7.1.6 Optical Isomers

• Stereoisomers are molecules that have the same structural formula but have the atoms arranged differently in space
• There are two types of stereoisomerism
  • Geometrical (cis/tras)
  • Optical

Optical isomerism

• A carbon atom that has four different atoms or groups of atoms attached to it is called a chiral carbon or chiral centre
• Compounds with a chiral centre (chiral molecules) exist as two optical isomers which are also known as enantiomers

A molecule has a chiral centre when the carbon atom is bonded to four different atoms or group of atoms; this gives rises to enantiomers

• The enantiomers are non-superimposable mirror images of each other
• Their physical and chemical properties are identical but they differ in their ability to rotate plane polarised light
  • Hence, these isomers are called ‘optical’ isomers
  • One of the optical isomers will rotate the plane of polarised in the clockwise direction
  • Whereas the other isomer will rotate it in the anti-clockwise direction

When unpolarised light is passed through a polariser, the light becomes polarised as the waves will vibrate in one plane only

Biological activity of enantiomers

• Enantiomers also differ from each other in terms of their biological activity
• Enzymes are chiral proteins that speed up chemical reactions by binding substrates
• They are very target-specific as they have a specific binding site (also called active site) and will only bind molecules that have the exact same shape
• Therefore, if one enantiomer binds to a chiral enzyme, the mirror image of this enantiomer will not bind nearly as well if at all
• It’s like putting a right-hand glove on the left hand!

Enantiomers differ from each other in their biological activity

• Enantiomers are optical isomers that are mirror images of each other and are non-superimposable
• They have similar chemical properties but differ from each other in their ability to rotate plane polarised light and in their biological activity

Optical activity

• Let’s suppose that in a solution, there is 20% of the enantiomer which rotates the plane polarised light clockwise and 80% of the enantiomer which rotates the plane of polarised light anticlockwise
• There is an uneven mixture of each enantiomer, so the reaction mixture is said to be optically active
  • The net effect is that the plane of polarised light will be rotated anticlockwise

• Similarly, if the percentages of the enantiomers are reversed, the reaction mixture is still optically active but now the plane of polarised light will be rotated clockwise
  • In this case, there is 20% of the enantiomer, which rotates the plane anticlockwise
  • And 80% of the enantiomer, which rotates the plane clockwise

• A racemic mixture is a mixture in which there are equal amounts of enantiomers present in the solution
• A racemic mixture is optically inactive as the enantiomers will cancel out each others effect and the plane of polarised light will not change

When one of the enantiomers is in excess, the mixture is optically active; when there are equal amounts of each enantiomer the mixture is optically inactive

• Molecules with a chiral centre exist as optical isomers
• These isomers are also called enantiomers and are non-superimposable mirror images of each other
• The major difference between the two enantiomers is that one of the enantiomers rotates plane polarised light in a clockwise manner and the other in an anticlockwise fashion
  • The enantiomer that rotates the plane clockwise is called the R enantiomer
  • The enantiomer that rotates the plane anticlockwise is called the S enantiomer

• These enantiomers are therefore said to be optically active
• Therefore, the rotation of plane polarised light can be used to determine the identity of an optical isomer of a single substance
  • For example, pass plane polarised light through a sample containing one of the two optical isomers of a single substance
  • Depending on which isomer the sample contains, the plane of polarised light will be rotated either clockwise or anti-clockwise
  • No effect will be observed when the sample is a racemic mixture

Each enantiomer rotates the plane of polarised light in a different direction