|
Interpreting the mass
spectrum of ethene (ethylene)
Doc
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 ethene
See also
comparing the infrared, mass, 1H NMR and 13C NMR spectra of ethane and
ethene
Ethene
C2H4
 displayed formula of ethene
 skeletal formula is only
Interpreting the fragmentation pattern of the mass spectrum of ethene
[M]+ is the molecular ion peak (M) with an m/z of
28 corresponding to [C2H6]+, the original ethene molecule minus an electron,
[CH2=CH2]+.
The small M+1 peak at m/z 29, corresponds to an ionised
ethene
molecule with one 13C atom in it i.e. an ionised ethene molecule of
formula [13C12CH4]+
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.
Ethene has 2 carbon atoms, so on
average, ~1 in 50 molecules will contain a 13C atom.
The most abundant ion of the molecule under mass
spectrometry investigation (ethene) 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 ethene.
Unless otherwise indicated, assume the carbon atoms in
ethene are the 12C isotope.
Some of the possible positive ions, [molecular fragment]+,
formed in the mass spectrometry of ethene.
The parent molecular ion of ethene
m/z 28:
[C2H4]+
Note that the
m/z 28 ion is both
the parent molecular ion
peak AND the base ion
peak.
| m/z value of
[fragment]+ |
27 |
26 |
25 |
24 |
14 |
| [molecular fragment]+ |
[C2H3]+ |
[C2H2]+ |
[C2H]+ |
[C2]+ |
[CH2]+ |
Analysing and explaining the principal ions in the
fragmentation pattern of the mass spectrum of ethene
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 ethene
(tabulated above)
Formation of m/z 27 to 24 ions:
[C2H4]+ ===> [C2H3]+
+ H
C-H bond scission of the parent molecular ion, mass
change 28 - 1 = 27.
Further C-H bond scissions will give m/z ions down
from 26 to 24.
You can also form the m/z 22 ion by elimination of a
hydrogen molecule from the parent molecular ion.
[C2H4]+ ===> [C2H2]+
+ H2
Some evidence for this comes from the presence of an
m/z 2 ion, which can only be an ionised hydrogen molecule - there is
often a chance that the 'other' fragment carries the positive
charge.
Formation of m/z 14 ion:
This could be formed by C-C bond scission of any
fragment containing at least two hydrogen atoms e.g.
[C2H3]+ ===> [CH2]+
+ CH
|
Comparing the infrared, mass, 1H NMR and 13C NMR
spectra of
ethane and ethene
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. |
 |
 |
|
INFRARED SPECTRA:
Apart from the significant differences in the fingerprint region at
wavenumbers 1500 to 400 cm-1, the most striking
differences are (i) the band at ~1900 cm-1 for ethene,
absent in the ethane spectrum, (ii) the bands at 800 cm-1
for ethane (CH3 vibrations), absent or much weaker in
ethene, and (iii) the strong absorptions at ~1000 cm-1
for ethene, completely absent in the ethane spectrum. |
 |
|
|
MASS SPECTRA: Both
ethane and ethene show some similarities in their mass
spectra e.g. m/z ions 25 to 28 for [C2Hx]+
(x = 1 to 4) ions and in both cases the base ion peak has an m/z
of 28. However, the molecular ion peaks will be different
because of their different relative molecular masses i.e. ethane
m/z 30 and ethene m/z 28. Ethane also has a prominent m/z ion
peak of 29, which is tiny in the ethene mass spectrum (and only
due to 1% 13C atoms in the parent molecular ion).
Ethene only shows a very tiny peak for m/z 15 ion. The mass
spectrum of ethene is a bit less complicated because of fewer
hydrogen atoms giving fewer possibilities of fragmentation ions. |
 |
 |
|
1H NMR SPECTRA: The 1H NMR spectra of
ethane and ethene are similar in that that both give one single
singlet resonance line in their proton NMR spectra. All the
protons in each molecule are equivalent to each other and occupy
the same chemical environment due to the symmetry of the
molecule, so no resonance splitting. However the two 1H
chemical shifts are significantly different due the different
shielding effects of the -CH3 and =CH2
groupings respectively. |
 |
 |
|
13C NMR SPECTRA: The
1C NMR spectra of ethane and ethene are similar in that that
both give one single resonance line in their carbon-13 NMR
spectra. In both molecules the two carbon atoms occupy the same
chemical environment due to the symmetry of the molecule.
However the two 13C chemical shifts are significantly
different due the different shielding effects of the -CH3
and =CH2
groupings respectively. |
Key words & phrases: C2H4 CH2=CH2 image diagram on how to interpret and explain the mass spectrum of
ethene m/z m/e base peaks, image and diagram of the mass spectrum of
ethene, details of the mass spectroscopy of ethene, low and high resolution mass
spectrum of ethene, prominent m/z peaks in the mass spectrum of ethene, comparative
mass spectra of ethene, the molecular ion peak in the mass spectrum of ethene,
analysing and understanding the fragmentation pattern of the mass spectrum
of ethene, characteristic pattern of peaks in the mass spectrum of ethene, relative
abundance of mass ion peaks in the mass spectrum of ethene, revising the mass
spectrum of ethene, revision of mass spectroscopy of ethene, most abundant ions in the
mass spectrum of ethene, how to construct the mass spectrum diagram for abundance
of fragmentation ions in the mass spectrum of ethene, how to analyse the mass
spectrum of ethene, how to describe explain the formation of fragmented ions in the
mass spectra of ethene equations for explaining the formation of the positive ions
in the fragmentation of the ionised molecule of ethene recognising the base ion
peak of ethene interpreting interpretation the mass spectrum of ethene
ethylene alkene
functional group
Links associated
with ethene
The chemistry of ALKENES
revision notes INDEX
The infrared spectrum of ethene ('ethylene')
The H-1 NMR spectrum of ethene ('ethylene')
The C-13 NMR spectrum of ethene ('ethylene')
Mass spectroscopy index
ALL SPECTROSCOPY INDEXES
All Advanced Organic
Chemistry Notes
Use My Google search site box
Email doc b:
chem55555@hotmail.com
TOP OF PAGE
|
