29.4 Isomerism: Optical

2026 Syllabus Objectives

By the end of these notes, you should be able to:

  1. Understand that enantiomers have identical physical and chemical properties, except for their ability to rotate plane-polarised light and their potential biological activity.
  2. Understand and use the terms optically active and racemic mixture.
  3. Describe the effect on plane-polarised light of the two optical isomers of a single substance.
  4. Explain the relevance of chirality to the preparation of drug molecules, including:
    • (a) The potential different biological activity of the two enantiomers
    • (b) The need to separate a racemic mixture into two pure enantiomers
    • (c) The use of chiral catalysts to produce a single pure optical isomer

Main Content

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1. What Is Optical Isomerism?

You already know that isomers are molecules that share the same molecular formula but are arranged differently. Optical isomerism is a special type of isomerism where two molecules are mirror images of each other — just like your left and right hands. They look the same at first glance, but you cannot superimpose (lay one perfectly on top of the other) one onto the other.

These two mirror-image molecules are called enantiomers.

2. What Is a Chiral Centre?

For a molecule to have optical isomers, it must contain a chiral centre (also called a chiral carbon).

A chiral centre is a carbon atom that is bonded to four completely different atoms or groups of atoms.

Because all four groups attached to the carbon are different, the molecule can be arranged in two ways that are mirror images of each other — and these two arrangements cannot be made identical no matter how you rotate them.

Example: Consider the molecule CHFClBr (a carbon bonded to hydrogen, fluorine, chlorine, and bromine). All four groups are different, so this carbon is a chiral centre, and the molecule exists as two enantiomers.

Another example: Amino acids like alanine (CH₃CH(NH₂)COOH) contain a chiral centre — the central carbon is bonded to –CH₃, –NH₂, –COOH, and –H, all four of which are different.

Note: A molecule can contain more than one chiral centre, which can lead to more than two optical isomers. You do not need to know the details of these more complex cases for your exam, just be aware that multiple chiral centres can exist.

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