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Interpreting the mass
spectrum of propane
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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 propane
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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
Propane C3H8,
 ,
,
,
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.
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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: C3H8 CH3CH2CH3 image diagram on how to interpret and explain the mass spectrum of
propane m/z m/e base peaks, image and diagram of the mass spectrum of
propane, details of the mass spectroscopy of propane, low and high resolution mass
spectrum of propane, prominent m/z peaks in the mass spectrum of propane, comparative
mass spectra of propane, the molecular ion peak in the mass spectrum of propane,
analysing and understanding the fragmentation pattern of the mass spectrum
of propane, characteristic pattern of peaks in the mass spectrum of propane, relative
abundance of mass ion peaks in the mass spectrum of propane, revising the mass
spectrum of propane, revision of mass spectroscopy of propane, most abundant ions in the
mass spectrum of propane, how to construct the mass spectrum diagram for abundance
of fragmentation ions in the mass spectrum of propane, how to analyse the mass
spectrum of propane, how to describe explain the formation of fragmented ions in the
mass spectra of propane equations for explaining the formation of the positive ions
in the fragmentation of the ionised molecule of propane recognising the base ion
peak of propane interpreting interpretation the mass spectrum of propane
alkane
functional group 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
ALL SPECTROSCOPY INDEXES
All Advanced Organic
Chemistry Notes
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