Advanced Organic Chemistry: Mass spectrum of ethene (ethylene) H2C=CH2

Interpreting and explaining the mass spectrum of ethene (ethylene)

[Author © Dr Phil Brown PhD: Doc Brown's advanced level organic chemistry exam revision notes suitable for students of UK A level chemistry courses & US K12 grade 11, grade 12 and AP honors chemistry courses: Molecular spectroscopy - analysing the mass spectra of ethene [spectra page updated April 3rd 2026 *]

 email doc brown Re-edit  mass spectrum of CH2=CH2

 Links associated with ethene  *  [privacy policy, cookies and disclaimer]

 This is a BIG chemistry website, PLEASE take time to explore it

 Mass spectrometry - spectra index

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

Introductory note on the mass spectrum of ethene

Students and teachers please note my explanation of the mass spectrum of ethene is designed for advanced, but pre-university, chemistry courses.

If M represents the ethene molecule, the initial ionisation to give the molecular ion is:

M(g) + high KE e-  ==> [M•]+(g) + 2e- and fragmentation equations assume [M]+ is the start of the processes and all species are in a gaseous state.

I've not usually shown an unpaired electron on e.g. an ion or a non-ionised alkyl radical R e.g.

[M•]+ ==> [X]+  +  R•, but you should be aware this is a more accurate depiction of some processes.

I've used simplified equations to show how some of the ions that might be formed in the fragmentation pattern for the mass spectrum of ethene and only the formation of singly charged positive are considered for the mass spectrum of ethene.

I've included a stick diagram and table of m/z ions for the mass spectrum of ethene and doing the mass spectrum analysis under standard conditions, databases can be compiled based on complex fingerprint patterns, often involving the relative intensities of many fragment ions, and used to identify compounds including ethene.

In selected cases, where two different fragment ions have the same integer m/z value, I've pointed out that modern mass spectrometers can measure relative ion mass to four decimal places. So, using accurate isotopic masses, I've calculated and compared the accurate ion masses if appropriate for ethene. BUT strictly speaking, 0.0005 should be deducted for singly charged ions to account for the loss of the electron in their formation. I have NOT done this for ethene, but the mass spectrometer software does!

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

The molecular structure and naming of alkenes

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 [C2H4]+, 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.

The base peak ion for the mass spectrum of ethene is the m/z 28 ion [C2H4]+

This means, unusually, the base ion peak for ethene is the same as the molecular ion peak.

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]+  or  [CH2=CH2]+

Note that the m/z 28 ion is both the parent molecular ion peak AND the base ion peak.
m/z value of [fragment]+ 28 27 26 25 24 14 13 12 2
[molecular fragment]+ [C2H4]+ [C2H3]+ [C2H2]+ [C2H]+ [C2]+ [CH2]+ [CH]+ [C]+ [H2]+

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

PLEASE NOTE I have found it difficult to find 'authentic' equations to explain mass spectra fragmentation patterns and it is complex chemistry! I've identified the formulae of the ionised fragments on the mass spectrum diagram, but the equations are from the internet or my conjecture as to how the ions might be formed - please take care in using the information, especially for assignments at university or pre-university level.

Atomic masses: H = 1;  C = 12 (~1% 13)

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 (M-1 ion peak)

Further C-H bond scissions will give m/z ions down from 26 to 24.

You can also form the m/z 26 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 to 12 ions:

This could be formed by C-C bond scission of any fragment containing at least two hydrogen atoms e.g.

[C2H3]+  ===>  [CH2]+  +  CH

or

[C2H4]+  ===>  [CH2]+  +  CH2

And the m/z 14 ion can lose protons to give the m/z 12 and 13 ions (see ion data table).
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.

Key words & phrases: C2H4 CH2=CH2 image diagram on how to interpret and explain the mass spectrum of ethene m/z m/e base peaks, image and diagram of the mass spectrum of ethene, details of the mass spectroscopy of ethene,  low and high resolution mass spectrum of ethene, prominent m/z peaks in the mass spectrum of ethene, comparative mass spectra of ethene, the molecular ion peak in the mass spectrum of ethene, analysing and understanding the fragmentation pattern of the mass spectrum of ethene, characteristic pattern of peaks in the mass spectrum of ethene, relative abundance of mass ion peaks in the mass spectrum of ethene, revising the mass spectrum of ethene, revision of mass spectroscopy of ethene, most abundant ions in the mass spectrum of ethene, how to construct the mass spectrum diagram for abundance of fragmentation ions in the mass spectrum of ethene, how to analyse the mass spectrum of ethene, how to describe explain the formation of fragmented ions in the mass spectra of ethene equations for explaining the formation of the positive ions in the fragmentation of the ionised molecule of ethene recognising the base ion peak of ethene interpreting interpretation the mass spectrum of ethene  ethylene alkene functional group How do you interpret the mass spectrum of ethene How to interpret the mass spectrum of ethene Explanatory diagram of the mass spectrum of the ethene molecule in terms of its molecular structure. Table listing data of the m/z ion prominent main peaks in the mass spectrum of ethene. How to explain the mass spectrum of ethene. The m/z value of the molecular ion peak in the mass spectrum of ethene. Identifying ethene from its mass spectrum pattern. The m/z m/e peak analysis interpretation diagram of the mass spectrum of the ethene molecule. The uses of the mass spectrum of the ethene molecule.  The distinctive features of the mass spectrum of the ethene molecule explained. explaining the fragmentation pattern of the mass spectrum of ethene equations showing the formation of the ionised fragments in the mass spectrum of ethene  what does the mass spectrum tell you about the structure and properties of the ethene molecule? Data table of ionised fragments in the mass spectrum of ethene and equations for their formation in the fragmentation of the ionised ethene molecule.

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 spectrometry index

ALL SPECTROSCOPY INDEXES

All Advanced Organic Chemistry Notes

Use My Google search site box

Email doc b: chem55555@hotmail.com

Website content © Dr Phil Brown 2000+. All copyrights reserved on revision notes, images, quizzes, worksheets etc. Copying of Doc Brown's pre-university advanced level chemistry website material is NOT permitted. Exam revision summaries & references to science course specifications are unofficial. These organic chemistry revision notes on spectroscopy (mass spectra of ethene) are suitable for use of pre-university students studying AQA advanced level chemistry, Edexcel advanced level chemistry, OCR advanced level chemistry, IB advanced level chemistry, WJEC (Eduqas) advanced level chemistry, CIE advanced level chemistry, CCEA advanced level chemistry, US grade 11-12 AP honors chemistry courses and they will also prove useful to 1st year undergraduate students of chemistry.

TOP OF PAGE