[Author © Dr Phil Brown PhD: Doc Brown's advanced level organic chemistry exam revision notes suitable for students of UK A level chemistry courses, IB chemistry & US K12 grade 11, grade 12 and AP honors chemistry courses: Molecular spectroscopy - analysing the mass spectra of methyl 2-hydroxybenzoate, methyl salicylate [spectra page updated April 4th 2026 *]

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Students and teachers please note my explanation of the mass spectrum of methyl 2-hydroxybenzoate (methyl salicylate) is designed for advanced, but pre-university, chemistry courses.

If M represents the methyl 2-hydroxybenzoate (methyl salicylate) molecule, the initial ionisation to give the molecular ion is:

M_(g) + high KE e^-  ==> [M•]⁺_(g) + 2e^- and fragmentation equations assume [M]⁺ is the start of the processes and all species are in a gaseous state.

I've not usually shown an unpaired electron on e.g. an ion or a non-ionised alkyl radical R e.g.

[M•]⁺ ==> [X]⁺  +  R•, but you should be aware this is a more accurate depiction of some processes.

I've used simplified equations to show how some of the ions that might be formed in the fragmentation pattern for the mass spectrum of methyl 2-hydroxybenzoate (methyl salicylate) and only the formation of singly charged positive are considered for the mass spectrum of methyl 2-hydroxybenzoate (methyl salicylate).

I've included a stick diagram and table of m/z ions for the mass spectrum of methyl 2-hydroxybenzoate (methyl salicylate) and doing the mass spectrum analysis under standard conditions, databases can be compiled based on complex fingerprint patterns, often involving the relative intensities of many fragment ions, and used to identify compounds including methyl 2-hydroxybenzoate (methyl salicylate).

In selected cases, where two different fragment ions have the same integer m/z value, I've pointed out that modern mass spectrometers can measure relative ion mass to four decimal places. So, using accurate isotopic masses, I've calculated and compared the accurate ion masses if appropriate for this ester. BUT strictly speaking, 0.0005 should be deducted for singly charged ions to account for the loss of the electron in their formation. I have NOT done this for this ester, but the mass spectrometer software does!

C₈H₈O₃, or , methyl 2-hydroxybenzoate (methyl salicylate, 'oil of wintergreen'

For more see The molecular structure and naming of carboxylic acids and derivatives

For more see The molecular structure and naming of aromatic compounds

Interpreting the fragmentation pattern of the mass spectrum of methyl 2-hydroxybenzoate

[M]⁺ is the parent molecular ion peak (M) with an m/z of 152 corresponding to [C₈H₈O₃]⁺, the original methyl 2-hydroxybenzoate molecule minus an electron, [HOC₆H₄COOCH₃]⁺.

The small M+1 peak at m/z 153, corresponds to an ionised methyl 2-hydroxybenzoate molecule with one ¹³C atom in it i.e. an ionised methyl 2-hydroxybenzoate molecule of formula [¹³C¹²C₇H₈O₃]⁺

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.

Methyl 2-hydroxybenzoate has 8 carbon atoms, so on average, ~1 in 12 molecules will contain a ¹³C atom.

Note the ratio of the m/z 152 ion to the m/z 153 ion is ~ 8 : 1 which fits in with the above argument.

This sort of argument also applies to fragment ions from the parent molecular ion of methyl 2-hydroxybenzoate - though the ratio will be greater.

The most abundant ion of the molecule under mass spectrometry investigation (methyl 2-hydroxybenzoate) 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 ion peak for methyl 2-hydroxybenzoate is the m/z 120 ion [C₇H₄O₂]⁺

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of methyl 2-hydroxybenzoate.

Unless otherwise indicated, assume the carbon atoms in methyl 2-hydroxybenzoate or its fragments are the ¹²C isotopes.

Some of the possible positive ions, [molecular fragment]⁺, formed in the mass spectrometry of methyl 2-hydroxybenzoate.

Parent molecular ion of methyl 2-hydroxybenzoate is m/z 152  [C₈H₈O₃]⁺ or [HOC₆H₄COOCH₃]⁺.

Data table of some of the molecular/fragment ions formed in the fragmentation pattern of the mass spectrum of methyl 2-hydroxybenzoate

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of methyl 2-hydroxybenzoate

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.

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

Bond enthalpies = kJ/mol: C-H = 412;  C-O = 360;  C=O  = 743;  O-H = 463; = 518 (benzene ring)

Possible equations to explain some of the most abundant ion peaks in the mass spectrum of methyl 2-hydroxybenzoate (tabulated above)

Formation of m/z 121 ion:

[C₈H₈O₃]⁺  ===>  [C₇H₅O₂]⁺  +  OCH₃

C-O bond scission in the parent molecular ion, loss of a methoxy group from the parent molecular ion,

mass change: 152 - 31 = 121 (M-31 ion peak)

Formation of m/z 120 ion:

[C₇H₅O₂]⁺  ===>  [C₇H₄O₂]⁺  +  H

Hydrogen atom loss from the m/z 121 ion. mass change: 121 - 1 = 120

The m/z 120 ion is the base peak ion, the most abundant and 'stable' ion fragment.

It might also be formed by elimination of methanol from the parent molecular ion.

[C₈H₈O₃]⁺  ===>  [C₇H₄O₂]⁺  +  CH₃OH

This is possible because the H atom of the OH phenol group can combine with the departing methoxy group.

Formation of m/z 93 ion:

[C₇H₅O₂]⁺  ===>  [C₆H₅O]⁺  +  CO

Loss of carbon monoxide from the m/z 121 ion, mass change: 121 - 28 = 93

Formation of m/z 92 ion:

[C₇H₄O₂]⁺  ===>  [C₆H₄O]⁺  +  CO

Loss of carbon monoxide from the m/z 120 ion, mass change: 120 - 28 = 92

Formation of m/z 65 ion:

[?]⁺  ===>  [C₅H₅]⁺  +  ?

The m/z 65 ion can lose protons to give the m/z 64 down to 62 ions (see table).

Formation of m/z 64 ion:

[C₆H₄O]⁺  ===>  [C₅H₄]⁺  +  CO

Loss of carbon monoxide from the m/z 92 ion, mass change: 92 - 28 = 64

Formation of m/z 53 ion:

[?]⁺  ===>   [C₃HO]⁺  or  [C₄H₅]⁺  +  ?

Formation or composition?

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

e.g. using accurate relative isotopic masses:

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

For m/z 53: [C₄H₅]⁺ = 53.0390  and [C₃HO]⁺ = 53.0027, a difference of 0.0363 in relative ion mass.

Formation of m/z 39 ion:

[?]⁺  ===>  [C₃H₃]⁺  +  ?

Formation of m/z 15 ion:

[?]⁺  ===>  [CH₃]⁺  +  ?

Methyl fragment formed from some larger fragment ion with at least two carbon atoms.

The infrared spectrum of methyl 2-hydroxybenzoate (methyl salicylate)

The H-1 NMR spectrum of methyl 2-hydroxybenzoate (methyl salicylate)

The C-13 NMR spectrum of methyl 2-hydroxybenzoate (methyl salicylate)

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