Optical Isomerism

Optical Isomerism is a type of stereoisomerism where compounds differ in how they rotate plane-polarized light due to chiral centers.

Definition of Optical Isomerism:

• Optical isomerism occurs when molecules can exist in two (or more) non-superimposable forms (stereoisomers) that differ in the way they interact with plane-polarized light.
• If a molecule can rotate the plane of polarized light, it is said to be “optically active.”

How Optical Activity is Measured

• Plane-Polarized Light: Ordinary light consists of waves vibrating in all planes. A polarizer (like a Nicol prism) filters out all but one plane of vibration, creating plane-polarized light.
• Polarimeter: An instrument used to measure how much a substance rotates plane-polarized light.
  • If the substance rotates the plane to the right (clockwise), it is called “dextrorotatory” (labeled as “+” or “d-”).
  • If it rotates the plane to the left (counterclockwise), it is called “levorotatory” (labeled as “−” or “l-”).

Types of Optical Isomers

1. Enantiomers

   • Non-superimposable mirror images
   • Identical physical properties (except for optical rotation)
   • Rotate plane-polarized light equally but in opposite directions
   • Represented as:
     • (+)-enantiomer (dextrorotatory)
     • (−)-enantiomer (levorotatory)

2. Diastereomers

   • Stereoisomers that are not mirror images
   • Occur when there are two or more chiral centers
   • Physical and chemical properties are different
   • Example: In a molecule with 2 chiral centers, (R,R) and (R,S) are diastereomers

3. Meso Compounds

   • Contain chiral centers
   • But are achiral overall due to an internal plane of symmetry
   • Are optically inactive
   • Superimposable on their mirror images
   • Example: Meso-tartaric acid

Racemic Mixtures

• A 1:1 mixture of two enantiomers
• Optical activities cancel each other out
• The mixture is optically inactive
• Symbolized as: (±)-compound

Number of Optical Isomers

• If a compound has n chiral centers, the maximum number of stereoisomers is:
• 2ⁿ, where:
• Half are enantiomers
• The rest may be diastereomers
• Subtract meso forms (if any) to find optically active forms

Example of Optical Isomerism:

• A molecule with 2 chiral centers → up to 4 isomers
  • (R,R), (S,S), (R,S), (S,R)
  • If (R,S) and (S,R) are meso, then only 3 unique stereoisomers exist

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