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Advanced Organic Chemistry: Mass spectrum of butanoic acid

Interpreting the mass spectrum of butanoic acid (butyric 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 butanoic acid

mass spectrum of butanoic acid C4H8O2 CH3CH2CH2COOH fragmentation pattern of m/z m/e ions for analysis and identification of butyric acid image diagram doc brown's advanced organic chemistry revision notes 

Butanoic acid (butyric acid(c) doc b  (c) doc b  (c) doc b a carboxylic acid

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.

m/z value of [fragment]+ 87 73 61 60 55 ? 45
[molecular fragment]+ [C4H8O2]+ [?]+ [C2H3O2]+ [C2H4O2]+ [C3H3O]+ [COOH]+
m/z value of [fragment]+ 43 42 41 39 29 27
[molecular fragment]+ [C3H7]+ [C3H6]+ [C3H5]+ [C3H3]+ [C2H5]+ [C2H3]+

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.


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