Optical Isomerism
Quick Notes
- Optical isomerism occurs in molecules with a chiral centre.
- A chiral centre is a carbon atom bonded to four different or unique groups.
- Chiral molecules exist as two non-superimposable mirror images called enantiomers.
- Enantiomers rotate plane-polarised light in opposite directions.
- A racemic mixture contains equal amounts of both enantiomers and does not rotate plane-polarised light.
Full Notes
Optical isomerism and rotation of plane polarised light has been covered in more detail here
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What Is Optical Isomerism?
Optical Isomerism is a form of stereoisomerism where molecules have the same structural formula but their atoms are arranged differently in space.
If a carbon atom is bonded to four different atoms or groups it is called a chiral carbon (giving a chiral centre in a molecule)
There are two possible ways the atoms or groups can be arranged giving two possible stereoisomers. The isomers are non-superimposable mirror images of one another and are called enantiomers.
Example 2-hydroxypropanoic acid (Lactic Acid)
The two enantiomers exist as mirror images.
Properties of Enantiomers
Enantiomers are physically identical except for:
- Rotation of plane-polarised light:
One enantiomer rotates light clockwise (+).
The other rotates light anticlockwise (−). - Interaction with other chiral molecules (e.g., in biological systems).
Racemic Mixtures
A racemic mixture contains equal amounts of both enantiomers.
Racemic mixtures do not rotate plane-polarised light because the rotations from each enantiomer cancel out.
They are formed in reactions that have equal probabilities of both enantiomers being produced.
For example, when a nucleophile attacks a C=O bond, there is an equal probability of the nucleophile attacking from above or below the plane of the bond - giving two possible configurations of the product (each an enantiomer).
Importance of Optical Isomerism in Pharmaceuticals
Many drugs are chiral and only one enantiomer is effective meaning chiral synthesis or separation is essential in drug development.
The active sites of many enzymes are stereospecific, meaning only one enantiomer can bind effectively. See Proteins and Enzymes.
Summary
- Optical isomerism arises from a chiral centre: a carbon bonded to four different groups.
- Such molecules exist as enantiomers, which are non-superimposable mirror images.
- Enantiomers rotate plane-polarised light in opposite directions; racemic mixtures show no overall rotation.
- Equal attack above/below a planar intermediate (e.g., C=O) can produce racemic mixtures.
- Chirality is critical in pharmaceuticals because different enantiomers can have different biological effects.