|
Interpreting
and explaining the mass
spectrum of propene
('propylene')
[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 - analysing the mass spectra of
propene
[spectra
page updated
April 4th 2026 *]
email doc
brown
Re-edit
mass spectrum of H2C=CHCH3
Links associated with propene
* [privacy policy, cookies
and disclaimer]
This is a BIG chemistry website, PLEASE take time to explore it
Mass spectrometry - spectra index
See also
comparing the infrared, mass, 1H
NMR and 13C NMR
spectra of propane, cyclopropane and propene
Introductory note on the mass spectrum of propene
Students and teachers please note
my explanation of the mass spectrum of propene is designed for
advanced, but pre-university, chemistry courses.
If M represents the
propene molecule, the initial ionisation to give the molecular ion is:
M(g) +
high KE e- ==> [M•]+(g) + 2e-
and 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 propene and only the formation of singly charged
positive are considered for the mass spectrum of propene.
I've included a stick diagram and table of m/z ions for the mass spectrum of
propene
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
propene.
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 and compared the accurate ion
masses if appropriate for propene. 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
propene,
but the mass spectrometer software does!
Propene,
 ,
 ,
 ,
 ,
 ,
An alkene
The molecular structure and naming of alkenes
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
molecular
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.
The base peak ion for
the mass spectrum of propene is m/z 41 ion
[C3H5]+
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
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 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
(M-1 ion peak)
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 (M-2 ion peak)
[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
(M-15 ion peak)
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
(M-27 ion peak)
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.
|
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. |
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. How do you interpret the mass spectrum of
propene How to interpret
the mass spectrum of propene Explanatory diagram of the mass spectrum of the
propene molecule in
terms of its molecular structure.
Table listing data of the m/z ion prominent main peaks in the mass spectrum of
propene. How to explain the mass spectrum of propene. The m/z value of the
molecular ion peak in the mass spectrum of propene. Identifying
propene from
its mass spectrum pattern. The m/z m/e peak analysis interpretation
diagram of the mass
spectrum of the propene molecule. The uses of the mass spectrum of the
propene molecule. The distinctive features of the mass spectrum of
the propene molecule explained. explaining the fragmentation pattern of the mass spectrum of
propene equations showing the
formation of the ionised fragments in the mass spectrum of propene
what does the mass spectrum tell you about the structure and
properties of the propene molecule? Data table of ionised fragments in
the mass spectrum of propene and equations for their formation in the
fragmentation of the ionised propene molecule.
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
Use My Google search site box
Email doc b:
chem55555@hotmail.com
Website content © Dr
Phil Brown 2000+. All copyrights reserved on revision notes, images,
quizzes, worksheets etc. Copying of Doc Brown's pre-university
advanced level chemistry website material is NOT
permitted. Exam revision summaries & references to science course specifications
are unofficial. These organic chemistry revision notes on
spectroscopy (mass spectra of propene) are
suitable for use of pre-university students studying AQA advanced level
chemistry, Edexcel advanced level chemistry, OCR advanced level
chemistry, IB advanced level chemistry, WJEC (Eduqas) advanced level
chemistry, CIE advanced level chemistry, CCEA advanced level chemistry, US grade 11-12 AP honors
chemistry courses and they will also prove useful to
1st year undergraduate students of chemistry. |
