Infrared Spectroscopy (WJEC GCSE Chemistry)

Higher Tier

What is infrared spectroscopy used for?

• In this topic the compounds that have been looked at are:
  • Alkanes
  • Alkenes
  • Alcohols
  • Carboxylic acids

Summary of organic molecules

Organic molecules containing two carbon atoms will begin with eth-

• These organic molecules contain different functional groups
  • For example alcohols contain an –OH group and carboxylic acids contain a –COOH group
• The bonds in the functional groups can be identified using infrared spectroscopy in compounds
• Substances are put into an infrared spectrometer and infrared light is shone through them
• Different chemical bonds absorb different wavelengths of light and bonds can be identified
  • For example the infrared spectrum of propanoic acid would show the following

Infrared spectrum of propanoic acid

The infrared spectrum of propanoic acid contains key peaks for the O–H and C=O bonds

• Different bonds can be identified by the peaks on the spectrum at different points
• Some of these are shown in the table below

Table of Infrared Absorptions

Bond	Wavenumber / cm-1
O-H (alcohols)	3230-3550
C-H	2850-3300
O-H (carboxylic acids)	2500-3000
C=O	1680-1750
C=C (alkenes)	1620-1680
C-C	750-1110

What is the fingerprint region?

• The fingerprint region is the more complex part of the spectrum which used to identify a particular molecule
• It is unique for different molecules