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

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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)

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 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.

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-C = 348; 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 proton loss from the molecular ion.

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.

Note that the m/z ion is much more abundant than the m/z 31 ion, the electronegative oxygen tens to keep the electrons and give a neutral fragment whereas the alkyl group tends to carry the positive charge from the parent molecular ion (see m/z 15 formation equation below).

Formation of m/z 31 ion:

[CH₃OCH₂]⁺ ===> [CHO]⁺ + CH₄ ????

Not sure how this is formed?, just a suggestion!

Formation of m/z 15 ion:

[CH₃OCH₃]⁺ ===> [CH₃]⁺ + CH₃O

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

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