Edexcel International A Level Chemistry

Revision Notes

4.6.1 Chirality in Optical Isomers

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

An Introduction to AS Level Organic Chemistry Enantiomers and Chiral Centre, downloadable AS & A Level Chemistry revision notes

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 = 2 where n= the number of chiral centres

Exam Tip

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

20-3-3-drawing-optical-isomers-exam-tip  

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