CHM210 Class 10/01

Enantiomers and Stereoisomers

• Enantiomer Definition: Does not apply in all cases; they are stereoisomers because they differ in the arrangement of atoms in space.

  • Key Point: The order of connectivity of atoms is the same.

• Diastereomers: New definition introduced.

  • Definition: Stereoisomers that are not related as mirror images.

  • Examples:

  • Pair 1 and Pair 3 are diastereomers; similarly, Pair 2 and Pair 3 are diastereomers.

  • They are stereoisomers, but not constitutional isomers.

Chiral Centers and RS Assignments

• Discussions regarding assigning priorities and RS designations to chirality centers in molecules.

  • Chirality Centers: Key concepts to identify and understand for assignments.

  • Ability to write relationships between pairs of molecules is critical for examinations.

• RS Designation:

  • For enantiomers: They have opposite configurations at all chirality centers.

  • Example:

    • Compound 1: 2R, 3R

    • Compound 2: 2S, 3S (both with opposite configurations at both centers)

  • For chiral diastereomers: Same configuration at some chirality centers and opposite configuration at others.

  • Example:

    • Compounds 1 and 3 share the same configuration at C2 (both R) but differ at C3 (one is R, the other is S).

Special Class of Diastereomers: Epimers

• Epimers:

  • Definition: A type of diastereomer where two diastereomers have the same configuration at all chirality centers except one.

  • Example: Discussion surrounding cholesterol, noting its multiple chirality centers and configuration similarities.

Chirality Centers in Molecules

• Investigation into specific molecules with chirality centers: Identification and demonstration of their configurations.

  • Chirality and Achiral Molecules:

  • Absence of a plane of symmetry in a molecule suggests that it is chiral, but the presence of such a symmetry guarantees it is achiral.

  • Practical exercises demonstrate distinguishing features of chirality using 3D models for clarity.

• Testing Chiral Properties:

  • Mirror images of molecules are examined for superimposability, establishing whether the tested molecules are chiral or achiral.

  • Example: Molecule 1 established as chiral through comparisons with mirror images.

Examination of Multiple Chirality Centers

• A deeper exploration of molecules with multiple chirality centers shows that:

  • Compounds with two chirality centers can either be chiral or achiral.

• Specific Cases: Identification of superimposable configurations in compounds demonstrating both chiral and achiral properties.

The Importance of Chirality in Biological Context

• Implications of chirality in real-world scenarios - particularly in pharmaceuticals where enantiomers might exhibit different biological effects.

  • Example: The discussion centered on lactic acid with respect to potential separation issues of non-superimposable enantiomers.

• FDA Historical Perspective:

  • Prior regulations allowed for approval of drug mixtures, focusing on safety data over separation requirements due to the inherent challenges.

  • Evolution of Standards: Recent trends reflect a shift in regulatory expectations towards the approval of single enantiomer drugs, yielding implications for pharmaceutical practices and methodologies.