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Enantiomers, Diastereomers, and Meso Compounds

These are types of stereoisomers, which are molecules with the same molecular formula and connectivity but different spatial arrangements of atoms. Here’s a detailed explanation of each:

1. Enantiomers

  • Definition:
  • Enantiomers are non-superimposable mirror images of each other. They are a type of stereoisomer that are chiral and optically active.
  • Properties:
  • They have identical physical properties (e.g., melting point, boiling point) except for their interaction with plane-polarized light.
  • They rotate plane-polarized light in equal but opposite directions:
    • One enantiomer rotates light to the right (dextrorotatory, +).
    • The other rotates light to the left (levorotatory, –).
  • They have identical chemical properties in non-chiral environments but may react differently in chiral environments (e.g., biological systems).
  • Example:
  • Lactic acid has two enantiomers:
    • L-lactic acid (found in muscles)
    • D-lactic acid (produced by bacteria)
    Structure:
       COOH             COOH
        |                |
   H - C - OH        HO - C - H
        |                |
       CH3              CH3

2. Diastereomers

  • Definition:
  • Diastereomers are stereoisomers that are not mirror images of each other. They arise when a molecule has two or more chiral centers and the configuration differs at one or more (but not all) of these centers.
  • Properties:
  • They have different physical properties (e.g., melting point, boiling point, solubility).
  • They may have different chemical reactivity.
  • They are not optically active as a pair, but each individual diastereomer may be optically active.
  • Example:
  • Tartaric acid has two chiral centers and three stereoisomers:
    1. L-tartaric acid (one enantiomer)
    2. D-tartaric acid (the other enantiomer)
    3. Meso-tartaric acid (a diastereomer that is achiral due to an internal plane of symmetry)
    Structure:
       COOH             COOH             COOH
        |                |                |
   HO - C - H        H - C - OH        HO - C - H
        |                |                |
   HO - C - H        H - C - OH        H - C - OH
        |                |                |
       COOH             COOH             COOH

3. Meso Compounds

  • Definition:
  • Meso compounds are achiral molecules that have multiple chiral centers but also possess an internal plane of symmetry. This symmetry makes them superimposable on their mirror image.
  • Properties:
  • They are optically inactive because the internal plane of symmetry cancels out the optical activity.
  • They have identical physical properties to their enantiomers and diastereomers.
  • Example:
  • Meso-tartaric acid is a meso compound because it has an internal plane of symmetry:
    • The two halves of the molecule are mirror images, making the molecule achiral.
    Structure:
       COOH
        |
   HO - C - H
        |
   H - C - OH
        |
       COOH

Comparison of Enantiomers, Diastereomers, and Meso Compounds:

FeatureEnantiomersDiastereomersMeso Compounds
Mirror ImageNon-superimposable mirror images.Not mirror images.Superimposable mirror images.
ChiralityChiral.Chiral or achiral.Achiral.
Optical ActivityOptically active (rotate light).May or may not be optically active.Optically inactive.
Physical PropertiesIdentical.Different.Identical.
ExampleLactic acid enantiomers.Tartaric acid diastereomers.Meso-tartaric acid.

Practice Problems:

  1. Identify Enantiomers and Diastereomers:
  • Consider the molecule 2,3-dichlorobutane:
    • Draw all possible stereoisomers and identify enantiomers and diastereomers.
    Answer:
  • There are four stereoisomers:
    1. (2R,3R)-2,3-dichlorobutane
    2. (2S,3S)-2,3-dichlorobutane
    3. (2R,3S)-2,3-dichlorobutane
    4. (2S,3R)-2,3-dichlorobutane
  • (2R,3R) and (2S,3S) are enantiomers.
  • (2R,3S) and (2S,3R) are enantiomers.
  • (2R,3R) and (2R,3S) are diastereomers.
  1. Identify Meso Compounds:
  • Consider the molecule 2,3-dibromobutane:
    • Draw all possible stereoisomers and identify any meso compounds.
    Answer:
  • There are three stereoisomers:
    1. (2R,3R)-2,3-dibromobutane
    2. (2S,3S)-2,3-dibromobutane
    3. (2R,3S)-2,3-dibromobutane (meso compound, due to internal symmetry).

Summary:

  • Enantiomers: Non-superimposable mirror images.
  • Diastereomers: Non-mirror image stereoisomers.
  • Meso Compounds: Achiral molecules with chiral centers and internal symmetry.

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