Infrared Spectroscopy (WJEC GCSE Chemistry)

Bonds in Organic Molecules

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

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