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Advanced Organic Chemistry: Mass spectrum of propanoic acid

Interpreting the mass spectrum of propanoic acid (propionic acid)

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 propanoic acid

mass spectrum of propanoic acid C3H6O2 CH3CH2COOH fragmentation pattern of m/z m/e ions for analysis and identification of propionic acid image diagram doc brown's advanced organic chemistry revision notes 

Propanoic acid  (propionic acid) C3H6O2   (c) doc b   (c) doc b   (c) doc b

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.

m/z value of [fragment]+ 73 57 56 55 47 46 45 43 ? 42
[molecular fragment]+ [C3H5O2]+ [C3H5O]+ [C3H4O]+ [C3H3O]+ [?]+ [?]+ [COOH]+ [CH3CO]+ [?]+
m/z value of [fragment]+ 31 30 29 29 28 28 27 26 15
[molecular fragment]+ [CH3O]+ [CH2O]+ [CHO]+ [C2H5]+ [C2H4]+ [CO]+ [C2H4]+ [C2H2]+ [CH3]+

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.


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