Optical Isomerism and Chirality: Key Concepts and Definitions

What is optical isomerism?

Optical isomerism is a type of stereoisomerism where molecules have the same structural formula but different arrangements in 3D space that affect plane-polarised light.

What causes optical isomerism?

A chiral (asymmetric) carbon atom.

What is a chiral carbon atom?

A carbon atom bonded to four different atoms or groups.

What are optical isomers also called?

Enantiomers.

What are enantiomers?

2 Molecules that are non-superimposable mirror images of each other.

What does "non-superimposable mirror images" mean?

They look like mirror images but cannot be exactly placed on top of each other so that all atoms match.

How do enantiomers differ in their physical properties?

They have identical physical properties except for their effect on plane-polarised light.

What does optically active mean?

A sample which rotates the plane of plane polarised light.

How do enantiomers affect plane-polarised light?

One rotates it clockwise (dextrorotatory), the other rotates it anticlockwise (laevorotatory).

What is a racemic mixture?

A mixture containing equal amounts of both enantiomers.

Why is a racemic mixture optically inactive?

Because the rotations of plane-polarised light cancel each other out.

How is a racemic mixture formed?

When a reaction produces both enantiomers in equal amounts.

How do you identify a chiral centre in a molecule?

Look for a carbon atom bonded to four different groups.

Can a molecule with a plane of symmetry be chiral?

No, it is not chiral if it has a plane of symmetry.

What is plane-polarised light?

Light that vibrates in only one plane.

Why does optical activity occur?

Because enantiomers interact differently with plane-polarised light due to their 3D structure.