Optical Isomerism (Oxford AQA International A Level Chemistry)

Optical Isomerism

• Stereoisomers are molecules that have the same structural formula but have the atoms arranged differently in space

• There are two types of stereoisomerism

  • E / Z

  • Optical

What is 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

  • Chiral comes from a Greek word meaning 'hand', so we talk about these molecules having a handedness

• A chiral carbon can also be described as an asymmetric carbon atom

• Compounds with a chiral centre (chiral molecules) exist as two optical isomers

Chiral centres in molecules

• The optical isomers are non-superimposable mirror images of each other just like your left and right hand

• These two mirror-image molecules can be known as:

  • Optical isomers

  • R and S (optical) isomers

  • Enantiomers

  • Racemate / racemic mixtures - if both isomers are present in equal quantities

• Two optical isomers have the same physical properties with the exception of their effect on plane polarised light

• One of the optical isomers rotates plane polarised light in a clockwise manner and the other in an anticlockwise fashion

• This property is used to distinguish between two different optical isomers

How optical isomers affect plane polarised light

Drawing optical isomers

• Optical isomers are drawn using dot and wedge bonds to give a 3D representation of the molecule

• Start by drawing:

  1. The chiral carbon

  2. Add two standard single bonds in the plane

  3. Add one wedge bond to show that an atom or group of atoms is coming out of the plane of the molecule

  4. Add one dashed bond to show that an atom or group of atoms is going behind the plane of the molecule

  5. Draw the mirror image of the structure

  6. Finally, add the four different atoms or groups of atoms

Racemic Mixtures

• A racemic mixture (or racemate) is a mixture containing equal amounts of each enantiomer

• Racemic mixtures are optically inactive

• Half of the enantiomers rotate polarised light clockwise and half rotate polarised light anti-clockwise

• Overall, this means they will cancel each other out and have no effect on the polarised light

Optical inactivity in racemic mixtures