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Interpreting the mass
spectrum of cyclopropane
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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 cyclopropane
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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
Cyclopropane C3H6,
 ,
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.
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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 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
ALL SPECTROSCOPY INDEXES
All Advanced Organic
Chemistry Notes
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