Interpreting and explaining the infrared spectrum of propene ('propylene')

[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 - analysing the infrared spectra of propene [spectra page updated April 4th 2026 *]

See also comparing the infrared, mass, ¹H NMR and ¹³C NMR spectra of propane, cyclopropane and propene

Introductory note on the infrared spectrum of propene

Students and teachers please note my explanation of the infrared spectrum of propene is designed for advanced, but pre-university, chemistry courses.

Based in the infrared spectrum diagram for propene, only some of the most prominent peaks for particular bond vibrations are discussed, particularly if propene has a functional group with a particular characteristic wavenumber peak.

The infrared spectrum of propene is unique and the whole, or selected wavenumbers, can be used to fingerprint its identity, sometimes analysing a mixture containing propene or following its change of concentration in a reaction.

Spectra obtained from a liquid film of propene. The right-hand part of the of the infrared spectrum of propene, wavenumbers ~1500 to 400 cm^-1 is considered the fingerprint region for the identification of propene and most organic compounds. It is due to a unique set of complex overlapping vibrations of the atoms of the molecule of propene.

An alkene The molecular structure and naming of alkenes

Interpretation of the infrared spectrum of propene

The most prominent infrared absorption lines of propene

There are prominent bands of C-H stretching vibration absorptions peaking at wavenumbers ~3095 to 3010 cm^-1, and also 995 to 905 cm^-1, associated with the C-H bond in a RCH=CH₂ grouping e.g. as in propene.

Overlapping with the above, are prominent bands due to C-H stretching vibration absorptions peak at wavenumbers ~2975 to 2860 cm-1, typical of molecules with a methyl alkyl group.

There is a very strong characteristic absorption for C=C stretching vibrations, peaking at wavenumbers 1645 to 1640 cm^-1, typical of an alkene like propene.

Around wavenumbers ~1470 to 1370 cm^-1 are absorptions dues to C-H deformation vibrations of the C-CH₃ grouping in propene.

The absence of other specific functional group bands will show that a particular functional group is absent from the propene molecular structure.

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.
	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.
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.
	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.
	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.
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..
	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.
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.

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