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Interpreting the mass
spectrum of propene
('propylene')
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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
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Mass spectroscopy - spectra index
See also
comparing the infrared, mass, 1H
NMR and 13C NMR
spectra of propane, cyclopropane and propene
Propene,
 ,
 ,
 ,
 ,
 ,
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 m/z 15 can lose hydrogen atoms to give
the m/z 14 and 13 ions.
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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.. |
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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. |
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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
ALL SPECTROSCOPY INDEXES
All Advanced Organic
Chemistry Notes
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