CHEM 231 Lab T e c h n i q u e P r i m e

CHEM 231 Lab

Technique Primer

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.


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.


C=O stretch

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

100

4000-2500

1900-1500

CºC

C=O
C=C
C=N

CºN


1500-400
Fingerprint region
C-O, NO2, SO 2

O-H
(broad)
CO2
(artefact!)

C=C

N-H

50

aldehyde C-H

aliphatic C-H


C=N

CºC
(weak)

C-H

aromatic C-H

transmittance (%)

O-H, N-H, C-H

2500-1900

C-O
(strong)

C=O
(strong)

CºN

0
4000

3000

1500

2000
wavenumber (cm-1)

Figure 3. Functional group regions in the infrared spectrum

1000

500