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Advanced Organic Chemistry: Mass spectrum of 1-methylethyl methanoate

Interpreting the mass spectrum of 1-methylethyl 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 1-methylethyl methanoate

mass spectrum of 1-methylethyl methanoate C4H8O2 HCOOCH(CH3)2 fragmentation pattern of m/z m/e ions for analysis and identification of isopropyl formate image diagram doc brown's advanced organic chemistry revision notes 

HCOOCH(CH3)2  1-methylethyl methanoate, propan-2-yl methanoate,

is an example of an ester, some old names: isopropyl methanoate, isopropyl formate

Interpreting the fragmentation pattern of the mass spectrum of 1-methylethyl methanoate

[M]+ is the molecular ion peak (M) with an m/z of 88 corresponding to [C4H8O2]+, the original 1-methylethyl methanoate molecule minus an electron, [HCOOCH(CH3)2]+.

The peak is tiny, suggesting the molecular ion is relatively unstable compared to many of the fragmentation ions.

The most abundant ion of the molecule under mass spectrometry investigation (1-methylethyl 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 1-methylethyl methanoate.

Unless otherwise indicated, assume the carbon atoms in 1-methylethyl methanoate are the 12C isotope.

Some of the possible positive ions, [molecular fragment]+, formed in the mass spectrometry of 1-methylethyl methanoate.

m/z value [fragment]+ 73 59 47 ? 45 43 42
[molecular fragment]+ [HCOOCHCH3]+ [OCH(CH3)2]+ [CH3O2]+ [HCOO]+ [C3H7]+ [C3H6]+
m/z value [fragment]+ 41 39 31 29 29 28 27 15
[molecular fragment]+ [C3H5]+ [C3H3]+ [CH3O]+ [C2H5]+ [CHO]+ [C2H4]+ [C2H3]+ [CH3]+

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of 1-methylethyl 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 1-methylethyl methanoate (tabulated above)

Formation of m/z 73 ion:

[HCOOCH(CH3)2]+  ===>  [HCOOCHCH3]+  +  CH3

C-C bond scission of the parent molecular ion, loss of methyl group, mass change 88 - 15 = 73.

Note the 'connection' between the m/z 73 and 15 ion equations, either fragment can be ionised, but only one of the two fragments can be ionised to carry the positive charge.

Formation of m/z 59 ion:

[HCOOCH(CH3)2]+  ===>  [OCH(CH3)2]+  +  HCO

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

Formation of m/z 45 ion:

[HCOOCH(CH3)2]+  ===>  [HCOO]+  +  CH(CH3)2

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

The m/z 45 ion is the base peak ion, the most abundant and 'stable' ion fragment.

Formation of m/z 43 ion:

[HCOOCH(CH3)2]+  ===>  [CH(CH3)2]+  +  HCOO

C-O bond scission of the parent molecular ion, loss of HCO group, mass change 88 - 45 = 43. A secondary carbocation is formed.

Note the 'connection' between the m/z 43 and 45 ion equations, either fragment can be ionised, but only one of the two fragments can be ionised to carry the positive charge.

The m/z 43 ion can lose hydrogen atoms/molecule to give a succession of lighter fragment ions e.g. m/z values of 42, 41 and 39

Formation of m/z 15 ion:

[HCOOCH(CH3)2]+  ===>  [CH3]+  +  HCOOCHCH3

The methyl ion can be formed by C-C bond scission of the parent molecular ion or from several of the larger fragment ions.


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