Detailed Notes on Stereochemistry and Chirality

Stereochemistry Overview

• Stereochemistry: Study of the spatial arrangement of atoms within molecules.
• Key to understanding reactions like SN1 (substitution nucleophilic unimolecular) and SN2 (substitution nucleophilic bimolecular).
• Chirality: Important concept in biological chemistry. A molecule is said to be chiral if it cannot be superimposed on its mirror image. Key for biomolecular interactions.

Chirality and Stereogenic Centers

• Chiral Molecules: Cannot be superimposed on their mirror images (e.g., amino acids).
• Achiral Molecules: Superimposable on their mirror images.
• A stereogenic center is a carbon atom bonded to four different substituents, leading to chirality.

Importance of Chirality

• Many crucial biomolecules exist as single enantiomers (e.g., L-alanine is the biologically active form).
• Enantiomers display identical physical properties (melting point, boiling point) but differ in how they interact with chiral environments.
• Can rotate plane-polarized light, denoted as (+) for dextrorotatory and (-) for laevorotatory.
• Historically, D- (right) and L- (left) nomenclature has been used in specific biological contexts.

SN1 vs. SN2 Reactions

SN1 Reactions:

• Involves formation of a planar carbocation intermediate.
• Can be attacked from either side, yielding a racemic mixture (50:50 of both enantiomers).

SN2 Reactions:

• Involves direct attack of a nucleophile at 180° to the leaving group.
• Causes inversion of stereochemistry at the reacting center.

Cahn-Ingold-Prelog (CIP) Priority Rules

1. Assign priority based on atomic number of atoms directly attached to the stereocenter.
2. If tied, promote to the next set of atoms in the substituents.
3. Orient the molecule so that the group with the lowest priority is directed away.
4. Trace the path from highest to lowest priority: clockwise = (R), counterclockwise = (S).

Biological Significance of Chirality and Examples

• Enantiomers can have very different biological activities:
  • Epinephrine: (+)-Epinephrine is biologically active; (-)-Epinephrine is less active.
• Thalidomide: Example of disastrous effects of chirality in drugs: one enantiomer is a sedative while the other is a teratogen. Significant birth defects were caused when the teratogenic form was used during pregnancy.

Summary and Applications

• Understanding chirality is crucial in fields like drug design, as the biological activity of enantiomers can differ dramatically.
• Mastery of CIP rules assists in accurately determining and communicating the stereochemistry of molecules.

Interactive Learning

• For further practice with stereochemistry and CIP rules, see R/S Chemistry at rschemistry.com (free for non-commercial use).
• Engage with exercises and resources to solidify understanding of stereochemical assignments.