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Interpreting
and explaining the mass
spectrum of ethane
[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
spectrometry - analysing the mass spectra of ethane
[spectra
page updated
April 3rd 2026 *]
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Re-edit
mass spectrum of
CH3CH3
Links associated with ethane
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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 ethane
Students and teachers please note
my explanation of the mass spectrum of ethane is designed for
advanced, but pre-university, chemistry courses.
If M represents the
ethane 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 ethane and only the formation of singly charged
positive are considered for the mass spectrum of ethane.
I've included a stick diagram and table of m/z ions for the mass spectrum of
ethane
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
ethane.
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 ethane. 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
ethane,
but the mass spectrometer software does!
The alkane
ethane
 yes! just a dash!
The molecular structure and
naming of alkanes
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 tiny M+1 ion 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.
The base ion peak
for the mass spectrum of ethane is m/z 28 ion
[C2H4]+
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
is the m/z 30 ion
[C2H6]+
or
[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
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 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
(M-1 ion peak)
Formation of m/z 28 ion:
[C2H6]+ ===> [C2H4]+
+ H2
Elimination of hydrogen molecule from parent
molecular ion,
mass loss 30 - 2 = 28
(M-2 ion peak)
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
(M-15 ion peak)
Formation of m/z ions 14 to 13
These can be formed by further hydrogen loss from
the
m/z 15 ion.
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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. |
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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. |
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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. |
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 |
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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. |
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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: C2H6 CH3CH3 image diagram on how to interpret and explain the mass spectrum of
ethane m/z m/e base peaks, image and diagram of the mass spectrum of
ethane, details of the mass spectroscopy of ethane, low and high resolution mass
spectrum of ethane, prominent m/z peaks in the mass spectrum of ethane, comparative
mass spectra of ethane, the molecular ion peak in the mass spectrum of ethane,
analysing and understanding the fragmentation pattern of the mass spectrum
of ethane, characteristic pattern of peaks in the mass spectrum of ethane, relative
abundance of mass ion peaks in the mass spectrum of ethane, revising the mass
spectrum of ethane, revision of mass spectroscopy of ethane, most abundant ions in the
mass spectrum of ethane, how to construct the mass spectrum diagram for abundance
of fragmentation ions in the mass spectrum of ethane, how to analyse the mass
spectrum of ethane, how to describe explain the formation of fragmented ions in the
mass spectra of ethane equations for explaining the formation of the positive ions
in the fragmentation of the ionised molecule of ethane recognising the base ion
peak of ethane interpreting interpretation the mass spectrum of ethane formula
alkane
functional group How do you interpret the mass spectrum of
ethane How to interpret
the mass spectrum of ethane Explanatory diagram of the mass spectrum of the
ethane molecule in
terms of its molecular structure.
Table listing data of the m/z ion prominent main peaks in the mass spectrum of
ethane. How to explain the mass spectrum of ethane. The m/z value of the
molecular ion peak in the mass spectrum of ethane. Identifying
ethane from
its mass spectrum pattern. The m/z m/e peak analysis interpretation
diagram of the mass
spectrum of the ethane molecule. The uses of the mass spectrum of the
ethane molecule. The distinctive features of the mass spectrum of
the ethane molecule explained. explaining the fragmentation pattern of the mass spectrum of
ethane equations showing the
formation of the ionised fragments in the mass spectrum of ethane
what does the mass spectrum tell you about the structure and
properties of the ethane molecule? Data table of ionised fragments in
the mass spectrum of ethane and equations for their formation in the
fragmentation of the ionised ethane molecule.
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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spectroscopy (mass spectra of ethane) 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. |
