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Interpreting and
explaining the infrared
spectrum of propane
[Author
©
Dr Phil Brown PhD:
Doc Brown's advanced level organic chemistry exam revision notes
suitable for students of UK A level chemistry courses & US K12 grade
11, grade 12 and AP honors chemistry courses:
Molecular
spectroscopy analysis of
propane
[spectra page
updated Mar 26th 2026 *]
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doc brown re-edit infrared spectrum of
C3H8
CH3CH2CH3
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Infrared spectroscopy - spectra index
Links associated
with propane
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and disclaimer]
See also
comparing the infrared, mass, 1H
NMR and 13C NMR
spectra of propane, cyclopropane and propene
Introductory note on the infrared spectrum of propane
Students and teachers please note
my explanation of the infrared
spectrum of propane is designed
for advanced, but pre-university, chemistry courses.
Based in
the infrared spectrum diagram for propane, only some of the most
prominent peaks for particular bond vibrations are discussed,
particularly if propane has a functional group with a particular
characteristic wavenumber peak.
The infrared spectrum of
propane is
unique and the whole, or selected wavenumbers, can be used to
fingerprint its identity, sometimes analysing a mixture
containing propane or following its change of concentration in a
reaction.
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,
 ,
,
,
alkane
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. |
|
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. How do you interpret the infrared absorption spectrum of
propane How
to interpret the infrared spectrum of propane Explanatory diagram of the
infrared spectrum of the propane molecule in terms of its molecular
structure. Listing data of the prominent main wavenumber peaks
troughs in the infrared spectrum of propane. How to explain the infrared
spectrum of propane. Use of the infrared spectrum of propane, identification
of propane from its infrared spectrum - fingerprint wavenumber pattern
to identify the propane molecule. The uses of the infrared spectrum of
the propane molecule. The distinctive features of the infrared spectrum
of the propane molecule explained interpretation diagram explaining the
peaks-trough of the transmittance of the infrared spectrum of
propane
what does the infrared spectrum tell you about the structure and
properties of the propane molecule? How is infrared spectrum of
propane used
to identify propane?
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
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Chemistry Notes
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permitted. Exam revision summaries & references to science course specifications
are unofficial. These organic chemistry revision notes on
spectroscopy (on the infrared spectrum of propane) are
suitable for use of pre-university students studying AQA advanced level
chemistry, Edexcel advanced level chemistry, OCR advanced level
chemistry, IB advanced level chemistry, WJEC (Eduqas) advanced level
chemistry, CIE advanced level chemistry, CCEA advanced level chemistry, US grade 11-12 AP honors
chemistry courses and they will also prove useful to
1st year undergraduate students of chemistry. |
