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Advanced Organic Chemistry: Mass spectrum of methyl propanoate

Interpreting the mass spectrum of methyl propanoate (methyl propionate)

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 propanoate

mass spectrum of methyl propanoate C4H8O2 CH3CH2COOCH3 fragmentation pattern of m/z m/e ions for analysis and identification of methyl propionate image diagram doc brown's advanced organic chemistry revision notes 

Methyl propanoate   (methyl propionate)   (c) doc b   (c) doc b   (c) doc b  an ester

Interpreting the fragmentation pattern of the mass spectrum of methyl propanoate

[M]+ is the molecular ion peak (M) with an m/z of 88 corresponding to [C4H8O2]+, the original methyl propanoate molecule minus an electron, [CH3CH2COOCH3]+.

The small M+1 peak at m/z 89, corresponds to an ionised methyl propanoate molecule with one 13C atom in it i.e. an ionised methyl propanoate 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.

methyl propanoate has 4 carbon atoms, so on average, ~1 in 25 molecules will contain a 13C atom.

The most abundant ion of the molecule under mass spectrometry investigation (methyl propanoate) 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 propanoate.

Unless otherwise indicated, assume the carbon atoms in methyl propanoate are the 12C isotope.

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

m/z value of [fragment]+ 87 59 58 ? 57 56 55
[molecular fragment]+ [C4H8O2]+ [C2H3O2]+ [C3H6O]+ [C3H5O]+ [C3H4O]+ [C3H3O]+
m/z value of [fragment]+ 45 ? 31 29 27 26 15
[molecular fragment]+ [?]+ [CH3O]+ [C2H5]+ [C2H3]+ [C2H2]+ [CH3]+

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of methyl propanoate

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 methyl propanoate (tabulated above)

Formation of m/z 87 ion:

[CH3CH2COOCH3]+  ===>  [C4H7O2]+  +  H

C-H bond scission, hydrogen atom lost, mass change 88 - 1 = 87.

Formation of m/z 59 ion:

[CH3CH2COOCH3]+  ===>  [COOCH3]+  +  C2H5

C-C bond scission of the parent molecular ion, loss of ethyl group, mass change 88 - 29 = 59.

Formation of m/z 57 ion:

[CH3CH2COOCH3]+  ===>  [CH3CH2CO]+  +  OCH3

C-O bond scission of the parent molecular ion, mass change 88 - 31 = 57.

The m/z 57 ion is the base peak ion, the most abundant and 'stable' ion fragment.

Formation of m/z 31 ion:

[CH3CH2COOCH3]+  ===>  [OCH3]+  +  CH3CH2CO

C-O bond scission of the parent molecular ion, mass change 88 - 57 = 31.

Formation of m/z 29 ion:

[CH3CH2COOCH3]+  ===>  [C2H5]+  +  COOCH3

C-C bond scission of the parent molecular ion, mass change 88 - 59 = 29.

Formation of m/z 15 ion:

[C4H8O2]+  ===>  [CH3]+  +  C3H5O2

C-C or C-O bond scission to form a methyl ion, mass change 88 - 73 = 15.


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