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Advanced Organic Chemistry: Infrared spectrum of Pentane

Interpreting the infrared spectrum of Pentane

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

See also comparing the infrared, mass, 1H NMR and 13C NMR spectra of the 3 alkane isomers of C5H12

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

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

Pentane  C5H12, alkanes structure and naming (c) doc b , alkanes structure and naming (c) doc b , alkanes structure and naming (c) doc b

For more see The molecular structure and naming of alkanes

Interpretation of the spectrum of pentane

The most prominent infrared absorption lines of pentane

Most of these are due to the vibrations of the C-H bonds e.g. stretching and bending deformations at ~2900 cm-1 and ~1500 cm-1.

There is no characteristic absorption band for a functional group, as alkanes don't have one.

The absence of a specific functional group band will show that particular functional group is absent from the pentane molecular structure.

Comparing the infrared, mass, 1H NMR and 13C NMR spectra of the 3 alkane isomers of C5H12

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 pentane, 2-methylbutane and 2,2-dimethylpropane image sizes.

Comparing the infrared spectra of pentane, 2-methylbutane and 2,2-dimethylpropane

Pentane, 2-methylbutane and 2,2-dimethylpropane are structural isomers of molecular formula C5H12

Pentane, 2-methylbutane and 2,2-dimethylpropane exemplify infrared spectra of  the alkane homologous series CnH2n+2  hydrocarbon molecules, where n = 5

INFRARED SPECTRA (above): There are, as expected, differences in the fingerprint region at wavenumbers 1500 to 400 cm-1, but there is no specific infrared absorption band for a functional group. The infrared spectra of pentane and 2-methylbutane seem very similar, but that of 2,2-dimethylpropane seems much simpler.

Comparing the mass spectra of pentane, 2-methylbutane and 2,2-dimethylpropane

Pentane, 2-methylbutane and 2,2-dimethylpropane are structural isomers of molecular formula C5H12

Pentane, 2-methylbutane and 2,2-dimethylpropane exemplify the mass spectra of  the alkane series CnH2n+2  hydrocarbon molecules, where n = 5

MASS SPECTRA (above): All three hydrocarbons show some similarities in their mass spectra e.g. m/z ions 27 to 29 for [C2Hx]+ (x = 2 and 4). The molecular ion peaks will be the same for all three isomers (m/z 72), but it is very tiny for 2,2-dimethypropane. The pattern ratios for m/z 39 to 43 are similar for pentane and 2-methylbutane, but m/z 42 and 43 ions are almost absent from the 2,2-dimethylpropane spectrum. The base peak ion for pentane is m/z 43, but for 2-methylbutane and 2,2-dimethylpropane it is m/z 57.

Comparing the 1H proton NMR spectra of pentane, 2-methylbutane and 2,2-dimethylpropane

Pentane, 2-methylbutane and 2,2-dimethylpropane are structural isomers of molecular formula C5H12

Pentane, 2-methylbutane and 2,2-dimethylpropane exemplify the 1H proton NMR spectra of the alkane homologous series CnH2n+2  hydrocarbon molecules where, n = 5

1H NMR SPECTRA (above): The 1H NMR spectra of all three molecules give different proton ratios for the different 1H chemical environments i.e. pentane's proton ratio is 3:2:1 (from 6:4:2 H's in the molecule). 2-methylbutane's proton ratio is 6:1:2:3 and 2,2-dimethylpropane's doesn't have a proton ratio, all hydrogen atoms are equivalent. This means all three isomeric C5H12 hydrocarbons can be distinguished from their 1H NMR spectra.

Comparing the carbon-13 NMR spectra of pentane, 2-methylbutane and 2,2-dimethylpropane

Pentane, 2-methylbutane and 2,2-dimethylpropane are structural isomers of molecular formula C5H12

Pentane, 2-methylbutane and 2,2-dimethylpropane exemplify the carbon-13 NMR spectra of members of  the alkane homologous series CnH2n+2  hydrocarbon molecules, where n = 5

13C NMR SPECTRA (above): The 13C NMR spectra of the three molecules show different numbers of carbon-13 chemical environments i.e different numbers of 13C NMR resonance lines. So, pentane gives three 13C chemical shifts, 2-methylbutane four and 2,2-dimethylpropane two. This means all three isomeric C5H12 hydrocarbons can be distinguished from their 13C NMR spectra.

Key words & phrases: image and diagram of the infrared spectrum of pentane, complete infrared absorption spectrum of pentane, comparative spectra of pentane, prominent peaks/troughs for identifying functional groups in the infrared spectrum of pentane, important wavenumber values in cm-1 for peaks/troughs in the infrared spectrum of pentane, revision of infrared spectroscopy of pentane, fingerprint region analysis of pentane, how to identify pentane from its infrared spectrum, identifying organic compounds like pentane from their infrared spectrum

Diagram of absorption of wavenumber peaks in the infrared spectrum of pentane. Characteristic peak wavenumbers in the infrared spectrum of pentane. Finger print identification pattern using the infrared spectrum of pentane. Revision notes on the infrared spectrum of pentane. Matching and deducing the structure of the pentane molecule from  its infrared spectrum. Infrared spectroscopy of alkane, infrared spectra of pentane, an isomer of molecular formula C5H12


Associated links

The chemistry of ALKANES revision notes INDEX

The mass spectrum of Pentane

The H-1 NMR spectrum of pentane

The C-13 NMR spectrum of Pentane

Infrared spectroscopy index

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All Advanced Organic Chemistry Notes

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