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Advanced Organic Chemistry: Mass spectrum of propane

Interpreting the mass spectrum of propane

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 propane

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

mass spectrum of propane C3H8 CH3CH2CH3 fragmentation pattern of m/z m/e ions for analysis and identification of propane image diagram doc brown's advanced organic chemistry revision notes 

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Interpreting the fragmentation pattern of the mass spectrum of propane

[M]+ is the molecular ion peak (M) with an m/z of 44 corresponding to [C3H8]+, the original propane molecule minus an electron, [CH3CH2CH3]+.

The small M+1 peak at m/z 45, corresponds to an ionised propane molecule with one 13C atom in it i.e. an ionised propane molecule of formula [13C12C2H8]+

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.

Propane 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 (propane) 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 propane.

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

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

The parent molecular ion of propane m/z44?[CH3CH2CH3]+

m/z value of [fragment]+ 43 42 41 39 38 29 28 27 26 15
[molecular fragment]+ [C3H7]+ [C3H6]+ [C3H5]+ [C3H3]+ [C3H2]+ [C2H5]+ [C2H4]+ [C2H3]+ [C2H2]+ [CH3]+

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

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 propane (tabulated above)

Formation of m/z 43 ion:

[CH3CH2CH3]+  ===>  [CH3CH2]+  +  H

C-H bond scission, loss hydrogen, mass change 44 - 1 = 43.

Formation of m/z 42 to 37 ions:

These can be formed by success hydrogen atom loss starting with the m/z 43 ion.

In fact the m/z 42 ion could be formed by elimination of hydrogen from the parent molecular ion.

[CH3CH2CH3]+  ===>  [CH2CH2]+  +  H2

Formation of m/z 29 ion:

[CH3CH2CH3]+  ===>  [CH3CH2]+  +  CH3

C-C bond scission of the parent molecular ion, loss methyl group, mass change 44 - 15 = 29.

The m/z 29 ion is the base peak ion, the most abundant and 'stable' ion fragment.

The m/z 30 ion is probably [13C12CH5]+. i.e. the C2H5 fragment with a carbon-13 isotope.

Formation of m/z 27 ion:

[C2H5]+  ===>  [C2H3]+  +  H2

Elimination of a hydrogen molecule from the m/z 29 ion, mass change 29 - 2 = 27.

In fact the m/z 28 to 26 ions are formed by successive hydrogen loss from the m/z 29 ion.

Formation of m/z 15 ion:

[CH3CH2CH3]+  ===>  [CH3]+  +  CH3CH2

C-C bond scission, loss of ethyl group from parent molecular ion, mass change 44 - 29 = 15, but this time the methyl group carries the positive charge.

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.

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


Links associated with propane

The chemistry of ALKANES revision notes INDEX

The infrared spectrum of propane

The H-1 NMR spectrum of propane

The C-13 NMR spectrum of propane

Mass spectroscopy index

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All Advanced Organic Chemistry Notes

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