Infra-red (IR) spectroscopy
Aims
• What is the fingerprint region in an infrared spectrum?
• Which functional groups give
characteristic absorption peaks?
• How are the spectra of ketones,
aldehydes, carboxylic acids and esters
similar and how are they different?
Infra-red (IR) spectroscopy
•
An infra-red spectrum is essentially a graph of transmission/% (vertical axis)
against wavenumber/cm-1, (horizontal axis). A dip (or trough) on the spectrum is
called a peak.
•
Infra-red spectroscopy is used to identify functional groups in organic compounds.
Most functional groups give rise to characteristic infra-red absorption which do
not change much from one compound to another.
•
Absorption of infra-red radiation by a molecule causes bond stretching and bond
bending (but not bond breaking!) resulting in the appearance of absorption peaks at
particular wavenumbers.
Note 1: The x-axis on an infra-red spectrum must be read from right to left!
Note 2:There is a change of scale on the x-axis.
Fingerprinting
The area of an IR spectrum below about 1500cm-1 usually has many
peaks. The shape of this region is unique for any particular substance.
It can therefore be used to identify an unknown compound by matching
with spectra of known samples using a computer database.
fingerprint region
transmission (%)
100
80
60
40
20
0
3000
2000
1000
wavenumber (cm-1)
Impurities
• An IR spectrum can be used to spot the presence of impurities.
The spectrum of a compound is matched against that of the pure
compound. Extra peaks indicate the presence of impurities in the
compound.
Identifying Functional Groups
• Particular bonds / functional groups produce peaks in particular
areas of the spectrum.
Bond
Wavenumber/ cm
C-H
2850-3300
C-C
750-1100
C=C
1620-1680
C=O
1680-1750
C-O
1000-1300
O-H (alcohols)
3230-3550
O-H (carboxylic acids)
2500-3000
-1
Infra-red spectra of alcohols
Wavenumber of peak/ cm
-1
Bond causing peak
3230-3550 (broad)
O-H
1000-1300 (narrow)
C-O
Note 1: The peak caused by the O—H bond (between 3230cm-1 and 3550cm-1) appears
broad due to the presence of hydrogen bonding between the alcohol molecules.
Note 2: The peak caused by the C—O bond (between 1000cm-1 and 1300cm-1) is often
clearly identifiable, despite appearing in the fingerprint region.
Infra-red spectra of compounds containing the C=O group
• The C=O bond is found in aldehydes, ketones, carboxylic acids, esters,
etc. It shows as an absorption peak at about 1700cm-1. The actual
value varies slightly, depending on the type of compound it is found in.
1. Aldehydes and Ketones:
Wavenumber of peak/cm
1700
-1
Bond causing peak
C=O
2. Carboxylic Acids:
Wavenumber of peak/cm
-1
Bond causing peak
1700
C=O
3000 (broad)
O-H
Note: The O—H absorption peak is broad (as in alcohols) due to
hydrogen bonding
3. Esters:
Wavenumber of peak/cm
-1
Bond causing peak
1700
C=O
1000-1300
C-O
1000-1300
C-O
Note: There is no O—H absorption peak but there are often two C—O absorption peaks
visible as there are two C—O bonds in slightly different molecular environments within
the molecule.
To do
• Summary questions on page 138 and 141