Overview
This lecture introduces monosaccharide structures and explores stereoisomerism, focusing on concepts like chirality, chiral centers, enantiomers, diastereomers, and epimers.
Stereoisomerism Basics
- Stereoisomers have the same molecular formula and atomic connections but different three-dimensional shapes.
- Stereoisomers can only be converted into each other by breaking bonds.
- Cis and trans forms (such as cis-2-butene and trans-2-butene) are examples due to hindered bond rotation.
Chirality and Chiral Objects
- "Handedness" means an object and its mirror image are non-superimposable, like left and right hands.
- Superimposable images match at all points; non-superimposable do not.
- Chiral objects cannot be superimposed onto their mirror images.
- Common chiral objects include left and right shoes and handed golf clubs.
Chiral Centers in Molecules
- A chiral center is a carbon with four different groups in a tetrahedral arrangement.
- Glyceraldehyde is a simple chiral carbohydrate with a chiral center.
- A molecule with at least one chiral center is chiral and has handedness.
Guidelines for Identifying Chiral Centers
- Carbons with multiple bonds or two identical groups are not chiral centers.
- Carbons in rings may be chiral if their attached groups are different.
- For each carbon in a molecule, check if it is bonded to four different groups to determine chirality.
- The presence of even one chiral center makes the molecule chiral.
Types of Stereoisomers: Enantiomers, Diastereomers, and Epimers
- Enantiomers are stereoisomers that are non-superimposable mirror images with identical formulas but different arrangements.
- D (dextro) and L (levo) denote the handedness of enantiomers.
- The number of possible stereoisomers increases with the number of chiral centers (2ⁿ rule, where n = chiral centers).
- Diastereomers are stereoisomers that are not mirror images of each other.
- Epimers are a type of diastereomer differing only at one chiral center.
Key Terms & Definitions
- Stereoisomer — Molecules with the same formula and atomic connections but different 3D arrangements.
- Chiral Center — Carbon atom bonded to four different groups in a tetrahedral shape.
- Chiral Molecule — Molecule with at least one chiral center; not superimposable on its mirror image.
- Enantiomer — Pair of stereoisomers that are non-superimposable mirror images.
- Diastereomer — Stereoisomers not related as mirror images.
- Epimer — Diastereomers differing in configuration at only one chiral center.
Action Items / Next Steps
- Review molecular structures to practice identifying chiral centers.
- Memorize key definitions and the rule for calculating stereoisomer numbers.
- Prepare example molecules for identifying enantiomers, diastereomers, and epimers.