There are two types of stereoisomerism:
- cis–trans isomerism
- optical isomerism.
Unlike in a carbon-carbon single bond, there is no free rotation about a carbon-carbon double bond. This results in the existence of type of isomerism called cis-trans isomerism in unsaturated organic compounds. For example, dibromoethene has two such isomers:
In cis-1,2-dibromoethene both the Br atoms are on the same side of the carbon-carbon double bond.
In trans-1,2-dibromoethene, the Br atoms on opposite sides of the carbon-carbon double bond.
These two stereoisomers have different arrangements of the atoms in space which means they are different compounds with different physical properties and some different chemical properties.
A molecule with a carbon atom that is bonded to four different atoms or groups of atoms can form two optical isomers. Optical isomers are mirror images of each other and cannot be superimposed.
Optical isomers are said to be chiral and the two isomers arc called a pair of enantiomers. The central carbon bonded to the four different groups is called the chiral centre or the asymmetric carbon atom, and is often indicated on formulae by an asterisk (*).
You can easily identify a chiral molecule because it contains at least one carbon atom that has four different groups attached to it.
Optical activity of chiral molecules
Light is made up of electric and magnetic fields vibrating in all directions at mutual right angles to each other. Since it is vibrating in the components of light are vibrating in all directions, normal light is said to be unpolarised.
Passing unpolarised light through a polariser results in polarised light, which vibrates in one plane only. A pair of optical isomers will affect polarised light in different ways. They will rotate the plane of polarisation of the light by equal amounts but in opposite directions.
One of the optical isomers will rotate polarised light clockwise and the other anticlockwise.