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Advanced Organic Chemistry: Mass spectrum of cyclopropane

Interpreting the mass spectrum of cyclopropane

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 cyclopropane

See also comparing the infrared, mass, 1H NMR and 13C NMR spectra of propane, cyclopropane and propene

mass spectrum of cyclopropane C3H6 fragmentation pattern of m/z m/e ions for analysis and identification of cyclopropane image diagram doc brown's advanced organic chemistry revision notes 

Cyclopropane C3H6, alkanes structure and naming (c) doc b , alkanes structure and naming (c) doc b

Interpreting the fragmentation pattern of the mass spectrum of cyclopropane

[M]+ is the molecular ion peak (M) with an m/z of 41 corresponding to [C3H6]+, the original cyclopropane molecule minus an electron.

The small M+1 peak at m/z 43, corresponds to an ionised cyclopropane molecule with one 13C atom in it i.e. an ionised cyclopropane molecule of formula [13C12C2H6]+

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.

Cyclopropane has 3 carbon atoms, so on average, ~1 in 33 molecules will contain a 13C atom.

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

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

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

The parent molecular ion of cyclopropane m/z 42: [C3H6]+, it is also the base peak ion.

m/z value of [fragment]+ 41 40 39 38 37 28 27 26 15 14
[molecular fragment]+ [C3H5]+ [C3H4]+ [C3H3]+ [C3H2]+ [C3H]+ [C2H4]+ [C2H3]+ [C2H2]+ [CH3]+ [CH2]+

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

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 cyclopropane (tabulated above)

Formation of m/z 41 ion:

[C3H6]+  ===>  [C3H5]+  +  H

C-H bond scission in the parent molecular ion, loss hydrogen, mass change 42 - 1 = 42.

You can then get successive hydrogen atom/molecule losses to give m/z ions 40 to 37.

Formation of m/z 40 ion:

[C3H6]+  ===>  [C3H4]+  +  H2

Elimination of a hydrogen molecule from the parent molecular ion, mass change 42 - 2 = 40.

Again, you can then get successive hydrogen atom/molecule losses to give m/z ions 39 to 37.

Formation of m/z 27 ion:

[C3H6]+  ===>  [C2H3]+  +  CH3

C-C bond scission in the parent molecular ion and loss, by proton rearrangement, a methyl group, mass loss 42 - 15 = 27.

You could also have an alternative ionisation of the methyl group fragment - see m/z 15 which can lose hydrogen atoms to give m/z ions of 14 and 13.

Hydrogen atom loss from the  [C2H3]+ ion will give the m/z 26 and 25 ions.

Comparing the infrared, mass, 1H NMR and 13C NMR spectra of propane, cyclopropane and propene

NOTE: The images are linked to their original detailed spectral analysis pages AND can be doubled in size with touch screens to increase the definition to the original ethane and ethene image sizes.

Comparing the infrared spectra of propane, propene and cyclopropane.

Cyclopropane and propene are structural isomers of molecular formula C3H6.

Propane and propene exemplify the infrared spectra of lower members of  the alkane and alkene homologous series of CnH2n+2 and CnH2n hydrocarbon molecules where n = 3.

INFRARED SPECTRA (above): Apart from the significant differences in the fingerprint region at wavenumbers 1500 to 400 cm-1, the most striking differences are: (i) propene shows the characteristic absorption at ~1700 cm-1 for the C=C stretching vibrations, absent in the other two spectra, (ii) cyclopropane shows an absorption band at 2200 cm-1, absent in the other two spectra, (iii) propane has an absorption band at ~750 cm-1, absent in the other two spectra.

Comparing the mass spectra of propane, propene and cyclopropane.

Cyclopropane and propene are structural isomers of molecular formula C3H6.

Propane and propene exemplify the mass spectra of lower members of  the alkane and alkene homologous series of CnH2n+2 and CnH2n hydrocarbon molecules where n = 3.

MASS SPECTRA (above): All three hydrocarbons show some similarities in their mass spectra e.g. m/z ions 26 to 28 for [C2Hx]+ (x = 2 to 4) and m/z 14 and 15 ions - but these are found in most aliphatic hydrocarbon spectra. The molecular ion peaks will be the same for the isomeric propene and cyclopropane (m/z 42) but that of propane will be 2 mass units higher at m/z 44. The base ion peak m/z values are all different, propane 29, propene 41 and cyclopropane 42.

Comparing the 1H proton NMR spectra of propane, propene and cyclopropane.

Cyclopropane and propene are structural isomers of molecular formula C3H6.

Propane and propene exemplify the 1H proton NMR spectra of lower members of  the alkane and alkene homologous series of CnH2n+2 and CnH2n hydrocarbon molecules where n = 3.

1H NMR SPECTRA (above): The 1H NMR spectra of all three molecules give different proton ratios i.e. propane 3:1 (actually 6:2 in the molecule), propene 2:1:3 (spectrum and molecule) and cyclopropane just a singlet for the six protons, so all three can be distinguished from each other by their 1H NMR spectra..

Comparing the carbon-13 NMR spectra of propane, propene and cyclopropane.

Cyclopropane and propene are structural isomers of molecular formula C3H6.

Propane and propene exemplify the carbon-13 NMR spectra of lower members of  the alkane and alkene homologous series of CnH2n+2 and CnH2n hydrocarbon molecules where n = 3.

13C NMR SPECTRA (above): The 13C NMR spectra of the three molecules show different numbers of carbon-13 chemical environments i.e propane 2, propene 3 and cyclopropane only 1, so all three could be distinguished from each other.

Key words & phrases: C3H6 image diagram on how to interpret and explain the mass spectrum of cyclopropane m/z m/e base peaks, image and diagram of the mass spectrum of cyclopropane, details of the mass spectroscopy of cyclopropane,  low and high resolution mass spectrum of cyclopropane, prominent m/z peaks in the mass spectrum of cyclopropane, comparative mass spectra of cyclopropane, the molecular ion peak in the mass spectrum of cyclopropane, analysing and understanding the fragmentation pattern of the mass spectrum of cyclopropane, characteristic pattern of peaks in the mass spectrum of cyclopropane, relative abundance of mass ion peaks in the mass spectrum of cyclopropane, revising the mass spectrum of cyclopropane, revision of mass spectroscopy of cyclopropane, most abundant ions in the mass spectrum of cyclopropane, how to construct the mass spectrum diagram for abundance of fragmentation ions in the mass spectrum of cyclopropane, how to analyse the mass spectrum of cyclopropane, how to describe explain the formation of fragmented ions in the mass spectra of cyclopropane equations for explaining the formation of the positive ions in the fragmentation of the ionised molecule of cyclopropane recognising the base ion peak of cyclopropane interpreting interpretation the mass spectrum of cyclopropane cycloalkane functional group

Stick diagram of the relative abundance of ionised fragments in the fingerprint pattern of the mass spectrum of cyclopropane. Table of the m/e m/z values and formula of the ionised fragments in the mass spectrum of cyclopropane. The m/e m/z value of the molecular ion peak in the mass spectrum of cyclopropane.  The m/e m/z value of the base ion peak in the mass spectrum of cyclopropane. Possible examples of equations showing the formation of the ionised fragments in cyclopropane. Revision notes on the mass spectrum of cyclopropane. Matching and deducing the structure of the cyclopropane molecule from its mass spectrum.


Links associated with cyclopropane

The chemistry of ALKANES revision notes INDEX

The infrared spectrum of cyclopropane

The H-1 NMR of cyclopropane

The C-13 NMR spectrum of cyclopropane

Mass spectroscopy index

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All Advanced Organic Chemistry Notes

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