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Advanced Organic Chemistry: Infrared spectrum of ethene (ethylene)

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Interpreting the infrared 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 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

infrared spectrum of ethene C2H4 CH2=CH2 wavenumbers cm-1 functional group detection fingerprint pattern identification of ethylene doc brown's advanced organic chemistry revision notes 

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 alkenes structure and naming (c) doc b  displayed formula of ethene alkenes structure and naming (c) doc b skeletal formula is only alkenes structure and naming (c) doc b

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.

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. 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.

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).

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 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

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