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Advanced Organic Chemistry: Mass spectrum of 2-methoxypropane

Interpreting the mass spectrum of 2-methoxypropane

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 2-methoxypropane

mass spectrum of 2-methoxypropane C4H10O CH3CH(OCH3)CH3 fragmentation pattern of m/z m/e ions for analysis and identification of methyl isopropyl ether image diagram doc brown's advanced organic chemistry revision notes 

2-methoxypropane    C4H10    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 2-methoxypropane

[M]+ is the molecular ion peak (M) with an m/z of 74 corresponding to [C4H10O]+, the original 2-methoxypropane molecule minus an electron, [CH3OCH(CH3)2]+.

The small M+1 peak at m/z 75, corresponds to an ionised 2-methoxypropane molecule with one 13C atom in it i.e. an ionised 1-methoxypropane molecule of formula [13C12C3H10O]+

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.

2-methoxypropane has 4 carbon atoms, so on average, ~1 in 25 molecules will contain a 13C atom.

You can also get an m/z 75 (M+1) ion from a collision between a hydrogen radical and the molecular ion.

[M]+  +  H  ==>  [MH]+

The most abundant ion of the molecule under mass spectrometry investigation (2-methoxypropane) 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 2-methoxypropane.

Unless otherwise indicated, assume the carbon atoms in 2-methoxypropane are the 12C isotope.

Some of the possible positive ions, [molecular fragment]+, formed in the mass spectrometry of 2-methoxypropane.

m/z value of [fragment]+ 73 59 43 41 39 31 29 27 15
[molecular fragment]+ [C4H10O]+ [C3H7O]+ [C3H7]+ [C3H5]+ [C3H3]+ [CH3O]+ [C2H5]+ [C2H3]+ [CH3]+

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of 2-methoxypropane

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 2-methoxypropane (tabulated above)

Formation of m/z 73 ion:

[CH3OCH(CH3)2]+  ===>  [C4H9O]+  +  H

C-H bond fission leading to H atom loss, mass change 74 - 1 = 73.

The m/z 73 ion most likely has the structure [CH2OCH(CH3)2]+.

Formation of m/z 59 ion:

[CH3OCH(CH3)2]+  ===>  [OCH(CH3)2]+  +  CH3

The m/z 59 ion is the base peak ion, the most abundant and 'stable' ion fragment, loss of methyl group, mass change 74 - 15 = 59..

Formation of m/z 43 ion:

[CH3OCH(CH3)2]+  ===>  [CH(CH3)2]+  +  CH3O

C-O bond scission, loss of methoxy fragment, mass change 74 - 31 = 43.

Formation of m/z 31 ion:

[CH3OCH(CH3)2]+  ===>  [CH3O]+  +  CH(CH3)2

C-O bond scission, loss of the alkyl fragment, mass change 74 - 43 = 31.

Note m/z 31 and 43 ions are formed by the same C-O bond scission, but the m/z 43 ion is more stable - a secondary carbocation (+I effect of methyl groups)

Formation of m/z 15 ion:

[CH3OCH(CH3)2]+  ===>  [CH3]+  +  OCH(CH3)2

C-O bond scission, loss of methoxy fragment, mass change 74 - 59 = 15.

The methyl ion can also be formed from other fragments by C-C or C-O bond scission.


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