Advanced Organic Chemistry: Mass spectrum of benzoic acid

Interpreting the mass spectrum of benzoic acid  (re-edit)

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 benzoic acid

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Mass spectroscopy - spectra index

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mass spectrum of benzoic acid C7H6O2 C6H5COOH fragmentation pattern of m/z m/e ions for analysis and identification of benzoic acid image diagram doc brown's advanced organic chemistry revision notes 

Benzoic acid (benenecarboxylic acid),  C7H6O2,  C6H5COOH  ,  (c) doc b , (c) doc b , (c) doc b

Interpreting the fragmentation pattern of the mass spectrum of benzoic acid

[M]+ is the molecular ion peak with an m/z of 122 corresponding to [C6H6O]+, the original benzoic acid molecule minus an electron, [C6H5COOH]+.

The small M+1 peak at m/z 123, corresponds to an ionised benzoic acid molecule with one 13C atom in it i.e. an ionised benzoic acid molecule of formula [13C12C6H6O2]+

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.

Benzoic acid has 7 carbon atoms, so on average, ~1 in 14 molecules will contain a 13C atom.

There is even a tiny peak for M+2, m/z 123, due to two 13C atoms in the molecule?

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

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

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

The parent molecular ion of benzoic acid m/z 122: [C6H5COOH]+
m/z value of [fragment]+ 106 with 13C atom 105 94 78 with 13C atom 77 76 75 74
[molecular fragment]+ [C6H5C=O]+ [C6H5C=O]+ [C6H6O]+ [C6H5]+ [C6H5]+ [C6H4]+ [C6H3]+ [C6H2]+
m/z value of [fragment]+ 52 51 50 39 38 37 27
[molecular fragment]+ [C4H4]+ [C4H3]+ [C4H2]+ [C3H3]+ [C3H2]+ [C3H]+ [C2H3]+

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

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

Bond enthalpies = kJ/mol: aryl C-H = 412;  C-O = 360;  C=O  = 743;  O-H = 463

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

Formation of m/z 105 ion:

[C6H5COOH]+  ===>  [C6H5CO]+  +  OH

C-O bond scission in the parent molecular ion, mass change 122 - 17  = 105.

The m/z 105 ion is the base peak ion, the most abundant and 'stable' ion fragment.

Formation of m/z 77 ion:

[C6H5COOH]+  ===>  [C6H5]+  +  COOH

C-C bond scission of parent molecular ion, loss of the carboxylic acid group, the appearance of the m/z 77 'phenyl' ion is very characteristic of aromatic monosubstituted benzene compounds mass, change 122 - 45 = 77.

Formation of m/z 45 ion:

[C6H5COOH]+  ===>  [COOH]+  +  C6H5

C-C bond scission of parent molecular ion, loss of the phenyl group, change 122 - 77 = 45.

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