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Advanced Organic Chemistry: Mass spectrum of ethene (ethylene)

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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

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Mass spectroscopy - spectra index

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

mass spectrum of ethene C2H4 CH2=CH2 fragmentation pattern of m/z m/e ions for analysis and identification of ethylene image diagram doc brown's advanced organic chemistry revision notes 

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

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.

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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

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