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Lecture on Isomerism

Jul 8, 2024

Lecture Notes on Isomerism

Introduction and Basic Concepts

  • Isomerism: The phenomenon of isomerism occurs when compounds have the same molecular formula but different arrangements of atoms or bonds.
  • Types of Isomerism: Structural and Geometrical.
  • Key Concept: Connectivity and arrangement play a crucial role in determining the type of isomerism.
  • Examples: Provided analogies like friends moving to different cities, wife arranging furniture differently to explain the concept of connectivity and arrangement change.

Structural Isomers

  • Types of Structural Isomers: Functional group, Chain, Position, Ring-Chain.
    • Functional group isomers: Compounds with the same molecular formula but different functional groups.
    • Chain isomers: Compounds with differing carbon chain arrangements.
    • Position isomers: Different positions of functional groups or multiple bonds.
    • Ring-Chain isomers: Ring and open-chain forms of the same compound.

Geometrical Isomers (Cis-Trans)

  • Found in systems with restricted rotation, commonly double bonds, rings, spiro compounds.
  • Criteria for Geometrical Isomers: At least one restricted rotation system (RS) and different groups attached to the carbons in the system.
  • Distinguishing Cis and Trans: Based on arrangement of atoms/groups on either side of the double bond or ring system.

Important Rules and Concepts

1. Atomic Number

  • Priority rule: Higher atomic number gets higher priority.
  • Example: Iodine > Bromine > Chlorine > Sulfur > Phosphorus > Carbon > Hydrogen

2. Atomic Mass

  • Used if atomic numbers are the same.
  • Example: Cl-35 vs. Cl-37

3. Next Atom Check

  • If direct atoms are the same, check atoms attached to them.
  • Example: Compare carbon chains or other attached groups.

4. Handling Multiple Bonds and Rings

  • Dummy Atoms: Used to simplify priority determination.
  • Example Setup: Carbon is attached to an oxygen, dummy atoms placed accordingly in both directions.

Symmetrical Compounds

  • Check for symmetry when naming geometrical isomers.
  • Use: Such as giving priority to symmetrical compounds when determining cis/trans isomers.

E-Z Naming System

  • E-Z system: Used based on the priority of groups attached to double bonds or rings.
  • E: Opposite sides, Z: Same sides.
  • Not Applicable: When groups attached are the same.

Example Challenges

  • Types of questions, approaches to determining geometrical or other types of isomerism.

Special Cases

Polyphenylene Systems

  • Analysis of systems with multiple rings and their geometrical isomers.
  • Example: Spirane systems with four and multiple rings.

Cycloalkenes

  • GII shown only if a cycloalkene has 8 or more carbons in the ring.
  • Stability Check: Eight to eleven carbon rings show GII more stably.

Quiz and Practice Questions

  • Extensive list of practice questions related to isomerism identification, geometrical isomers, and priority rules application.

Conclusion

  • Review: Key concepts on structural and geometrical isomerism, their identification and practical application of rules.
  • Next Class: Opticals and conformations to be discussed.

End of Class Note: Practice and understanding of isomerism are essential for mastering organic chemistry, with an upcoming focus on optical isomers and conformations.

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