[Author ©  Dr Phil Brown PhD: Doc Brown's advanced level organic chemistry exam revision notes suitable for students of UK A level chemistry courses & US K12 grade 11, grade 12 and AP honors chemistry courses: Molecular spectroscopy - analysing the mass spectrum of benzoic acid [spectra page updated April 3rd 2026 *]

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Some practice questions based on the mass spectrum of benzoic acid

Benzoic acid (benenecarboxylic acid),  C₇H₆O₂,  C₆H₅COOH  ,  , ,

The molecular structure and naming of aromatic compounds

Interpreting the fragmentation pattern of the mass spectrum of benzoic acid

[M]⁺ is the molecular ion peak with an m/z of 122 (M) corresponding to [C₇H₆O₂]⁺, the original benzoic acid molecule minus an electron, [C₆H₅COOH]⁺

The small M+1 peak at m/z ion 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.

The base peak ion for the mass spectrum of benzoic acid is the m/z 105 ion [C₆H₅C=O]⁺

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 ion  [C₇H₆O₂]⁺  or  [C₆H₅COOH]⁺

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

PLEASE NOTE I have found it difficult to find 'authentic' equations to explain mass spectra fragmentation patterns and it is complex chemistry! I've identified the formulae of the ionised fragments on the mass spectrum diagram, but the equations are from the internet or my conjecture as to how the ions might be formed - please take care in using the information, especially for assignments at university or pre-university level.

Atomic masses: H = 1;  C = 12 (~1% 13);  O = 16;

Bond enthalpies = kJ/mol: aryl = 518, aryl C-H = 412;  C-O = 360;  C=O  = 743;  O-H = 463

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

Formation of m/z 105 ion:

[C₆H₅COOH]⁺  ===>  [C₆H₅C=O]⁺  +  OH

C-O bond scission in the parent molecular ion,

mass change 122 - 17  = 105 (M-17 ion peak)

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

The m/z 106 ion [¹³C¹²C₅H₅C=O]⁺ will be formed in the same way, but the ionised fragment contains a carbon-13 isotope (not likely to be [C₇H₅O]⁺).

Note that an accurate mass spectrometer can sort them out, it can measure relative fragment ion masses to four decimal places e.g. using very accurate relative isotopic masses,

¹²C = 12.0000  ¹³C = 13.0034, ¹H = 1.0078,  ¹⁶O = 15.9949, you can then calculate (predict) that the accurate relative ion masses are:

For m/z 106: [¹³C¹²C₅H₅CO]⁺ = 106.0373 and [C₆H₆CO]⁺ = 106.0417, difference of 0.0044 in ion relative mass, no problem! 106

Formation of m/z 94 ion:

[C₆H₅COOH]⁺  ===>  [C₆H₆O]⁺  +  CO

Loss of CO from parent molecular ion.

mass change 122 - 28  = 94 (M-28 ion peak)

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,

mass, change 122 - 45 = 77 (M-45 ion peak)

The appearance of the m/z 77 'phenyl' ion is very characteristic of aromatic monosubstituted benzene compounds.

Or, it can be formed by loss of CO from the m/z 105 ion.

[C₆H₅CO]⁺  ===>  [C₆H₅]⁺  +  CO

Loss of protons from the m/z 77 ion can give m/z ions 76, 75 and 74 (see table).

The m/z ion 78 [¹³C¹²C₅H₅]⁺ will be formed in the same way, but the ionised fragment contains a carbon-13 isotope (not likely to be [C₆H₆]⁺).

Note that an accurate mass spectrometer can sort them out, it can measure relative fragment ion masses to four decimal places e.g. using very accurate relative isotopic masses,

¹²C = 12.0000  ¹³C = 13.0034, ¹H = 1.0078, you can then calculate (predict) that the accurate relative ion masses are:

For m/z 78: [¹³C¹²C₅H₅]⁺ = 78.0424  and  [C₆H₆]⁺ = 78.0468, difference of 0.0044 in ion relative mass, no problem!

Formation of m/z 51 ion:

[C₆H₅]⁺  ===>  [C₄H₃]⁺  +  C₂H₂

C-C bond scission of the m/z 77 ion,

mass change 77 - 26 = 51

Formation of m/z 45 ion:

[C₆H₅COOH]⁺  ===>  [COOH]⁺  +  C₆H₅

C-C bond scission of parent molecular ion, loss of the phenyl group,

mass change 122 - 77 = 45 (M-77 ion peak)

Formation of m/z 27 ion:

[C₆H₅]⁺  ===>  [C₆H₅]⁺  +  C₂H₂

C-C bond scission of parent molecular ion, loss of the phenyl group,

mass change 122 - 77 = 45 (M-77 ion peak)

Formation of m/z 17 ion:

[C₆H₅COOH]⁺  ===>  [OH]⁺  +  C₆H₅CO

C-O bond scission in the parent molecular ion,

mass change 122 - 105  = 17 (M-105 ion peak)

Far lower probability than the formation of the m/z 105 ion (see above).

INDEX of all AROMATIC COMPOUND chemistry revision notes

The physical and chemical properties of benzoic acid and selected derivatives

Mass spectrometry index

All Advanced Organic Chemistry Notes