CHEM 231 Lab
T e c h n i q u e P r i m e r
Infrared Spectroscopy
Once a reaction has been carried out, it is necessary to determine the yield, as well as to provide proof that the desired transformation has occurred. One source of proof is infrared (IR) spectroscopy. The IR spectrum of a compound reveals stretching and bending modes within the molecule. Conveniently, certain functionalities give rise to characteristic IR stretches (Figure 3). For example, the O-H stretch occurs at around 3300 cm-1, the C=O stretch at around 1740 cm-1, and the C=C stretch at around 1600 cm-1. Therefore, compounds can be characterized by diagnostic absorbances, such as the carbonyl stretch in acetone (Figure 1), and reactions can be followed by observing the disappearance of one band and the appearance of another.
Figure 1. IR spectrum of acetone (image from the Spectral Database for Organic Compounds)
Samples for IR spectroscopy (Figure 2) are easily prepared by sandwiching a small amount of pure
sample between two plates of sodium chloride, which is largely transparent to IR radiation. Alternatively, a thin film can be deposited on a single plate by slowly dripping an organic solution of the compound onto the surface, allowing the solvent to evaporate.
Figure 2. IR Sample preparation using NaCl plates (image from organicchem.org)
Of course, sodium chloride is soluble in water, so it is important to protect the plates from any sources of water. All washing and cleaning should be done with methylene chloride (or other similar solvent), and care should be taken to ensure that the sample is properly dried. Otherwise, the plates quickly become clouded and useless. Please note that infrared spectroscopy is non-destructive, which means you can recover your entire sample if desired.
500 1000
1500 wavenumber (cm-1)
2000 3000
4000 0 50 100
tr
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m
itt
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%
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4000-2500 2500-1900 1900-1500 1500-400
O-H, N-H, C-H CºC
CºN
C=O C=C C=N
Fingerprint region
C-O, NO2, SO2
a
ro
m
a
tic
C
-H
a
lip
h
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ti
c
C
-H
O-H (broad)
N-H
C-H
CºC (weak)
CºN
a
ld
e
h
y
d
e
C
-H
C=O (strong)
C=C
C=N CO2
(artefact!)
C-O (strong)
Figure 3. Functional group regions in the infrared spectrum
