Stereoisomerism and Optical Activity

Introduction to Stereoisomerism and Optical Activity

• Lecture focuses on stereoisomerism, especially optical activity, crucial in pharmaceutical organic chemistry.
• Stereoisomerism involves compounds sharing a molecular formula but differing in their atoms' spatial arrangements.
• Recommends watching lectures in sequence for clear understanding.
• The lecture will cover geometric and optical isomerism within the current unit.

Understanding Stereoisomers

• Stereoisomers share the same molecular and structural formula.
• Stereoisomers differ in their arrangement of atoms in space, such as group orientations on a carbon atom.
• Rotation around single bonds yields different spatial arrangements, altering the compound's appearance.

Optical Isomerism: Definition and Key Differences

• Optical isomerism, a stereoisomerism subset, features isomers that have identical molecular and structural formulas.
• Physical and chemical properties are generally the same for optical isomers, with exceptions for optically active compounds.
• Their behavior towards light, especially plane-polarized light, is the key differentiating factor.

Ordinary Light vs. Plane-Polarized Light

• Ordinary light's waves oscillate in every direction.
• A polarizer, like a Nicol prism made of caco3, transforms ordinary light into plane-polarized light by aligning light waves in one plane.
• The Nicol prism concentrates and emits light as a thin layer of plane-polarized light.

Polarimeter and Optical Activity Detection

• A polarimeter assesses a substance's optical activity.
• A compound solution is placed inside the polarimeter.
• Plane-polarized light is directed through the solution.
• The light exiting the solution will be unaffected (straight), rotated clockwise, or rotated counterclockwise.
• These outcomes determine a compound's optical activity.

Interpretation of Results: Optically Active vs. Inactive Compounds

• If light passes through unchanged, the compound is optically inactive, meaning it doesn't rotate plane-polarized light.
• Light rotation indicates an optically active compound.
• Clockwise rotation is "dextrorotatory," denoted by a plus (+) sign.
• Counterclockwise rotation is "levorotatory," denoted by a minus (-) sign.
• Notes on this will be provided.

Summary of Optical Activity

• If a compound rotates plane-polarized light, it is optically active.
• Dextrorotatory describes rightward (clockwise) rotation.
• Levorotatory describes leftward (counterclockwise) rotation.
• No rotation suggests an optically inactive compound.