Introduction to Geometrical Isomerism & Conformational Isomerism
Geometrical isomerism occurs in compounds with restricted rotation, primarily around double bonds or cyclic structures. This unit explores how to name these isomers (Cis/Trans, E/Z, Syn/Anti) and how to determine their configuration. It also delves into Conformational Isomerism—the different 3D shapes formed by free rotation around single bonds in alkanes like ethane and butane, and cyclic systems like cyclohexane. Finally, it explores atropisomerism in biphenyls and the concepts of stereoselectivity.
Syllabus & Topics
- 1Nomenclature of geometrical isomers (Cis-Trans, E/Z, Syn-Anti systems).
- 2Methods of determination of configuration of geometrical isomers.
- 3Conformational isomerism in Ethane and n-Butane.
- 4Conformational isomerism in Cyclohexane (Chair, Boat forms).
- 5Stereoisomerism in biphenyl compounds (Atropisomerism) and conditions for optical activity.
- 6Stereospecific and stereoselective reactions.
Learning Objectives
Frequently Asked Questions (FAQs)
Q1. What is the Difference Between Cis/Trans and E/Z Nomenclature?
Cis/Trans nomenclature is used when there are two identical groups attached to the double-bonded carbons; Cis means the identical groups are on the same side, while Trans means they are on opposite sides. E/Z nomenclature is a more general system based on the Cahn–Ingold–Prelog (CIP) priority rules. If the two highest priority groups on each double-bond carbon are on the same side, the configuration is Z (Zusammen); if they are on opposite sides, it is E (Entgegen).
Q2. What is Atropisomerism?
Atropisomerism is a form of stereoisomerism that arises due to restricted rotation around a single bond, commonly seen in substituted biphenyl systems with bulky groups. When rotation is sufficiently hindered, the molecule becomes locked into stable conformations that can exhibit chirality.
Q3. Which Conformation of Cyclohexane is Most Stable and Why?
The chair conformation of cyclohexane is the most stable because its bond angles are close to 109.5°, minimizing angle strain, and all adjacent carbon–hydrogen bonds are staggered, minimizing torsional strain.
Q4. What’s the Difference Between Stereospecific and Stereoselective Reactions?
A stereospecific reaction is one in which different stereoisomeric reactants produce different stereoisomeric products, following a specific mechanistic pathway. A stereoselective reaction is one in which multiple stereoisomeric products are possible, but one is formed preferentially as the major product.