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The mass
spectrum of propan-2-ol
(2-propanol)
Doc
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 propan-2-ol
See also
comparing the infrared, mass, 1H NMR and 13C NMR
spectra of the 3 isomers of C3H8O
Propan-2-ol C3H8O,
,
,
 ,
Interpreting the fragmentation pattern of the mass spectrum of propan-2-ol
(2-propanol, isopropyl alcohol)
[M]+ is the molecular ion peak (M) with an m/z of
60 corresponding to [C3H8O]+, the original propan-2-ol molecule minus an electron,
[CH3CH(OH)CH3]+.
However, this is a highly unstable molecular ion,
only a trace is detected.
The small M+1 peak at m/z 61, corresponds to an ionised
propan-2-ol
molecule with one 13C atom in it i.e. an ionised propan-2-ol molecule of
formula 13C12C2H8O
Identifying the species giving the most prominent peaks
(apart from M) in the fragmentation pattern of propan-2-ol.
| m/z value of
[fragment]+ |
59 |
45 |
43 ? |
43 ? |
41 |
| [molecular fragment]+ |
[CH3CHOCH3]+ |
[CH3CHOH]+ |
[CH3CO]+ |
[C3H7]+ |
[C3H5]+ |
| m/z value of
[fragment]+ |
39 |
31 |
29 ? |
29 |
27 |
19 |
15 |
| [molecular fragment]+ |
[C3H3]+ |
[CH2OH]+ |
[CHO]+ |
[C2H5]+ |
[C2H3]+ |
[?]+ |
[CH3]+ |
Analysing and explaining the principal ions in the
fragmentation pattern of the mass spectrum of propan-2-ol
Examples of equations to explain some of the most abundant ion peaks of
the mass spectrum of propan-2-ol
Atomic masses: H = 1; C = 12; O = 16
Bond enthalpies kJ/mol: C-C = 348; C-H = 412;
C-O = 360; O-H = 463
Formation of m/z 59 ion:
[CH3CH(OH)CH3]+ ===> [CH3CHOCH3]+
+ H
C-H bond scission and proton loss from parent molecular ion m/z 60
of propan-2-ol, mass change 60 - 1 = 59..
Formation of m/z 45 ion:
[CH3CH(OH)CH3]+ ===> [CH3CHOH]+
+ CH3
C-C bond scission of the parent molecular ion, mass
change 60 - 15 = 45.
The m/z 45 ion is the base peak, the most
abundant stable ion fragment for propan-2-ol.
Formation of m/z 27 ion:
[CH3CHOH]+ ===> [CH3CHOH]+
+ H2O
Elimination of water from the m/z 45 ion, mass
change 45 - 18 = 27.
Formation of m/z 15 ion:
[CH3CH(OH)CH3]+ ===> [CH3]+
+ CH3CHOH
C-C bond scission of the parent molecular ion, mass
change 60 - 45 = 15, but the m/z 45 ion is more likely to be formed
from this fragmentation reaction.
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Comparing the infrared, mass, 1H NMR and 13C NMR
spectra of the 3 isomers of C3H8O
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 propan-1-ol,
propan-2-ol and methoxyethane image sizes. |
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I wasn't able to obtain an infrared
spectrum for methoxyethane, so I've added the infrared spectrum
of ethoxyethane to enable a few comparisons with two aliphatic
alcohols
Comparing the
infrared
spectra of
propan-1-ol,
propan-2-ol and
methoxyethane
Propan-1-ol,
propan-2-ol and methoxyethane
are structural isomers of molecular formula C3H8O
Propan-1-ol,
propan-2-ol and methoxyethane
exemplify infrared spectra of the lower members of the homologous series
of aliphatic alcohols and ethers |
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INFRARED SPECTRA
(above): There are, as expected, differences in the fingerprint region at
wavenumbers 1500 to 400 cm-1, but most absorptions
for all three molecules are the various C-O and the many C-H
vibrational modes. However, there is one characteristic distinguishing
absorption only present in the infrared spectra of alcohols, but
not in ethers, that is the broad O-H stretching vibration
peaking at ~3350 cm-1. There is also another broad
absorption band (origin?) peaking at ~650 cm-1 in the
alcohol spectra, but not in the ether spectra. |
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Comparing the
mass
spectra of
propan-1-ol,
propan-2-ol and
methoxyethane
Propan-1-ol,
propan-2-ol and methoxyethane
are structural isomers of molecular formula C3H8O
Propan-1-ol,
propan-2-ol and methoxyethane
exemplify the mass spectra of the lower members of the homologous series
of aliphatic alcohols and ethers |
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MASS SPECTRA (above):
The base ion peaks are m/z 45 for propan-2-ol and methoxyethane,
but that of propan-1-ol is m/z 31. Many of the fragmentation
ions are common to all three spectra. The m/z 45 ion is peak is
