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Interpreting the infrared
spectrum of propane
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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 infrared spectra of propane
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Infrared spectroscopy - spectra index
See also
comparing the infrared, mass, 1H
NMR and 13C NMR
spectra of propane, cyclopropane and propene
Spectra obtained from a liquid film of propane. The right-hand part of the of the
infrared spectrum of propane, wavenumbers
~1500 to 400
cm-1 is considered the fingerprint region for the
identification of propane and most organic compounds. It is due to a unique set
of complex overlapping vibrations of the atoms of the molecule of propane.
Propane C3H8,
 ,
,
,
Interpretation of
the infrared spectrum of propane
The most prominent infrared absorption lines of propane
C-H stretching vibration absorptions occur in bands
peaking at wavenumbers 2975 to 2845 cm-1.
C-H deformation-bending vibration absorptions occur
in bands peaking at wavenumbers 1470 to 1365 cm-1.
C-C-C skeletal vibrations absorptions occur in bands
peaking at wavenumbers 1175 to 1140 and 840 to 790 cm-1.
The absence of other specific functional group bands
will show that a particular functional group is absent from the
propane
molecular
structure.
Apart from the fingerprint pattern for propane, these are all
typical alkane/alkyl vibrations and offer little of value in
investigating molecular structure.
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Comparing the infrared, mass, 1H NMR and 13C NMR
spectra of propane, cyclopropane and propene
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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Comparing the
infrared
spectra of propane, propene and cyclopropane.
Cyclopropane and propene are structural isomers of molecular formula C3H6.
Propane
and propene exemplify the infrared spectra of lower members of the alkane
and alkene homologous series of CnH2n+2 and CnH2n
hydrocarbon
molecules where n = 3. |
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INFRARED SPECTRA
(above):
Apart from the significant differences in the fingerprint region at
wavenumbers 1500 to 400 cm-1, the most striking
differences are: (i) propene shows the characteristic absorption
at ~1700 cm-1 for the C=C stretching vibrations,
absent in the other two spectra, (ii) cyclopropane shows an absorption band
at 2200 cm-1, absent in the other two spectra, (iii)
propane has an absorption band at ~750 cm-1, absent
in the other two spectra. |
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Comparing the
mass
spectra of propane, propene and cyclopropane.
Cyclopropane and propene are structural isomers of molecular formula C3H6.
Propane
and propene exemplify the mass spectra of lower members of the alkane
and alkene homologous series of CnH2n+2 and CnH2n
hydrocarbon
molecules where n = 3. |
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MASS SPECTRA (above):
All three hydrocarbons show some similarities in their mass
spectra e.g. m/z ions 26 to 28 for [C2Hx]+
(x = 2 to 4) and m/z 14 and 15 ions - but these are found in
most aliphatic hydrocarbon spectra. The molecular ion peaks will
be the same for the isomeric propene and cyclopropane (m/z 42)
but that of propane will be 2 mass units higher at m/z 44. The
base ion peak m/z values are all different, propane 29, propene
41 and cyclopropane 42. |
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Comparing the
1H proton NMR
spectra of propane, propene and cyclopropane.
Cyclopropane and propene are structural isomers of molecular formula C3H6.
Propane
and propene exemplify the 1H proton NMR spectra of lower members of the alkane
and alkene homologous series of CnH2n+2 and CnH2n
hydrocarbon
molecules where n = 3. |
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1H NMR SPECTRA (above): The 1H NMR spectra of
all three molecules give different proton ratios i.e. propane
3:1 (actually 6:2 in the molecule), propene 2:1:3 (spectrum and
molecule) and cyclopropane just a singlet for the six protons,
so all three can be distinguished from each other by their
1H NMR spectra.. |
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Comparing the
carbon-13 NMR
spectra of propane, propene and cyclopropane.
Cyclopropane and propene are structural isomers of molecular formula C3H6.
Propane
and propene exemplify the carbon-13 NMR spectra of lower members of the alkane
and alkene homologous series of CnH2n+2 and CnH2n
hydrocarbon
molecules where n = 3. |
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13C NMR SPECTRA
(above): The
13C NMR spectra of the three molecules show different numbers of
carbon-13 chemical environments i.e propane 2, propene 3 and cyclopropane
only 1, so all three could be distinguished from each other. |
Key words & phrases: C3H8 CH3CH2CH3 image and diagram explaining the infrared spectrum
of propane, complete infrared absorption spectrum of propane, comparative spectra of
propane, prominent peaks/troughs for identifying functional groups in the infrared spectrum of
propane,
important wavenumber values in cm-1 for peaks/troughs in the infrared spectrum
of propane, revision of infrared spectroscopy of propane, fingerprint region analysis of
propane, how to identify propane from its infrared spectrum, identifying organic
compounds like propane from their infrared spectrum,
how to analyse the absorption bands in the infrared spectrum of propane detection of functional groups in the
propane molecule example of the infrared spectrum of a
molecule like propane with a
functional group
interpreting interpretation of the infrared spectrum of propane shows presence
of propane alkane functional group Diagram of absorption of wavenumber
peaks in the infrared spectrum of propane. Characteristic peak wavenumbers in the infrared
spectrum of propane. Finger print identification pattern using the infrared
spectrum of propane. Revision notes on the infrared spectrum of propane. Matching
and deducing the structure of the propane molecule from its infrared
spectrum.
Links associated with propane
The chemistry of ALKANES
revision notes INDEX
The mass
spectrum of propane
The H-1 NMR spectrum of propane
The C-13 NMR
spectrum of propane
Infrared spectroscopy index
ALL SPECTROSCOPY INDEXES
All Advanced Organic
Chemistry Notes
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