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Advanced Organic Chemistry: Mass spectrum of methoxyethane

Interpreting the mass spectrum of methoxyethane

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 methoxyethane

C3H8O CH3OCH2CH3 mass spectrum of methoxyethane fragmentation pattern of m/z m/e ions for analysis and identification of ethyl methyl ether image diagram doc brown's advanced organic chemistry revision notes 

Methoxyethane  alcohols and ether structure and naming (c) doc b  alcohols and ether structure and naming (c) doc b  alcohols and ether structure and naming (c) doc b  alcohols and ether structure and naming (c) doc b

Interpreting the fragmentation pattern of the mass spectrum of methoxyethane

[M]+ is the molecular ion peak (M) with an m/z of 60 corresponding to [C3H8O]+, the original methoxyethane molecule minus an electron, [CH3OCH2CH3]+.

The small M+1 peak at m/z 61, corresponds to an ionised methoxyethane molecule with one 13C atom in it i.e. an ionised methoxyethane molecule of formula [13C12C2H8O]+

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.

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

In the mass spectrum of ethers, an M+1 ion can also be formed by a hydrogen atom radical combining with the molecular ion i.e. m/z 61 can originate from [M]+  +  H  ===>  [MH]+

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

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

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

m/z value of [fragment]+ 59 45 43 31 29 28 27 15
[molecular fragment]+ [C3H7O]+ [C2H5O]+ [C2H3O]+ [CH3O]+ [CH3CH2]+ [C2H4]+ [C2H3]+ [CH3]+

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

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

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

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

Formation of m/z 59 ion:

[CH3OCH2CH3]+  ===>  [C3H7O]+  +  H

C-H bond scission and expulsion of a proton (mass change 60 - 1 = 59).

Formation of m/z 45 ion:

[CH3OCH2CH3]+  ===>  [C2H5O]+  +  CH3

Scission of a C-C or C-O bond to lose a methyl group from the parent molecular ion (see also m/z 15).

Mass change: 60 - 15 = 45

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

The m/z ion may have the structure CH3OCH2  or  OCH2CH3 or ?

Could also be formed from the m/z 59 ion?

[C3H7O]+  ===>  [C2H5O]+  +  CH2

Formation of m/z 43 ion:

[C3H7O]+  ===>  [C2H3O]+  +  CH4

Scission of a C-C or C-O bond to lose a methane from the m/z 59 ion ?

Mass change: 59 - 16 = 43

Formation of m/z 31 ion:

[CH3OCH2CH3]+  ===>  [CH3O]+  +  CH2CH3

C-O bond scission of the parent molecular ion (see also m/z 29 ion).

Mass change: 60 - 29 = 31

Formation of m/z 29 ion:

[CH3OCH2CH3]+  ===>  [CH2CH3]+  +  CH3O

C-O bond scission of the parent molecular ion.

Mass change: 60 - 31 = 29

Formation of m/z 15 ion:

[CH3OCH2CH3]+  ===>  [CH3]+  +  C2H5O

Scission of a C-C or C-O bond to lose a methyl group.

Mass change: 60 - 45 = 15


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