much smaller in the propan-1-ol spectrum compared to the other
two. The molecular ion peak height for propan-2-ol is relative
small compared to the other two spectra suggesting it is much
the less stable m/z 56 ion. |
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Comparing the
1H proton NMR
spectra of
propan-1-ol,
propan-2-ol and
methoxyethane
Propan-1-ol,
propan-2-ol and methoxyethane
are structural isomers of molecular formula C3H8O
Propan-1-ol,
propan-2-ol and methoxyethane exemplify the 1H proton NMR spectra of
the lower members of the homologous series of aliphatic alcohols and
ethers |
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1H NMR SPECTRA (above): The 1H NMR spectra of
all three molecules give different integrated proton ratios for the
different 1H chemical environments i.e. the proton
ratios are as follows: propan-1-ol
3:2:2:1; propan-2-ol 6:1:1 and methoxyethane
3:2:3. Therefore, all three can be distinguished by their
1H NMR spectra. |
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Comparing the
carbon-13 NMR
spectra of
propan-1-ol,
propan-2-ol and
methoxyethane
Propan-1-ol,
propan-2-ol and methoxyethane
are structural isomers of molecular formula C3H8O
Propan-1-ol,
propan-2-ol and methoxyethane exemplify the carbon-13 NMR spectra of
members of the lower members of the homologous series of aliphatic
alcohols and ethers |
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13C NMR SPECTRA
(above): The
13C NMR spectra of propan-1-ol and methoxyethane show
three different 13C NMR chemical shifts, but
propan-2-ol can be distinguished from the other two by
exhibiting only two chemical shift lines. You would need other
spectral data to distinguish propan-1-ol and methoxyethane. |
Key words & phrases: 2-propanol isopropyl
alcohol
image diagram on how to interpret and explain the mass spectrum of
propan-2-ol m/z m/e base peaks, image and diagram of the mass spectrum of
propan-2-ol, details of the mass spectroscopy of propan-2-ol, low and high resolution mass
spectrum of propan-2-ol, prominent m/z peaks in the mass spectrum of propan-2-ol, comparative
mass spectra of propan-2-ol, the molecular ion peak in the mass spectrum of
propan-2-ol,
analysing and understanding the fragmentation pattern of the mass spectrum
of propan-2-ol, characteristic pattern of peaks in the mass spectrum of
propan-2-ol, relative
abundance of mass ion peaks in the mass spectrum of propan-2-ol, revising the mass
spectrum of propan-2-ol, revision of mass spectroscopy of propan-2-ol, most abundant ions in the
mass spectrum of propan-2-ol, how to construct the mass spectrum diagram for abundance
of fragmentation ions in the mass spectrum of propan-2-ol, how to analyse the mass
spectrum of propan-2-ol, how to describe explain the formation of fragmented ions in the
mass spectra of propan-2-ol equations for explaining the formation of the positive ions
in the fragmentation of the ionised molecule of propan-2-ol recognising the base
ion peak of propan-2-ol 2-propanol isopropyl alcohol
isomer of molecular formula C3H8O Stick diagram of the relative abundance
of ionised fragments in the fingerprint pattern of the mass spectrum of
propan-2-ol. Table of the m/e m/z values and formula of the ionised fragments in the
mass spectrum of propan-2-ol. The m/e m/z value of the molecular ion peak in the
mass spectrum of propan-2-ol. The m/e m/z value of the base ion peak in the
mass spectrum of propan-2-ol. Possible examples of equations showing the formation
of the ionised fragments in propan-2-ol. Revision notes on the mass spectrum of
propan-2-ol.
Matching and deducing the structure of the propan-2-ol molecule from its mass
spectrum. Mass spectroscopy of
aliphatic alcohols,
mass spectra of propan-2-ol, an isomer of molecular formula C3H8O 2-propanol
Links associated
with propan-2-ol
The chemistry of ALCOHOLS
revision notes INDEX
The infrared spectrum of Propan-2-ol (2-propanol,
isopropyl alcohol)
The H-1 NMR spectrum of Propan-2-ol (2-propanol,
isopropyl alcohol)
The C-13 NMR spectrum
of Propan-2-ol (2-propanol,
isopropyl alcohol)
Mass spectroscopy index
ALL SPECTROSCOPY INDEXES
All Advanced Organic
Chemistry Notes
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