Diastereomers

Definition

• Diastereomers: Compounds with the same molecular formula and sequence of bonded elements, but are nonsuperimposable and non-mirror images.
• Under the broader concept of stereochemistry, they are a type of stereoisomers not related as mirror images.

Characteristics

• Physical Properties: Diastereomers have different physical properties such as:
  • Melting points
  • Boiling points
  • Densities
  • Solubilities
  • Refractive indices
  • Dielectric constants
  • Specific rotations
• Optical Activity: Diastereomers other than geometrical isomers may or may not be optically active.
• Chemical Properties:
  • Show similar but not identical chemical properties.
  • Reaction rates may differ with a given reagent.
• Separation Techniques: Can be separated by fractional crystallization, fractional distillation, chromatography due to differences in physical properties, unlike enantiomers.

Erythro and Threo Diastereomers

• Definitions:
  • Erythro: Similar groups on the same side in Fischer projection.
  • Threo: Similar groups on opposite sides in Fischer projection.
• Example:
  • Hydroxylation of trans-crotonic acid yields threo enantiomers.
  • Hydroxylation of cis-crotonic acid yields erythro enantiomers.

Properties Comparison: Enantiomers vs. Diastereomers

• Melting/Boiling Points:
  • Enantiomers: Identical
  • Diastereomers: Different
• Solubility:
  • Enantiomers: Same
  • Diastereomers: Different
• Optical Rotation:
  • Enantiomers: Same, but opposite sign
  • Diastereomers: Different values
• Chemical Properties:
  • Enantiomers: Similar
  • Diastereomers: May differ
• Free Energy:
  • Enantiomers: Same
  • Diastereomers: Different

FAQs

1. Difference between enantiomer and diastereomer:

   • Enantiomers are non-superimposable mirror images (chiral).
   • Diastereomers have non-superimposable chiral centers, not mirror images.

2. Example of diastereomers:

   • Compounds with ring structures having non-mirror image isomers.

3. Optical activity:

   • Enantiomers are optically active.

4. Chirality of diastereomers:

   • Often chiral, with distinct chiral centers.

5. Types of stereoisomers:

   • Diastereomerism
   • Optical isomerism (enantiomerism/chirality)

6. D and L enantiomers:

   • Configurational stereochemistry based on the position of the hydroxyl group.

7. Identifying diastereomers:

   • Presence of two or more chiral centers and non-mirror image stereoisomers.