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Advanced Organic Chemistry: Mass spectrum of cyclobutane

Interpreting the mass spectrum of cyclobutane

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 cyclobutane

mass spectrum of cyclobutane C4H8 fragmentation pattern of m/z m/e ions for analysis and identification of cyclobutane image diagram doc brown's advanced organic chemistry revision notes 

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Interpreting the fragmentation pattern of the mass spectrum of cyclobutane

[M]+ is the molecular ion peak (M) with an m/z of 56 corresponding to [C4H8]+, the original cyclobutane molecule minus an electron.

The small M+1 peak at m/z 57, corresponds to an ionised cyclobutane molecule with one 13C atom in it i.e. an ionised cyclobutane molecule of formula [13C12C3H8]+

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.

Cyclobutane 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 (cyclobutane) 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 cyclobutane.

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

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

The parent molecular ion of cyclobutane m/z 56: [C4H8]+

m/z value of [fragment]+ 55 54 53 41 40 39 29 28 27 15 14
[molecular fragment]+ [C4H7]+ [C4H6]+ [C4H5]+ [C3H5]+ [C3H4]+ [C3H3]+ [C2H5]+ [C2H4]+ [C2H3]+ [CH3]+ [CH3]+

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

Atomic masses: H = 1;  C = 12

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

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

Formation of m/z 55 and other ions:

[C4H8]+  ===>  [C4H7]+  +  H

C-H bond scission of the parent molecular ion, hydrogen atom loss, mass change 56 - 1 = 55.

The m/z 55 ion can then lose hydrogen atoms to give the m/z 54 to 50 ions.

The m/z 54 ion could be formed elimination of a hydrogen molecule from the parent molecular ion.

[C4H8]+  ===>  [C4H6]+  +  H2

Again, this fragment can also lose hydrogen atoms, retaining the C4 atoms to give m/z ions 53 to 50 (see table).

Formation of m/z 41 ion:

[C4H8]+  ===>  [C3H5]+  +  CH3

C-C bond scission of the parent molecular ion, proton rearrangement and a methyl group expelled, mass change 56 - 15 = 41.

Although much less probable, note that the methyl group could be the fragment that is ionised giving the m/z 15 ion.

The m/z 4 ion can further lose hydrogen atoms to give m/z 40 to 37 ions (see table).

Formation of m/z 28 ion:

[C4H8]+  ===>  [C2H4]+  +  C2H4

The elimination of ethene via a double C-C bond scission of the parent molecular ion, mass change 56 - 28 = 28.

The m/z 28 ion is the base peak ion, the most abundant and 'stable' ion fragment.


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