Infra-red (IR) spectroscopy

Aims• What is the fingerprint region in an infra-red

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.

0

20

40

60

80

100

3000 2000 1000

wavenumber (cm-1)

tran

sm

issio

n

(%)

fingerprint region

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 -1

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

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 -1

Bond causing peak

1700 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