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Interpreting the infrared
spectrum of ethene (ethylene)
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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 infrared spectra of ethene
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Infrared spectroscopy - spectra index
See also
comparing the infrared, mass, 1H NMR and 13C NMR spectra of ethane and
ethene
Spectra obtained from a liquid film of ethene. The right-hand part of the of the
infrared spectrum of ethene, wavenumbers
~1500 to 400
cm-1 is considered the fingerprint region for the
identification of ethene and most organic compounds. It is due to a unique set
of complex overlapping vibrations of the atoms of the molecule of ethene.
Ethene
C2H4
displayed formula of ethene
 skeletal formula is only
Interpretation of
the infrared spectrum of ethene
The most prominent infrared absorption lines of ethene
The absorption C-H stretching vibrations of the CH2
group peak at wavenumber ~3026
There are absorption bands at ~1400 cm-1
due to scissoring H-C-H
vibrations and ~3100 cm-1 from H-C-H asymmetric
stretching vibrations - the latter overlaps with the C-H stretching
vibrations.
Strong absorption band peaking at 900 cm-1
due to a H-C-H 'wagging' vibration.
Not sure what the absorption band at ~1900 cm-1
is due to?
Note: Unlike most other alkenes, it is a totally
symmetrical molecule and cannot absorb infrared radiation at the
characteristic C=C stretching vibration frequency of
~1690 to 1620 cm-1 - the there is no change in the
dipolar character of this molecule (this is university level
theory).
Lots of peaks appear due to rotation-vibration
interactions - but this is again, university level theory and is
observed much more clearly in the infrared spectra of gaseous
molecules.
The absence of other specific functional group bands
will show that a particular functional group is absent from the
ethene
molecular
structure.
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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. |
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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. Because of
its complete symmetry, the ethene infrared spectrum does not
show the characteristic C=C vibration absorption at wavenumber
~1680 to 1620 cm-1 of alkenes. |
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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). |
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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. |
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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 and diagram explaining the infrared spectrum
of ethene, complete infrared absorption spectrum of ethene, comparative spectra of
ethene, prominent peaks/troughs for identifying functional groups in the infrared spectrum of
ethene,
important wavenumber values in cm-1 for peaks/troughs in the infrared spectrum
of ethene, revision of infrared spectroscopy of ethene, fingerprint region analysis of
ethene, how to identify ethene from its infrared spectrum, identifying organic
compounds like ethene from their infrared spectrum,
how to analyse the absorption bands in the infrared spectrum of ethene detection of
alkene functional groups in the ethene molecule example of the infrared spectrum of a
molecule like ethene with a alkene
functional group
interpreting interpretation of the infrared spectrum of ethene shows presence
of alkene functional group
ethylene
Links associated with ethene
The chemistry of ALKENES
revision notes INDEX
The mass spectrum of ethene ('ethylene')
The H-1 NMR spectrum of ethene ('ethylene')
The C-13 NMR spectrum of ethene ('ethylene')
Infrared spectroscopy index
ALL SPECTROSCOPY INDEXES
All Advanced Organic
Chemistry Notes
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