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The mass
spectrum of ethanol
(ethyl alcohol)
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Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK IB
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Mass spectroscopy - spectra index
Ethanol C2H6O,
,
,
,
Revision notes on the structure and naming
(nomenclature) of aliphatic ALCOHOLS and ETHERS
Interpreting the mass spectrum of ethanol
[M]+ is the molecular ion peak (M) with an m/z of
46 corresponding to [C2H6O]+, the original
ethanol molecule minus an electron,
[CH3CH2OH]+.
The tiny M+1 peak at m/z 47, corresponds to an ionised
ethanol
molecule with one 13C atom in it i.e. an ionised ethanol molecule of
formula [13C12CH6O]+.
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.
Ethanol 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 (methoxymethane) 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 methoxymethane.
Unless otherwise indicated, assume the carbon atoms in
methoxymethane are the 12C isotope.
Some of the possible positive ions, [molecular fragment]+,
formed in the mass spectrometry of ethanol - identifying the species giving the most prominent peaks
(apart from M) in the fragmentation pattern of ethanol.
| m/z value of
[fragment]+ |
45 |
43 |
31 |
30 |
29 |
28 |
27 |
26 |
15 |
| [molecular fragment]+ |
[C2H5O]+ |
[C2H3O]+ |
[CH2OH]+ |
[CHOH]+ |
[C2H5]+ |
[C2H4]+ |
[C2H3]+ |
[C2H2]+ |
[CH3]+ |
Analysing and explaining the principal ions in the
fragmentation pattern of the mass spectrum of ethyl alcohol mass spectrum
ethanol
Atomic masses: H = 1; C = 12 (1 in ~100 is 13); O = 16
Bond enthalpies = kJ/mol: C-C = 348; C-H = 412;
C-O = 360; 743; O-H = 463
Explaining the principal fragments of the mass spectrum of
ethanol
Formation of m/z 43 and 45 ions:
The prominent peak at m/z 45 corresponds to the loss of
a hydrogen atom (actually a hydrogen radical H•) from the ionised ethanol molecule
- the parent molecular ion.
[C2H6O]+
===> [C2H5O]+
+ H
Mass change 46 - 1 = 45.
Loss of H2 from this ion forms the m/z 43
ion.
[C2H6O]+
===> [C2H3O]+
+ H2
Formation of m/z 31 ion:
The m/z 31 ion is formed by scission of the C-C bond
in the parent molecular ion.
[CH3CH2OH]+
===> [CH2OH]+ + CH3
Mass change 46 - 15 = 31.
The m/z 31 ion is the base peak, the most
stable fragment.
Formation of m/z 28 ion:
The m/z 28 ion is formed by the elimination of water
from the parent molecular ion of ethanol.
[CH3CH2OH]+
===> [C2H4]+ +
H2O
Mass change 46 - 18 = 28.
Formation of m/z 29 ion:
The m/z 29 ion is formed by scission of the C-O bond
in the parent molecular ion of ethanol.
[CH3CH2OH]+
===> [C2H5]+ +
OH
Mass change 46 - 17 = 29.
Formation of m/z 27 ion:
The m/z 27 ion can be formed by elimination of hydrogen
from the m/z 29 ion.
[C2H5]+ ===>
[C2H3]+ + OH
Formation of m/z 15 ion:
The m/z 15 ion formed by scission of [CH3CH2OH]+
===> [CH3]+ + CH2OH
Scission of the C-C bond in the parent molecular
ion of ethanol.
See also comparing the IR, mass, 1H
NMR and 13C NMR spectra of
isomers of C2H6O
below.
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Comparing the infrared, mass, 1H NMR and 13C NMR
spectra of the 2 isomers of C2H6O
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 ethanol (ethyl
alcohol) and
methoxymethane (dimethyl ether) 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
difference is the broad O-H stretching band ~3400 cm-1,
found in the infrared spectrum of alcohols, but absent in
the infrared spectrum of ethers. |
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MASS SPECTRA: Both
ethanol and methoxymethane show some similarities in their mass
spectra, their but their base ion peaks are quite different - for ethanol
it is m/z 31 and for methoxymethane it is m/z 45.
Ethanol gives the m/z 29 [C2H5]+
ion, which can be distinguished from the m/z 29 [C2H5]+
ion by high resolution spectroscopy. Ethanol has more abundant
peaks for m/z ions 26, 27 and 43. |
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1H NMR SPECTRA: The 1H NMR spectra of
ethanol and methoxymethane are quite significantly different.
Ethanol gives 3 peaks in the proton ratio 3:2:1 (3 different
chemical environments), whereas methoxymethane only gives one
1H chemical shift peak (all 6 protons in the same
chemical environment). |
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13C NMR SPECTRA: The 13C NMR spectra of
ethanol and methoxymethane are different. Ethanol gives two
13C resonances, but methoxymethane only one (2
different 13C chemical environments and a 13C
single chemical environment). |
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the mass spectra of ethanol explaining the m/z ion fragmentation pattern of the
mass spectrum of ethanol
Associated links
The infrared spectrum of Ethanol (ethyl alcohol)
The H-1 NMR spectrum of
Ethanol (ethyl alcohol)
The C-13 NMR spectrum Ethanol (ethyl alcohol)
The chemistry of ALCOHOLS
revision notes INDEX
Mass spectroscopy index
ALL SPECTROSCOPY INDEXES
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