Stereoisomers Explained: Enantiomers, Diastereomers, and Meso Compounds

Master stereoisomerism and optical activity. A clear guide to identifying enantiomers, diastereomers, and meso compounds with examples.

1. Optical Isomerism

Definition:

• Optical isomers are molecules that have a chiral center (a carbon attached to 4 different groups) and exist as non-superimposable mirror images.
• These isomers can rotate plane-polarized light, which is why they show optical activity.

Example:

• Lactic acid:CH3-CHOH-COOH
  • Central carbon (CHOH) has 4 different groups: CH3, H, OH, COOH
  • Two optical isomers:
    • D-(+)-Lactic acid → rotates light clockwise
    • L-(-)-Lactic acid → rotates light counterclockwise

Key Point:

• Optical isomers are non-superimposable mirror images of each other.

2. Optical Activity

Definition:

• When a chiral compound is exposed to plane-polarized light, it rotates the light.
• Clockwise rotation → dextrorotatory (+)
• Counterclockwise rotation → levorotatory (-)
• Measured using a polarimeter.

Example:

• D-(+)-Glucose → rotates light clockwise
• L-(-)-Glucose → rotates light counterclockwise

Important Note:

• Only chiral compounds are optically active.
• Achiral compounds → optically inactive

3. Enantiomerism

Definition:

• Enantiomers are pairs of optical isomers that are mirror images and non-superimposable.
• They have identical physical properties (mp, bp, solubility) but opposite optical rotations.

Example:

• 2-Butanol:CH3-CH(OH)-CH2-CH3
  • Enantiomers: (R)-2-Butanol and (S)-2-Butanol
  • Mirror images but not superimposable

Key Point:

• Enantiomers have opposite configurations at the chiral center.
• They rotate plane-polarized light in equal magnitude but opposite direction.

4. Diastereoisomerism

Definition:

• Diastereomers are stereoisomers that are not mirror images.
• They have two or more chiral centers.
• Their physical properties differ (mp, bp, solubility, reactivity).

Example:

• Tartaric acid:HOOC-CHOH-CHOH-COOH
  • Two chiral centers
  • (R,R) & (S,S) → enantiomers
  • (R,S) → diastereomer (also a meso compound, optically inactive)

Key Points:

• Diastereomers ≠ mirror images
• Physical and sometimes chemical properties differ

5. Meso Compounds

Definition:

• Molecules with two or more chiral centers but internally symmetric, making them achiral overall, are called meso compounds.
• They are optically inactive despite having chiral centers.

Example:

• Tartaric acid:(R,S)-Tartaric acid
  • Two chiral centers
  • Symmetric molecule
  • Optical activity = 0

Key Points:

• Meso compounds = chiral centers + symmetry → achiral overall
• Physical properties differ from their diastereomers

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