Overview
This lecture explains stereoisomers, distinguishing between diastereomers and enantiomers, and describes their properties and identification methods.
Stereoisomers Overview
- Stereoisomers have the same molecular formula and atom connectivity but different spatial arrangements.
- The two main types of stereoisomers are diastereomers and enantiomers.
Diastereomers
- Diastereomers are stereoisomers that are not mirror images of each other.
- Example: cis/trans isomers (like 1,2-dichloromethane) are diastereomers.
- They have distinct spatial configurations that cannot be arranged as mirror images.
- Diastereomers have different physical and chemical properties.
Enantiomers
- Enantiomers are stereoisomers that are non-superimposable mirror images of each other.
- Each molecule in an enantiomeric pair is called an enantiomorph.
- Enantiomers must have at least one chiral center—a central atom (usually carbon) where the arrangement of groups differs in the mirror image.
- Enantiomers have identical physical and chemical properties except for how they rotate plane-polarized light (optical activity); they rotate light in opposite directions.
Identifying Chiral Centers and Enantiomers
- Chiral centers are atoms, usually carbon, bonded to four different groups.
- If two molecules are mirror images and not superimposable, they are enantiomers.
- Example: Lactic acid (C₃H₆O₃) has two forms that are mirror images and classified as enantiomers; carbon 2 is the chiral center.
Stereoisomer Summary
- Both diastereomers and enantiomers have the same formula and bonding but differ in spatial arrangement.
- Diastereomers: not mirror images; different properties.
- Enantiomers: mirror images; same properties except optical activity.
Key Terms & Definitions
- Stereoisomer — Compounds with the same formula and atom connectivity but different spatial arrangements.
- Diastereomer — Stereoisomers that are not mirror images.
- Enantiomer — Stereoisomers that are non-superimposable mirror images.
- Chiral Center — An atom with four different groups attached, leading to non-superimposable mirror images.
- Enantiomorph — A molecule that is one of a pair of enantiomers.
Action Items / Next Steps
- Review examples of stereoisomers and practice identifying diastereomers vs. enantiomers.
- Identify chiral centers in provided molecules.