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Interpreting the mass
spectrum of ethane
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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
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Mass spectroscopy - spectra index
See also
comparing the infrared, mass, 1H NMR and 13C NMR spectra of ethane and
ethene
The alkane
ethane
 yes! just a dash!
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.
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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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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
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mass spectra of ethane, the molecular ion peak in the mass spectrum of ethane,
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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
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
ALL SPECTROSCOPY INDEXES
All Advanced Organic
Chemistry Notes
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