Hydrocarbons
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:
- C–H stretch from 3000–2850 cm-1
- C–H bend or scissoring from 1470-1450 cm-1
- C–H rock, methyl from 1370-1350 cm-1
- C–H rock, methyl, seen only in long chain alkanes, from 725-720 cm-1
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:
- C=C stretch from 1680-1640 cm-1
- =C–H stretch from 3100-3000 cm-1
- =C–H bend from 1000-650 cm-1
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:
- –C?C– stretch from 2260-2100 cm-1
- –C?C–H: C–H stretch from 3330-3270 cm-1
- –C?C–H: C–H bend from 700-610 cm-1
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:
- C–H stretch from 3100-3000 cm-1
- overtones, weak, from 2000-1665 cm-1
- C–C stretch (in-ring) from 1600-1585 cm-1
- C–C stretch (in-ring) from 1500-1400 cm-1
- C–H "oop" from 900-675 cm-1
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
Functional Groups Containing the C-O Bond
Alcohols have IR absorptions associated with both the O-H and the C-O stretching vibrations.
- O–H stretch, hydrogen bonded 3500-3200 cm-1
- C–O stretch 1260-1050 cm-1 (s)
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:
- C=O stretch - aliphatic ketones 1715 cm-1
- ?, ?-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.
- H–C=O stretch 2830-2695 cm-1
- C=O stretch:
- aliphatic aldehydes 1740-1720 cm-1
- alpha, beta-unsaturated aldehydes 1710-1685 cm-1
Figure 9. shows the spectrum of butyraldehyde.
Figure 7. Infrared Spectrum of Butyraldehyde
The carbonyl stretch C=O of esters appears:
- C=O stretch
- aliphatic from 1750-1735 cm-1
- ?, ?-unsaturated from 1730-1715 cm-1
- C–O stretch from 1300-1000 cm-1
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.
- O–H stretch from 3300-2500 cm-1
- C=O stretch from 1760-1690 cm-1
- C–O stretch from 1320-1210 cm-1
- O–H bend from 1440-1395 and 950-910 cm-1
Figure 11. shows the spectrum of hexanoic acid.
Figure 9. Infrared Spectrum of Hexanoic acid
Organic Nitrogen Compounds
- N–O asymmetric stretch from 1550-1475 cm-1
- N–O symmetric stretch from 1360-1290 cm-1
Figure 10. Infrared Spectrum of Nitomethane
Organic Compounds Containing Halogens
Alkyl halides are compounds that have a C–X bond, where X is a halogen: bromine, chlorine, fluorene, or iodine.
- C–H wag (-CH2X) from 1300-1150 cm-1
- C–X stretches (general) from 850-515 cm-1
- C–Cl stretch 850-550 cm-1
- C–Br stretch 690-515 cm-1
The spectrum of 1-chloro-2-methylpropane are shown below.
Figure 11. Infrared Spectrum of 1-chloro-2-methylpropane
