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Advanced Organic Chemistry: Mass spectrum of methyl 2-hydroxybenzoate

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Interpreting the mass spectrum of methyl 2-hydroxybenzoate

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 methyl 2-hydroxybenzoate (methyl salicylate)

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Mass spectroscopy - spectra index

mass spectrum of methyl 2-hydroxybenzoate C8H8O3 fragmentation pattern of m/z m/e ions for analysis and identification of methyl salicylate image diagram oil of wintergreen doc brown's advanced organic chemistry revision notes  

C8H8O3, (c) doc b 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.

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.

The parent molecular ion of methyl 2-hydroxybenzoate m/z ?: [?]+

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

m/z value of [fragment]+ 15 39 53 63 64 65
[molecular fragment]+ [CH3]+ [C3H3]+ ? [C3HO]+

? [C4H5]+

[C5H3]+ [C5H4]+ [C5H5]+
m/z value of [fragment]+ 92 93 120 121 152 153
[molecular fragment]+ [C6H4O]+ [C6H5O]+ [C7H4O2]+ [C7H5O2]+ [C8H8O3]+ [13CC7H8O3]+

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

Relative atomic masses: H = 1;  C = 12 (13 for ~1 in 100);  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

Loss of a methoxy group from the parent molecular ion, mass change: 152 - 31 = 121

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 can 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 64 ion:

[C6H4O]+  ===>  [C5H4]+  +  CO

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

Formation of m/z 15 ion:

[?]+  ===>  [CH3]+  +  ?

Methyl fragment formed from some larger fragment ion.


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Links associated with methyl 2-hydroxybenzoate

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)

INDEX of all revision notes on CARBOXYLIC ACIDS and DERIVATIVES

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