Interpreting and explaining the infrared spectrum of cyclopropane

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

Introductory note on the infrared spectrum of cyclopropane

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

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

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

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

Interpretation of the infrared spectrum of cyclopropane

The most prominent infrared absorption lines of cyclopropane

C-H stretching vibrations peak at wavenumbers ~3080 to 3040  cm^-1 for cyclopropane.

Not sure what vibrations cause the sharp band at ~2200 cm^-1.

-CH₂- deformation vibrations peak at wavenumbers 1480 to 1440 cm^-1 for cyclopropane.

Cyclopropane shows -CH₂- skeletal vibration absorption bands peaking at wavenumbers 1020 to 1000 cm^-1.

Cyclopropane shows many absorption bands in common with aliphatic hydrocarbon molecules.

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

Website content © Dr Phil Brown 2000+. All copyrights reserved on revision notes, images, quizzes, worksheets etc. Copying of Doc Brown's pre-university advanced level chemistry website material is NOT permitted. Exam revision summaries & references to science course specifications are unofficial. These organic chemistry revision notes on the spectroscopy of cyclopropane - its infrared spectrum is fully analysed and 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.