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Advanced Organic Chemistry: Mass spectrum of cyclohexene

Interpreting the mass spectrum of cyclohexene

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 cyclohexene

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Interpreting the fragmentation pattern of the mass spectrum of cyclohexene

[M]+ is the parent molecular ion peak (M) with an m/z of 82 corresponding to [C6H10]+, the original cyclohexene molecule minus an electron.

The small M+1 peak at m/z 83, corresponds to an ionised cyclohexene molecule with one 13C atom in it i.e. an ionised cyclohexene molecule of formula [13C12C5H10]+

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.

Cyclohexene has 6 carbon atoms, so on average, ~1 in 17 molecules will contain a 13C atom.

This sort of argument also applies to fragment ions from the parent molecular ion of cyclohexene - though the ratio will be greater e.g. m/z 68 could be [C5H8]+  or  [13C12C4H7]+.

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

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

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

The parent molecular ion of cyclohexene m/z 82: [C6H10]+

Data table of some of the ions formed in the fragmentation pattern of the mass spectrum of cyclohexene

m/z value of [fragment]+ 39 40 41 42 50 51 53 54 55
[molecular fragment]+ [C3H3]+ [C3H4]+ [C3H5]+ [C3H6]+ [C4H2]+ [C4H3]+ [C4H5]+ [C4H6]+ [C4H7]+
m/z value of [fragment]+ 65 67 68 77 79 81 82 83
[molecular fragment]+ [C5H5]+ [C5H7]+ [C5H8]+ [C6H5]+ [C6H7]+ [C6H9]+ [C6H10]+ [13C12C5H10]+

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

Atomic masses: H = 1;  C = 12 (13 for ~1 in 100)

Bond enthalpies = kJ/mol: C-C = 348;  C-H = 412;  C=C = 612

Possible equations to explain some of the most abundant ion peaks in the mass spectrum of cyclohexene (tabulated above)

Formation of m/z 81 ion:

[C6H10]+  ===>  [C6H9]+  +  H

Loss of a hydrogen atom from the parent molecular ion, mass change 82 - 1 = 81.

The m/z 81 or 82 ions can lose hydrogen atoms/molecule to give the m/z ions of 81 down to 77 (see data table and spectrum).

Formation of m/z 67 ion:

[C6H10]+  ===>  [C5H7]+  +  CH3

Formed by the loss of CH3 (CH2 + adjacent H) by ring scission of the parent molecular ion, mass change 82 - 15 = 67.

The m/z 67 ion is the base peak ion, the most abundant and 'stable' ion fragment.

The m/z 68 peak could have the same structure as the m/z ion, but be  [13C12C4H7] with a carbon-13 atom in it.

Formation of m/z 54 ion

[C6H10]+  ===>  [C4H6]+  +  C2H4

Perhaps loss of an unionised ethene molecule from the parent molecular ion?, mass change 82 - 28 = 54. This is the 2nd most abundant fragment in the spectrum.


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Links associated with cyclohexene

The infrared spectrum of cyclohexene

The H-1 NMR spectrum of cyclohexene

The C-13 NMR spectrum of cyclohexene

The chemistry of ALKENES revision notes INDEX

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