH]⁺  +  CH₃CH₂

C-C bond scission, loss of ethyl group, mass change 74 - 29 = 45.

Loss of protons leads to the m/z 43 and 42 ions.

Formation of m/z 29 ion:

[CH₃CH₂COOH]⁺  ===>  [CH₃CH₂]⁺  +  COOH

C-C bond scission, loss of carboxyl group, mass change 74 - 45 = 29.

Formation of m/z 28 ion:

[?]⁺  ===>  [C₂H₄]⁺  or  [CO]⁺  +  ?

Formation of an ionised ethene molecule (H loss from ethyl fragment)? or an ionised carbon monoxide molecule (COOH fragment split)?

The m/z 28 ion is the base ion peak, the most abundant ionised fragment.

Formation of m/z 15 ion:

[CH₃CH₂COOH]⁺  ===>  [CH₃]⁺  +  CH₂COOH

C-C bond scission, loss of ionised methyl group, mass change 74 - 57 = 15.

A ionised methyl group can be lost from other larger fragments e.g. the m/z 73 ion.

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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