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Infrared
Tutorial
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The region of the infrared
spectrum which is of greatest interest to organic chemists is
the wavelength range 2.5 to 15 micrometers (µ). In practice,
units proportional to frequency, (wave number in units of cm-1)
rather than wavelength, are commonly used and the region 2.5 to
15 µ corresponds to approximately 4000 to 600 cm-1.
See the following figure which describes the regions of the electromagnetic
spectrum
Alcohols and amines display strong broad O-H and N-H stretching
bands in the region 3400-3100 cm-1. The bands are broadened
due to hydrogen bonding and a sharp 'non-bonded' peak can often
be seen at around 3400 cm-1.
Alkene and alkyne C-H bonds display sharp stretching absorptions
in the region 3100-3000 cm-1
.
The bands are of medium intensity and are often obscured by other
absorbances in the region (i.e., OH).
Triple bond stretching absorptions occur in the region 2400-2200
cm-1. Absorptions from nitriles are generally of medium
intensity and are clearly defined. Alkynes absorb weakly in this
region unless they are highly asymmetric; symmetrical alkynes
do not show absorption bands.
Carbonyl stretching bands occur in the region 1800-1700 cm-1.
The bands are generally very strong and broad. Carbonyl compounds
which are more reactive in nucleophilic addition reactions (acyl
halides, esters) are generally at higher wave number than simple
ketones and aldehydes, and amides are the lowest, absorbing in
the region 1700-1650 cm-1.
Carbon-carbon double bond stretching occurs in the region around
1650-1600 cm-1. The bands are generally sharp and of
medium intensity. Aromatic compounds will typically display a
series of sharp bands in this region.
Carbon-oxygen single bonds display stretching bands in the
region 1200-1100 cm-1. The bands are generally strong
and broad. You should note that many other functional groups have
bands in this region which appear similar.
The spectra at the left is for
acetophenone.
3400-3200 cm-1: no OH or NH present 3100 cm-1:
moderate peak suggesting unsaturated CH 2900 cm-1:
weak peak indicating possible saturated CH 2200 cm-1:
no unsymmetrical triple bonds 1690 cm-1: strong carbonyl
absorbance
1610 cm-1: weak absorbance bands consistent with carbon-carbon
double bonds The spectrum seems to be consistent with a carbonyl
compound containing both single and double bonds. The carbonyl
absorbance occurs at a relatively low wave number, suggesting
the carbonyl is conjugated with a double bond or an aromatic ring.
The large number of degrees of unsaturation suggest the presence
of an aromatic ring.
