Optical Isomerism (AQA A Level Chemistry)

Optical Isomerism

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

  • Geometrical (E/Z)

  • Optical

• 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

• 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 just like your left and right hand

• 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

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 that one of the enantiomers 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

• These enantiomers are therefore said to be optically active

• 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