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Advanced Organic Chemistry: Mass spectrum of propyl methanoate

Interpreting the mass spectrum of propyl methanoate

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 propyl methanoate

mass spectrum of propyl methanoate C4H8O2 HCOOCH2CH2CH3 fragmentation pattern of m/z m/e ions for analysis and identification of propyl formate image diagram doc brown's advanced organic chemistry revision notes 

Propyl methanoate (propyl formate)   (c) doc b    (c) doc b    (c) doc b 

Interpreting the fragmentation pattern of the mass spectrum of propyl methanoate

[M]+ is the molecular ion peak (M) with an m/z of 88 corresponding to [C4H8O2]+, the original propyl methanoate molecule minus an electron, [HCOOCH2CH2CH3]+.

The molecular peak ion is very small, suggesting the ion is relatively unstable compared to many other smaller fragmentation ions.

Not here, but you might see an M+1 peak at m/z 89, corresponding to an ionised propyl methanoate molecule with one 13C atom in it i.e. an ionised propyl methanoate 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.

Propyl methanoate 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 (propyl methanoate) 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 propyl methanoate.

Unless otherwise indicated, assume the carbon atoms in propyl methanoate are the 12C isotope.

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

m/z value [fragment]+ 73 60 ? 59 57 47 ? 45 43 42
[molecular fragment]+ [HCOOCH2CH2]+ [C2H4O2]+ [OCH2CH2CH3]+ [C3H5O]+ [CH3O2]+ [HCOO]+ [C3H7]+ [C3H6]+
m/z value [fragment]+ 41 40 39 31 30 ? 29 29 28 27 26 18 15
[molecular fragment]+ [C3H5]+ [C3H4]+ [C3H3]+ [CH3O]+ [CH2O]+ [C2H5]+ [CHO]+ [C2H4]+ [C2H3]+ [C2H2]+ [H2O]+ [CH3]+

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of propyl methanoate

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

Bond enthalpies kJ/mol: C-C = 348;  C-H = 412;  C-O = 360;  C=O  743

Possible equations to explain the most abundant ion peaks of propyl methanoate (tabulated above)

Formation of m/z 73 ion:

[HCOOCH2CH2CH3]+  ===>  [HCOOCH2CH2]+  +  CH3

C-C bond scission, loss of methyl group, mass change 88 - 15 = 73

Formation of m/z 59 ion:

[HCOOCH2CH2CH3]+  ===>  [OCH2CH2CH3]+  +  HCO

C-O bond scission, loss of methyl group, mass change 88 - 29 = 73

or

[HCOOCH2CH2CH3]+  ===>  [HCOOCH2]+  +  CH2CH3

C-C bond scission, loss of ethyl group,  mass change 88 - 29 = 73

Formation of m/z 45 ion:

[HCOOCH2CH2CH3]+  ===>  [HCOO]+  +  CH2CH2CH3

C-O bond scission of parent molecular ion, mass change 88 - 43 = 45.

Formation of m/z 42 ion:

[?]+  ===>  [C3H6]+  +  ?

Propene ion formed in some way from the parent molecular ion of fragment ion.

Theoretically could be a [C2H2O]+ ion?

Formation of m/z 31 ion:

[?]+  ===>  [CH3O]+  +  ?

The m/z 31 ion is the base peak ion, the most abundant and 'stable' ion fragment - not sure how it is formed?

Formation of m/z 29 ion:

[HCOOCH2CH2CH3]+  ===>  [OCH2CH2CH3]+  +  HCO

C-O bond scission, 88 - 59 = 29

or

[HCOOCH2CH2CH3]+  ===>  [CH2CH3]+  +  HCOOCH2

C-C bond scission, ionisation of ethyl group, mass change 88 -  = 29

Hydrogen atom/molecule loss from the ethyl cation gives m/z ions of 28, 27 and 26.


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