Unit 1: Stereo Isomerism (Optical Isomerism)

Semester 4

BP401T

Introduction to Stereo Isomerism (Optical Isomerism)

Stereoisomerism arises when molecules have the same molecular formula and sequence of bonded atoms but differ in their 3D spatial arrangement. Optical isomerism is a crucial aspect of medicinal chemistry as drug receptors are highly stereoselective, often responding to only one specific enantiomer of a drug molecule. This unit covers the fundamental concepts of chirality, symmetry, nomenclature rules (R/S and D/L), and the methods used to separate enantiomeric mixtures.

Syllabus & Topics

1Optical activity, enantiomerism, diastereoisomerism, meso compounds.
2Elements of symmetry (plane, center, alternating axis).
3Chiral and achiral molecules.
4D/L system of nomenclature of optical isomers.
5Sequence rules and R/S system of nomenclature.
6Reactions of chiral molecules.
7Racemic modification and resolution of racemic mixtures.
8Asymmetric synthesis: partial and absolute.

Learning Objectives

Understand the fundamentals of optical isomerism and optical activity.

Differentiate between enantiomerism, diastereoisomerism, and meso compounds.

Identify elements of symmetry to determine molecular chirality.

Apply D/L and R/S systems of nomenclature effectively.

Understand racemic modifications, resolutions, and asymmetric synthesis.

Frequently Asked Questions (FAQs)

Q1. What is the Difference Between Enantiomers and Diastereomers?

Enantiomers are non-superimposable mirror images of each other and have identical physical properties except for the direction in which they rotate plane-polarized light. Diastereomers are stereoisomers that are not mirror images of each other and differ in physical and chemical properties such as melting point, boiling point, and solubility.

Q2. How Do You Assign R/S Configuration?

R/S configuration is assigned using the Cahn–Ingold–Prelog (CIP) priority rules. First, assign priorities (1 to 4) to the substituents attached to the chiral center based on atomic number. Then orient the molecule so that the lowest priority group (4) is pointing away from the observer. If the sequence 1 → 2 → 3 proceeds clockwise, the configuration is R (Rectus). If the sequence is counterclockwise, the configuration is S (Sinister).

Q3. What is a Meso Compound?

A meso compound is a molecule that contains two or more chiral centers but is overall achiral (optically inactive) because it has an internal plane of symmetry. This symmetry makes the molecule superimposable on its mirror image despite the presence of stereocenters.

Q4. What is Racemic Resolution?

Racemic resolution is the process of separating a racemic mixture, which contains equal amounts (50:50) of two enantiomers and is optically inactive, into its individual optically active enantiomers using chemical, mechanical, or biochemical methods.