Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 15.3 Optical activity-Study Notes - New Syllabus

15.3 Optical Activity

Optical activity is a characteristic property of substances containing chiral molecules. A single optical isomer can rotate the plane of plane-polarised light, demonstrating the presence of chirality.

Definition of Optical Activity

Optical activity is the ability of a substance to rotate the plane of polarisation of plane-polarised monochromatic light.

Conditions for Optical Activity

• Molecule must contain a single chiral centre.
• Substance must contain only one enantiomer.
• Light used is:
  • Plane-polarised
  • Monochromatic (single wavelength)

Plane-Polarised Light

Normal light vibrates in many planes.

Plane-polarised light vibrates in: One plane only

Effect of Enantiomers

• One enantiomer rotates light clockwise.
• The other rotates light anticlockwise.
• Rotation occurs by the same angle but in opposite directions.

Terms Used

• Dextrorotatory (+): rotates light clockwise
• Laevorotatory (−): rotates light anticlockwise

Important Note

• Direction of rotation cannot be predicted from structure alone at this level.
• It must be determined experimentally.

Racemic Mixture

A 50:50 mixture of both enantiomers is called a:

Racemic mixture

It is optically inactive because:

• Equal and opposite rotations cancel each other.

Examples of Optically Active Compounds

• 2-butanol
• Lactic acid
• Amino acids (except glycine)

Key Features

• Optical activity results from chirality.
• Single enantiomers rotate plane-polarised light.
• Enantiomers rotate light equally but oppositely.
• Racemic mixtures are optically inactive.