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Advanced Organic Chemistry: Mass spectrum of propene ('propylene')

Interpreting the mass spectrum of propene ('propylene')

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 propene

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

mass spectrum of propene C3H6 CH3CH=CH2 fragmentation pattern of m/z m/e ions for analysis and identification of propene image diagram doc brown's advanced organic chemistry revision notes 

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Interpreting the fragmentation pattern of the mass spectrum of propene

[M]+ is the molecular ion peak (M) with an m/z of 42 corresponding to [C3H6]+, the original propene molecule minus an electron, [CH3CH=CH2]+.

The small M+1 peak at m/z 43, corresponds to an ionised propene molecule with one 13C atom in it i.e. an ionised propene 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.

Propene 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 (propene) 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 propene.

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

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

The parent molecular ion of propene m/z 42: [CH3CH=CH2]+

m/z value of [fragment]+ 41 40 39 38 37 28 27 26 25 15 14
[molecular fragment]+ [C3H5]+ [C3H4]+ [C3H3]+ [C3H2]+ [C3H]+ [C2H4]+ [C2H3]+ [C2H2]+ [C2H2]+ [CH3]+ [CH2]+

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

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 propene (tabulated above)

Formation of m/z 41 to 37 ions:

[CH3CH=CH2]+  ===>  [C3H5]+  +  H

C-H bond scission of the parent molecular ion, hydrogen atom loss, mass change 42 - 1 = 41 giving m/z 41 ion.

The m/z 41 ion is the base peak ion, the most abundant and 'stable' ion fragment.

The m/z 41 ion can lose a hydrogen atom/molecule giving the m/z 40, 39, 38 and 37 ions.

The m/z 40 ion can also be formed by elimination of a hydrogen molecule from the parent molecular ion, mass change 42 - 2 = 40.

[CH3CH=CH2]+  ===>  [C3H4]+  +  H2

Formation of m/z 27 to 25 ions:

[CH3CH=CH2]+  ===>  [C2H3]+  +  CH3

C-C bond scission of the parent molecular ion, methyl group lost, mass change 42 - 15 = 27, to give the m/z 27 ion.

The m/z 27 ion can lose a hydrogen atom or molecule to give the m/z 26 and 25 ions.

Formation of m/z 15 and 14 ions:

[CH3CH=CH2]+  ===>  [CH3]+  +  C2H3

C-C bond scission of the parent molecular ion, methyl group released and ionised, mass change 42 - 27 = 15.

Note this is the alternative ionised fragment to the formation of the m/z 27 ion.

The methyl cation can lose hydrogen atoms to give the m/z 14 and 13 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 H2C=CHCH3 CH3CH=CH2 CH2=CHCH3 image diagram on how to interpret and explain the mass spectrum of propene m/z m/e base peaks, image and diagram of the mass spectrum of propene, details of the mass spectroscopy of propene,  low and high resolution mass spectrum of propene, prominent m/z peaks in the mass spectrum of propene, comparative mass spectra of propene, the molecular ion peak in the mass spectrum of propene, analysing and understanding the fragmentation pattern of the mass spectrum of propene, characteristic pattern of peaks in the mass spectrum of propene, relative abundance of mass ion peaks in the mass spectrum of propene, revising the mass spectrum of propene, revision of mass spectroscopy of propene, most abundant ions in the mass spectrum of propene, how to construct the mass spectrum diagram for abundance of fragmentation ions in the mass spectrum of propene, how to analyse the mass spectrum of propene, how to describe explain the formation of fragmented ions in the mass spectra of propene equations for explaining the formation of the positive ions in the fragmentation of the ionised molecule of propene recognising the base ion peak of propene interpreting interpretation the mass spectrum of propene alkene functional group propylene

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


Links associated with propene

The chemistry of ALKENES revision notes INDEX

The infrared spectrum of propene ('propylene')

The H-1 NMR spectrum of propene ('propylene')

The C-13 NMR spectrum of propene ('propylene')

Mass spectroscopy index

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