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Advanced Organic Chemistry: Mass spectrum of ethane

See also comparing the infrared, mass, 1H NMR and 13C NMR spectra of ethane and ethene

Interpreting the mass 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 mass spectra of ethane

See also comparing the infrared, mass, 1H NMR and 13C NMR spectra of ethane and ethene

mass spectrum of ethane C2H6 CH3CH3 fragmentation pattern of m/z m/e ions for analysis and identification of ethane image diagram doc brown's advanced organic chemistry revision notes 

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Interpreting the fragmentation pattern of the mass spectrum of ethane

[M]+ is the molecular ion peak (M) with an m/z of 30 corresponding to [C2H6]+, the original ethane molecule minus an electron, [CH3CH3]+.

The small M+1 peak at m/z 31, corresponds to an ionised ethane molecule with one 13C atom in it i.e. an ionised ethane molecule of formula [13C12CH6]+

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.

Ethane 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 (ethane) 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 ethane.

Unless otherwise indicated, assume the carbon atoms in ethane are the 12C isotope.

Some of the possible positive ions, [molecular fragment]+, formed in the mass spectrometry of ethane.

The parent molecular ion of ethane m/z 30 [CH3CH3]+

m/z value of [fragment]+ 29 28 27 26 25 15 14
[molecular fragment]+ [C2H5]+ [C2H4]+ [C2H3]+ [C2H2]+ [C2H]+ [CH3]+ [CH2]+

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of ethane

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 ethane (tabulated above)

Formation of m/z 29 ion:

[C2H6]+  ===>  [C2H5]+  +  H

C-H bond scission of parent molecular ion, hydrogen loss, mass change 30 - 1 = 29.

Formation of m/z 28 ion:

[C2H6]+  ===>  [C2H4]+  +  H2

Elimination of hydrogen molecule from parent molecular ion, mass loss 30 - 2 = 28.

The m/z 28 ion is the base peak ion, the most abundant and 'stable' ion fragment.

Formation of m/z ions 27 to 25

These can be formed by further hydrogen loss from the m/z 29 or 28 ions.

Formation of m/z 15 ion:

[C2H6]+  ===>  [CH3]+  +  CH3

C-C bond scission of parent molecular ion, methyl group loss, mass change 30 - 15 = 15.

Formation of m/z ions 14 to 13

These can be formed by further hydrogen loss from m/z 15 ion.

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). The mass spectrum of ethane is a bit more complicated because of the two extra hydrogen atoms giving more 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.

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Links associated with ethane

The chemistry of ALKANES revision notes INDEX

The infrared spectrum of ethane

The H-1 NMR spectrum of ethane

The C-13 NMR spectrum of ethane

Mass spectroscopy index

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