|
Interpreting the mass
spectrum of ethanol
(ethyl alcohol)
(re-edit)
Doc Brown's
Chemistry Advanced Level Pre-University Chemistry
Revision Study Notes for UK IB
KS5 A/AS GCE advanced level organic chemistry students US K12 grade 11 grade 12
honors organic chemistry
courses involving molecular spectroscopy analysing mass spectra
Use your
mobile phone in 'landscape'?
Mass spectroscopy - spectra index
Other
associated links
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
Note that the ionised ethanol molecule [C2H6O]+
can be written as [C2H5OH]+
or [CH3CH2OH]+.
BUT,
the ionised original molecule, or, any other ionised fragment, may not
necessarily have the same structure as it had in the original
ethanol molecule.
This is a general situation in the mass
spectra of organic compounds like ethanol.
this is why mass spectrum fragments are often listed as
simple molecular formulae as well as showing the actual bonding
arrangement in the fragment, which may not fit in with the 'usual'
organic molecular structures you are used to!
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.
|
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. |
 |
 |
|
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. |
 |
 |
|
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. |
 |
 |
|
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). |
 |
 |
|
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). |
Key words & phrases: how to interpret and explain the mass spectrum of
ethanol, image and diagram of the mass spectrum of ethanol, details of the mass
spectroscopy of ethanol, low and high resolution mass spectrum of ethanol,
prominent m/z peaks in the mass spectrum of ethanol, comparative mass spectra of
ethanol, the molecular ion peak in the mass spectrum of ethanol, analysing and
understanding the fragmentation pattern of the mass spectrum of ethanol,
characteristic pattern of peaks in the mass spectrum of ethanol, relative
abundance of mass ion peaks in the mass spectrum of ethanol, revising the mass
spectrum of ethanol, revision of mass spectroscopy of ethanol, most abundant
ions in the mass spectrum of ethanol, how to construct the mass spectrum diagram
for abundance of fragmentation ions in the mass spectrum of ethanol, how to
analyse the mass spectrum of ethanol, how to describe explain the formation of
fragmented ions in the mass spectra of ethanol explaining the m/z ion
fragmentation pattern of the mass spectrum of ethanol Explanatory diagram of the
mass spectrum of the ethanol molecule. Listing data of the prominent main peaks
in the mass spectrum of ethanol. How to explain the mass spectrum of ethanol.
The m/z value of the molecular ion peak in the mass spectrum of ethanol.
Identifying ethanol from its mass spectrum pattern. The m/z m/e peak analysis of
the mass spectrum of the ethanol molecule. The uses of the mass spectrum of the
ethanol molecule explaining the fragmentation pattern of the mass spectrum of ethanol equations showing the formation of the ionised fragments in 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
All Advanced Organic
Chemistry Notes
Use My Google search site box
Email doc b:
chem55555@hotmail.com
|
