Understanding Optical Activity and Chirality

Mar 6, 2025

Mindmap

5.4: Optical Activity

Introduction to Optical Activity

  • Enantiomers: Mirror images with identical properties (e.g., b.p., m.p., density) except in interaction with plane-polarized light.
  • Plane-polarized light: Light oscillating in a single plane, created by a polarizing filter.

Interaction with Chiral Molecules

  • Plane-polarized light is rotated when interacting with chiral molecules.
  • Optical Activity: The ability of a compound to rotate plane-polarized light, discovered by Jean-Baptiste Biot in 1815.
    • Chiral compounds: Optically active.
    • Achiral compounds: Optically inactive.
  • Enantiomers: Rotate light in opposite directions (clockwise or counterclockwise).
    • Dextrorotatory (d or +): Clockwise rotation.
    • Levorotatory (l or -): Counterclockwise rotation.
    • R/S Configuration: Unrelated to optical activity direction; it indicates spatial arrangement.

Measurement of Optical Rotation

  • Polarimeter: Instrument used to measure the angle and direction of light rotation.
  • Specific Rotation: Standardized measure to allow comparison between substances, defined for 1.0 g/mL concentration in a 1.0 dm tube at 20°C.
    • Formula: Specific rotation is calculated from observed rotation.

Optical Activity in Samples

  • Enantiomerically Pure: Sample with only one enantiomer.
  • Racemic Mixture: Equal mix of enantiomers, optically inactive with zero net rotation.
    • Enantiomeric Excess (ee): Measures the excess of one enantiomer over the other in a mix.

Chirality in Biological Properties

  • Biological Selectivity: Enantiomers can have different effects due to chiral environments, like enzymes and receptors.
  • Examples:
    • Limonene: Different enantiomers have distinct smells (orange vs. lemon).
    • Carvone: Different odors from (R)- and (S)-isomers.
    • Ibuprofen: Only (S)-enantiomer is active.

Historical Example: Thalidomide

  • Thalidomide tragedy highlighted the importance of understanding chirality in drugs.
    • R-enantiomer: Effective.
    • S-enantiomer: Teratogenic, caused birth defects.
    • Led to greater emphasis on testing enantiomeric effects in pharmaceuticals.

These notes cover the concepts of optical activity, measurement of optical rotation, implications of chirality in biological systems, and historical context emphasizing the importance of chirality in pharmaceuticals.