20.3.3 Optical Isomers

Optical isomers

• A carbon atom that has four different atoms or groups of atoms attached to it is called a chiral carbon or chiral centre
  • Chira comes from a Greek word meaning hand, so we talk about these molecules having a handedness

• The carbon atom is described as being asymmetric, i.e. there is no plane of symmetry in the molecule
• Compounds with one chiral centre (chiral molecules) exist as two optical isomers, also known as enantiomers
• Just like the left hand cannot be superimposed on the right hand, enantiomers are non-superimposable
  • Enantiomers are mirror images of each other

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

Diastereomers

• Diastereomers are compounds that contain more than one chiral centre
  • Diastereomers are not mirror images of each other because each chiral carbon has two isomers
  • This also means that they have different physical and chemical properties
    
    

2-bromo-3-chlorobutane exists as a diastereomer due to 2 chiral centres

Properties of optical isomers

• The chemical properties of optical isomers are generally identical, with one exception
  • Optical isomers interact with biological sensors in different ways
    • For example, one enantiomer of carvone smells of spearmint, while the other smells of caraway
      
      

                         Carvone optical isomers have distinctive smells

• Optical isomers have identical physical properties, with one exception
  • Isomers differ in their ability to rotate the plane of polarised light

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

• The major difference between the two enantiomers is:
  • One enantiomer rotates plane polarised light in a clockwise manner and the other in an anticlockwise fashion
  • A common way to differentiate the isomers is to use (+) and (-), but there are other systems using d and l, D and L, or R and S

• 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 by a fixed number of degrees

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

• A racemic mixture (or racemate) is a mixture containing equal amounts of each enantiomer
  • One enantiomer rotates light clockwise, the other rotates light anticlockwise

• A racemic mixture is optically inactive as the enantiomers will cancel out each others effect
  • This means that the plane of polarised light will not change
    
    

           Racemic mixtures are optically inactive

Racemic mixtures and drugs

• In the pharmaceutical industry, it is much easier to produce synthetic drugs that are racemic mixtures than producing one enantiomer of the drug
• Around 56% of all drugs in use are chiral and of those 88% are sold as racemic mixtures
• Separating the enantiomers gives a compound that is described as enantiopure, it contains only one enantiomer
• This separation process is very expensive and time consuming, so for many drugs it is not worthwhile, even though only half the of the drug is pharmacologically active
• For example, the pain reliever ibuprofen is sold as a racemic mixture

The structure of ibuprofen showing the chiral carbon that is responsible for the racemic mixture produced in the synthesis of the drug