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Advanced Organic Chemistry: Infrared spectrum of ethane

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Interpreting the infrared spectrum of ethane

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 ethane

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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 ethane C2H6 CH3CH3 wavenumbers cm-1 functional group detection fingerprint pattern identification of ethane doc brown's advanced organic chemistry revision notes 

Spectra obtained from a liquid film of ethane. The right-hand part of the of the infrared spectrum of ethane, wavenumbers ~1500 to 400 cm-1 is considered the fingerprint region for the identification of ethane and most organic compounds. It is due to a unique set of complex overlapping vibrations of the atoms of the molecule of ethane.

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Interpretation of the infrared spectrum of ethane

The most prominent infrared absorption lines of ethane

Typical alkyl C-H stretching vibrations occur at wavenumbers peaking ~2900 cm-1.

Typical alkyl H-C-H deformation vibrations occur at wavenumbers peaking ~1450 cm-1.

Other alkyl CH3 vibrations occur at wavenumbers peaking ~800 cm-1.

Ethane is a very small symmetrical molecule and hence the relatively simple infrared spectrum.

The absence of other specific functional group bands will show that a particular functional group is absent from the ethane 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. The infrared spectrum of ethane looks simple, but this may lack of resolution.

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.

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Links associated with ethane

The chemistry of ALKANES revision notes INDEX

The mass spectrum of ethane

The H-1 NMR spectrum of ethane

The C-13 NMR spectrum of ethane

Infrared spectroscopy index

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