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الكيمياء الصناعية | Spectral Interpretation by Application of Group Frequencies |
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One of the most common application of infrared spectroscopy is to the identification of organic compounds. The major classes of organic molecules are shown in this category and also linked on the bottom page for the number of collections of spectral information regarding organic molecules.
Hydrocarbons compounds contain only C-H and C-C bonds, but there is plenty of information to be obtained from the infrared spectra arising from C-H stretching and C-H bending.
In alkanes, which have very few bands, each band in the spectrum can be assigned:
Figure 1. shows the IR spectrum of octane. Since most organic compounds have these features, these C-H vibrations are usually not noted when interpreting a routine IR spectrum. Note that the change in dipole moment with respect to distance for the C-H stretching is greater than that for others shown, which is why the C-H stretch band is the more intense.
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Figure 1. Infrared Spectrum of Octane
In alkenes compounds, each band in the spectrum can be assigned:
Figure 2. shows the IR spectrum of 1-octene. As alkanes compounds, these bands are not specific and are generally not noted because they are present in almost all organic molecules.
Figure 2. Infrared Spectrum of 1-Octene
In alkynes, each band in the spectrum can be assigned:
The spectrum of 1-hexyne, a terminal alkyne, is shown below.
Figure 3. Infrared Spectrum of 1-Hexyne
In aromatic compounds, each band in the spectrum can be assigned:
Note that this is at slightly higher frequency than is the –C–H stretch in alkanes. This is a very useful tool for interpreting IR spectra. Only alkenes and aromatics show a C–H stretch slightly higher than 3000 cm-1.
Figure 4. shows the spectrum of toluene.
Figure 4. Infrared Spectrum of Toluene
Alcohols have IR absorptions associated with both the O-H and the C-O stretching vibrations.
Figure 5. shows the spectrum of ethanol. Note the very broad, strong band of the O–H stretch.
Figure 5. Infrared Spectrum of Ethanol
The carbonyl stretching vibration band C=O of saturated aliphatic ketones appears:
- ?, ?-unsaturated ketones 1685-1666 cm-1
Figure 6. shows the spectrum of 2-butanone. This is a saturated ketone, and the C=O band appears at 1715.
Figure 6. Infrared Spectrum of 2-Butanone
If a compound is suspected to be an aldehyde, a peak always appears around 2720 cm-1 which often appears as a shoulder-type peak just to the right of the alkyl C–H stretches.
Figure 9. shows the spectrum of butyraldehyde.
Figure 7. Infrared Spectrum of Butyraldehyde
The carbonyl stretch C=O of esters appears:
Figure 10. shows the spectrum of ethyl benzoate.
Figure 8. Infrared Spectrum of Ethyl benzoate
The carbonyl stretch C=O of a carboxylic acid appears as an intense band from 1760-1690 cm-1. The exact position of this broad band depends on whether the carboxylic acid is saturated or unsaturated, dimerized, or has internal hydrogen bonding.
Figure 11. shows the spectrum of hexanoic acid.
Figure 9. Infrared Spectrum of Hexanoic acid
Figure 10. Infrared Spectrum of Nitomethane
Alkyl halides are compounds that have a C–X bond, where X is a halogen: bromine, chlorine, fluorene, or iodine.
The spectrum of 1-chloro-2-methylpropane are shown below.
Figure 11. Infrared Spectrum of 1-chloro-2-methylpropane
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