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Interpreting and
explaining the mass
spectrum of cyclopropane
[Author
©
Dr Phil Brown PhD:
Doc Brown's advanced level organic chemistry exam revision notes
suitable for students of UK A level chemistry courses & US K12 grade
11, grade 12 and AP honors chemistry courses:
Molecular
spectroscopy analysis of
cyclopropane
[spectra page updated
Mar 23rd 2026 *]
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Re-edit
mass spectrum of
cyclo-C3H6
Links associated with cyclopropane
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Mass spectrometry - spectra index
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See also
comparing the infrared, mass, 1H
NMR and 13C NMR
spectra of propane, cyclopropane and propene
Introductory note on the mass spectrum of cyclopropane
Students and teachers please note
my explanation of the mass spectrum of cyclopropane is designed for
advanced, but pre-university, chemistry courses.
If M represents the
cyclopropane molecule, the initial ionisation to give the molecular ion is:
M(g) +
high KE e- ==> [M•]+(g) + 2e-
and for fragmentation equations assume [M]+ is the start of the
processes and all species are in a gaseous state.
I've not usually shown an unpaired electron on e.g. an ion or a non-ionised
alkyl radical R e.g.
[M•]+ ==> [X]+ + R•,
but you should be aware this is a more accurate depiction of some
processes.
I've used simplified equations to show how some of
the ions that might be formed in the fragmentation pattern for the
mass spectrum of cyclopropane and only the formation of singly charged
positive are considered for the mass spectrum of cyclopropane.
I've included a stick diagram and table of m/z ions for the mass spectrum of
cyclopropane
and doing the mass spectrum analysis under standard conditions,
databases can be compiled based on complex fingerprint patterns, often involving
the relative intensities of many fragment ions, and used to identify compounds including
cyclopropane.
In selected cases, where two
different fragment ions have the same integer m/z value,
I've pointed out that modern mass spectrometers can measure
relative ion mass to four decimal places. So, using
accurate isotopic masses, I've calculated the accurate ion
masses, BUT strictly speaking, 0.0005 should be deducted
for singly charged ions to account for the loss of the
electron in their formation. I have NOT done this for
cyclopropane,
but the mass spectrometer software does!
Cyclopropane C3H6,
 ,
The molecular structure and
naming of alkanes
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.
Bothe parent molecular
ion and the base peak ion for
the mass spectrum of cyclopropane is the m/z 42 ion
[C3H6]+
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.
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.
The base peak ion for
the mass spectrum of cyclobutane is the m/z 42 ion
[C3H6]+
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
PLEASE NOTE
I have found it difficult to find 'authentic' equations to explain mass
spectra fragmentation patterns and it is complex chemistry! I've identified
the formulae of the ionised fragments on the mass spectrum diagram, but the
equations are from the internet or my conjecture as to how the ions might be
formed - please take care in using the information, especially for
assignments at university or pre-university level.
Atomic masses: H = 1; C = 12
(~1% 13)
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 (M-1 ion peak)
You can then get successive hydrogen atom/molecule
losses to give m/z ions 40 down 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 (M-2 ion peak)
Again, you can then get successive hydrogen
atom/molecule losses to give m/z ions 39 to 37 see diagram and data
table.
Formation of m/z 25 to 29 ions:
Several routes available
e.g. from the parent molecular ion
(i) m/z 29 [C3H6]+ ===> [C2H5]+
+ CH
(ii) m/z 28 [C3H6]+ ===> [C2H3]+
+ CH2
(iii) m/z 27 [C3H6]+ ===> [C2H3]+
+ CH3
All involve C-C bond
scission in the parent molecular ion and possible proto rearrangement
too.
For (iii) 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.
|
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. |
|
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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 |
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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. |
|
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 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. How do you interpret the mass spectrum of cyclopropane How to interpret
the mass spectrum of cyclopropane Explanatory diagram of the mass spectrum of the
cyclopropane molecule in
terms of its molecular structure.
Listing data of the prominent main peaks in the mass spectrum of
cyclopropane. How to explain the mass spectrum of cyclopropane. The m/z value of the
molecular ion peak in the mass spectrum of cyclopropane. Identifying
cyclopropane from
its mass spectrum pattern. The m/z m/e peak analysis interpretation
diagram of the mass
spectrum of the cyclopropane molecule. The uses of the mass spectrum of the
cyclopropane molecule. The distinctive features of the mass spectrum of
the cyclopropane molecule explained. explaining the fragmentation pattern of the mass spectrum of
cyclopropane equations showing the
formation of the ionised fragments in the mass spectrum of
cyclopropane
what does the mass spectrum tell you about the structure and
properties of the cyclopropane molecule? Data table of ionised fragments in
the mass spectrum of cyclopropane and equations for their formation in the
fragmentation of cyclopropane molecules
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 spectrometry index
ALL SPECTROSCOPY INDEXES
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Chemistry Notes
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