Chirality in Optical Isomers (Edexcel IAL Chemistry)

Spec point — 15.1

Chirality in 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

• You always obtain two optical isomers from one chiral centre
• Where a molecule has two chiral centres, then there are two pairs of optical isomers
• The isomers will only differ in two characteristics; how they interact with plane polarised light and how they react with other chiral molecules
• If a molecule has three chiral centres there will be eight stereoisomers
• This means the relationship is: number of isomers = 2ⁿ where n= the number of chiral centres

Exam Hint When drawing optical isomers, always draw mirror images including wedge and dashed bonds