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Advanced Organic Chemistry: Mass spectrum of methoxymethane

The mass spectrum of methoxymethane (dimethyl ether)

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 methoxymethane

C2H6O CH3OCH3 mass spectrum of methoxymethane fragmentation pattern of m/z m/e ions for analysis and identification of dimethyl ether image diagram doc brown's advanced organic chemistry revision notes 

Methoxymethane (dimethyl ether) 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 alcohols and ether structure and naming (c) doc b

Revision notes on the structure and naming (nomenclature) of aliphatic ALCOHOLS and ETHERS

Interpreting the fragmentation pattern of the mass spectrum of methoxymethane

[M]+ is the molecular ion peak (M) with an m/z of 46 corresponding to [C2H6O]+, the original methoxymethane molecule minus an electron, [CH3OCH3]+.

The small M+1 peak at m/z 47, corresponds to an ionised methoxymethane molecule with one 13C atom in it i.e. an ionised methoxymethane molecule of formula [13C12CH6O]+

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.

methoxymethane has 2 carbon atoms, so on average, ~1 in 50 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 47 can originate from [M]+  +  H  ===>  [MH]+

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

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

Some of the possible positive ions, [molecular fragment]+, formed in the mass spectrometry of methoxymethane based on [CH3OCH3]+.

m/z value of [fragment]+ 45 31 29 15
[molecular fragment]+ [CH3OCH2]+ [CH3O]+ [CHO]+ [CH3]+

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

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

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

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

Formation of m/z 45 ion:

[CH3OCH3]+  ===>  [CH3OCH2]+  +  H

Formed by hydrogen radical loss from the parent molecular ion of methoxymethane, involves scission of a C-H bond, mass change 46 - 1 = 45.

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

Formation of m/z 31 ion:

[CH3OCH3]+  ===>  [CH3O]+  +  CH3

Formed by scission of the C-O bond, mass change 46 - 15 = 31.

Formation of m/z 29 ion:

[CH3O]+  ===>  [CHO]+  +  H2

Elimination of hydrogen from the m/z 31 ion, mass change 31 - 2 = 29.

Formation of m/z 15 ion:

[CH3OCH3]+  ===>  [CH3]+  +  CH3O

Formed by scission of the C-O bond (see note on m/z 31 ion).

 

See also comparing the IR, mass, 1H NMR and 13C NMR spectra of isomers of C2H6O below.

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Comparing the infrared, mass, 1H NMR and 13C NMR spectra of the 2 isomers of C2H6O

NOTE: The images are linked to their original detailed spectral analysis pages AND can be doubled in size with touch screens to increase the definition to the original ethanol (ethyl alcohol) and methoxymethane (dimethyl ether) image sizes.

INFRARED SPECTRA: Apart from the significant differences in the fingerprint region at wavenumbers 1500 to 400 cm-1, the most striking difference is the broad O-H stretching band ~3400 cm-1, found in the infrared spectrum of alcohols, but absent in the infrared spectrum of ethers.

MASS SPECTRA: Both ethanol and methoxymethane show some similarities in their mass spectra, but their base ion peaks are quite different - for ethanol it is m/z 31 and for methoxymethane it is m/z 45. Methoxymethane gives the m/z 29 [CHO]+ ion, which can be distinguished from the m/z 29 [C2H5]+ ion by high resolution spectroscopy. Methoxymethane has less abundant peaks for m/z ions 26, 27 and 43.

1H NMR SPECTRA: The 1H NMR spectra of ethanol and methoxymethane are quite significantly different. Ethanol gives 3 peaks in the proton ratio 3:2:1 (3 different chemical environments), whereas methoxymethane only gives one 1H chemical shift peak (all 6 protons in the same chemical environment).

13C NMR SPECTRA: The 13C NMR spectra of ethanol and methoxymethane are different. Ethanol gives two 13C resonances, but methoxymethane only one (2 different 13C chemical environments and a 13C single chemical environment).


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