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Advanced Organic Chemistry: Mass spectrum of ethyl methanoate

Interpreting the mass spectrum of ethyl methanoate (ethyl formate)

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 ethyl methanoate

mass spectrum of ethyl methanoate C3H6O2 HCOOCH2CH3 fragmentation pattern of m/z m/e ions for analysis and identification of ethyl formate image diagram doc brown's advanced organic chemistry revision notes 

Ethyl methanoate (ethyl formate) C3H6O2  (c) doc b  (c) doc b  (c) doc b  (c) doc b

Interpreting the fragmentation pattern of the mass spectrum of ethyl methanoate

[M]+ is the molecular ion peak (M) with an m/z of 74 corresponding to [C3H6O2]+, the original ethyl methanoate molecule minus an electron, [HCOOCH2CH3]+.

The small M+1 peak at m/z 75, corresponds to an ionised ethyl methanoate molecule with one 13C atom in it i.e. an ionised ethyl methanoate molecule of formula [13C12C2H6O2]+

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.

Ethyl methanoate has 3 carbon atoms, so on average, ~1 in 33 molecules will contain a 13C atom.

The most abundant ion of the molecule under mass spectrometry investigation (ethyl 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 ethyl methanoate.

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

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

m/z value of [fragment]+ 73 56 47 ? 46 45 45 43 31
[molecular fragment]+ [C3H6O]+ [C3H4O]+ [?]+ [?]+ [CH3CH2O]+ [HCO2]+ [?]+ [CH3O]+
m/z value of [fragment]+ 30 29 29 28 28 27 26 18 15
[molecular fragment]+ [CH2O]+ [HCO]+ [CH3CH2]+ [C2H4]+ [CO]+ [C2H3]+ [C2H2]+ [H2O]+ [CH3]+

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of ethyl 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 ethyl methanoate (tabulated above)

Formation of m/z 73 ion:

[HCOOCH2CH3]+  ===>  [COOCH2CH3]+  +  H

C-H bond scission and loss of hydrogen atom.

Formation of m/z 56 ion:

[HCOOCH2CH3]+  ===>  [C3H4O]+  +  H2O

Elimination of water , mass change 74 - 18 = 56.

Formation of m/z 45 ion:

[HCOOCH2CH3]+  ===>  [HCOO]+  +  CH2CH3

[HCOOCH2CH3]+  ===>  [OCH2CH3]+  +  HCO

C-O bond scission of the parent molecular ion, two possibilities of bond breaking leading to two possibilities of ionised fragment, both mass changes 74 - 29 = 45

Formation of m/z 31 ion:

[OCH2CH3]+  ===>  [CH3O]+  +  CH2  ???

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

Formation of m/z 29 ion:

[HCOOCH2CH3]+  ===>  [CHO]+  +  OCH2CH3

[HCOOCH2CH3]+  ===>  [CH2CH3]+  +  HCOO

Two possibilities of C-O bond scission of the parent molecular ion.

Mass change 74 - 45 = 29.

Formation of m/z 28 ion:

[CH2CH3]+  ===>  [C2H4]+  +  H

Hydrogen atom loss from ethyl ion (above), further hydrogen atom losses give the m/z 27 and 26 ions.

It may be possible the [CO]+ ion be formed from other fragment ions?

Formation of m/z 15 ion:

[HCOOCH2CH3]+  ===>  [CH3]+  +  C2H3O2

C-C bond scission of the parent molecular ion, mass change 74 - 59 = 15, or from one of the larger fragment ions


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