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Advanced Organic Chemistry: Mass spectrum of 1,3-dioxane

Interpreting the mass spectrum of 1,3-dioxane

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 1,3-dioxane

mass spectrum of 1,3-dioxane C4H8O2 fragmentation pattern of m/z m/e ions for analysis and identification of meta-dioxane image diagram doc brown's advanced organic chemistry revision notes 

Interpreting the fragmentation pattern of the mass spectrum of 1,3-dioxane

[M]+ is the molecular ion peak (M) with an m/z of 88 corresponding to [C4H8O2]+, the original 1,3-dioxane molecule minus an electron.

The small M+1 peak at m/z 89, corresponds to an ionised 1,3-dioxane molecule with one 13C atom in it i.e. an ionised 1,3-dioxane molecule of formula [13C12C3H8O2]+

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.

1,3-dioxane has 4 carbon atoms, so on average, ~1 in 25 molecules will contain a 13C atom.

The most abundant ion of the molecule under mass spectrometry investigation (1,3-dioxane) 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 1,3-dioxane.

Unless otherwise indicated, assume the carbon atoms in 1,3-dioxane are the 12C isotope.

Some of the possible positive ions, [molecular fragment]+, formed in the mass spectrometry of 1,3-dioxane.

m/z value of [fragment]+ 87 59 ? 58 57 41 39
[molecular fragment]+ [C4H7O2]+ [C2H3O2]+ [C3H6O]+ [C3H5O]+ [C3H5]+ [C3H3]+
m/z value of [fragment]+ 31 30 29 28 28 27 26 15 ?
[molecular fragment]+ [CH3O]+ [CH2O]+ [C2H5]+ [C2H4]+ [CO]+ [C2H3]+ [C2H2]+ [CH3]+

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of 1,3-dioxane

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 1,3-dioxane (tabulated above)

Formation of m/z 87 ion:

[C4H8O2]+  ===>  [C4H7O2]+  +  H

C-H bond scission, loss of hydrogen atom, mass change 88 - 1 = 87.

This might be preceded by the breaking of a weaker C-C or C-O bond in the ring?

The m/z 87 ion is the base peak ion, the most abundant and 'stable' ion fragment.

It is unusual for the base ion peak to be the largest fragment.

Formation of m/z 58 ion:

[C4H8O2]+  ===>  [CH2CH2CH2O]+  or  [CH2CH2OCH2]+  +  CH2O

Part of the original ring system, C-C or C-O bond scission to open ring and the loss of CH2O but I'm not sure? Mass change 88 - 30 = 58.

Formation of m/z 30 ion:

[CH2CH2CH2O]+  or  [CH2CH2OCH2]+  ===> [CH2O]+  +  C2H4

C-C or C-O bond scission of the m/z 58 ion, mass change 58 - 28 = 30.

OR

[C4H8O2]+  ===>  [CH2O]+  +  C3H6O

Part of the original ring system, C-C or C-O bond scission to open ring and the loss of CH2O but I'm not sure? Mass change 88 - 58 = 30.

Formation of m/z 28 ion:

[CH2CH2CH2O]+  or  [CH2CH2OCH2]+  ===> [C2H4]+  +  CH2O

C-C or C-O bond scission of the m/z 58 ion, mass change 58 - 30 = 28


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