1. Infrared spectroscopy
C2.8 Analytical Techniques
23 November 2018

2. Infrared spectroscopy
Infrared spectroscopy is used to identify functional groups (e.g. alcohol ROH, carboxylic acid ROOH) present in organic molecules.
All covalent bonds will vibrate at a characteristic frequency.
How it works:
Sample is placed inside spectrometer
A beam of IR radiation in the range of cm-1 is passed through the sample
The bonds inside the molecule absorb the radiation at their characteristic frequencies.
The spectrometer is linked to a computer which plots the spectrum.

3. What do we see?
The molecule’s spectrum has a number of troughs (called peaks). Each of peak represents the vibration of a particular bond.
The presence of these peaks signifies the presence of a particular functional group.
Anything below 1500cm-1 is known as the fingerprint region

4. Applications of infrared spectroscopy
Infrared spectroscopy has many everyday uses:
It is used extensively in forensic science (eg. To analyse paint fragments from vehicles in hit-and-run offences)
Monitoring the degree of unsaturation in polymers
Quality control in perfume manufacture
Drug analysis
Testing the breath of suspected drunken drivers for ethanol

5. Interpreting spectra
The information generated from an infrared spectrum can be used to identify the functional groups in a molecule. You will need to identify the presence of functional groups on your data booklet.

6. Narrow O-H peak in alcohols
C-H peak

7. Aldehydes and ketones
C=O peak
C-H peak

8. Broad O-H peak in carboxylic acids
C=O peak
Broad O-H peak in carboxylic acids

9. Knowledge Check
Complete the past paper questions. We will complete question 2 as a worked example.