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Advanced Organic Chemistry: Infrared spectrum of propane

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Interpreting the infrared spectrum of propane

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 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

infrared spectrum of propane C3H8 CH3CH2CH3 wavenumbers cm-1 functional group detection fingerprint pattern identification of propane doc brown's advanced organic chemistry revision notes 

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.

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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.

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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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

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