• Stereoisomerism is a form of isomerism where the molecules have the same molecular formula and the sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in space. This kind of isomerism can be divided into two types: geometric and optical.

Geometric Isomerism

  • Occurs in compounds which have a c=c double bond.
  • The restriction of rotation around the c=c bond leads to the existence of distinct configurational isomers.
  • The isomers are termed as cis(if similar groups are on the same side) and trans(if similar groups are on the opposite side).

Optical Isomerism

  • Optical isomers are molecules that differ by being mirror images of each other, much like left and right hands.
  • This property is due to a chiral centre, typically a carbon atom bonded to four different groups.
  • Optical isomers can rotate plane-polarised light. The isomer that rotates light in a clockwise direction is dextrorotatory (d or +), while the one that rotates it counter-clockwise is laevorotatory (l or -).
  • A solution with an equal amount of d and l isomers is called a racemic mixture, and it does not rotate plane-polarised light (it is optically inactive).
  • Optical isomerism is important in many biological systems. The two isomers can have very different properties: for instance, one isomer of a drug might have the desired effects, while the other may be harmful.

Testing for optical activity

  • A sample is placed in a polarimeter, a device which can measure the rotation of plane-polarised light.
  • A compound that is optically inactive will not rotate the plane of polarized light, while an optically active compound will.

Importance of Stereoisomers

  • Stereoisomers can have significantly different chemical and physical properties.
  • The understanding of their behaviour is crucial in many fields, especially in drug design and medicinal chemistry, as the two isomers can have different or even opposite effects in the body.