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Lecture on Isomerism and Geometrical Isomerism

Jul 8, 2024

Lecture on Isomerism and Geometrical Isomerism

Introduction

  • Welcome and introduction by RS Sir.
  • Today's class is divided into two parts due to the extensive time required.
  • Emphasis on the advanced level of questions to mirror exam standards.
  • Focus on isomerism, particularly geometrical isomerism and its types.

Isomerism

  • Definition: Structurally different compounds having the same molecular formula.
  • Example: Benzyl, benzyl, and benzo names used to explain isomerism.
  • Types: Structural and Stereoisomerism.

Types of Structural Isomerism

  1. Chain Isomerism: Different carbon chain structures.
  2. Position Isomerism: Different positions of functional groups or multiple bonds.
  3. Functional Isomerism: Different functional groups (e.g., aldehyde, ketone).
  4. Ring-Chain Isomerism: Difference between a ring and an open-chain structure.
  5. Metamerism: Different alkyl groups on either side of a polyvalent functional group.

Geometrical Isomerism (GI)

Definition and Conditions

  • Geometrical: Spatial arrangement around a double bond (restricted rotation) or cyclic structures.
  • Conditions: Restricted rotation system (RS), different groups/atoms around at least two terminals.
  • Types: Cis (same side) and trans (opposite side).

Examples and Explanations

  • Single Bonds vs. Double Bonds: Double bonds don't allow free rotation, leading to GI.
  • RS Consideration: Terminals (T1, T2 must have different groups/atoms).

Case Studies and Exercises

  • Various molecular structures discussed, analyzing RS for the presence of GI.
  • Special cases like spiro compounds, polyenes, and cumulated dienes.

Common Pitfalls and Misconceptions

  • Cylcopropane systems example (no GI due to rigid structure).
  • Errors in identifying cis/trans configurations due to not considering RS criteria.
  • Cycloalkenes GI: Valid only in rings of 8+ carbons due to ring strain affecting lower sizes.

Key Rules for Geometrical Isomerism

  1. Atomic Number Priority: Higher atomic number atoms/groups get priority (Z-configuration rules for complex cases).
  2. Multiplicity Consideration: Using dummy atoms for multiple bonds/rings in GI comparison.
  3. Symmetrical Compound Consideration: Unique rules for symmetrical compounds (cycloalkanes, specific polyenes).
  4. Stability Factors: Influence of thermodynamic stability in forming more stable geometrical isomers.

Special Focus on Polyenes and Spiro Compounds

  • Polyenes: Alternating double bonds and their influence on GI (planar vs. non-planar structures).
  • Spiro Compounds: Unique structures where two rings are connected through a single common atom.

Practice Questions and Discussion

  • Extensive practice questions embedded throughout the session for reinforcing concepts.
  • Students encouraged to solve and discuss GI configurations, especially in complex molecules (e.g., polyenes, spiro compounds).

Recap and Next Session

  • Summary of key points discussed in today's lecture: isomerism types, geometrical isomerism, conditions, and configuration rules.
  • Announcing the focus of the next lecture on optical isomerism and configuration.

Full transcript