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Understanding Elimination Reactions

Dec 2, 2024

Lecture Notes: Alkyl Halide and Elimination Reactions

Introduction to Elimination Reactions

  • Types of Reactions: Similar to nucleophilic substitution reactions; types include E1 and E2 (similar to SN1 and SN2).
  • Mechanism: Involves removal of groups to form double bonds, with the use of a base instead of a nucleophile.
  • Historical Context: Quinine synthesis showcased elimination reactions. Quinine is a natural product with antimalarial properties.

General Mechanism of Elimination Reactions

  • Role of Base: Attacks hydrogen, leaving behind electrons, and a leaving group departs with electrons from the bond.
  • Formation of Double Bond: Base abstracts proton, and a new double bond is formed.
  • Example Compounds: Discussed potassium and sodium as metals forming strong bases.

Specific Examples

  • Base Action: Strong base like potassium or sodium attacks protons leading to double bond formation and leaving group departure.
  • Elimination in Lab: Used in aldol condensation to form double bonds through heat and removal of water as a leaving group.

Terminology

  • Alpha (α) and Beta (β) Carbons:
    • Alpha Carbon: The carbon attached to the leaving group (often a halogen).
    • Beta Carbon: Adjacent carbon where the base abstracts a hydrogen.
  • Identification of Bonds: Double bonds form between alpha and beta carbons.

Examples and Applications

  • 2-Bromone-2-methylpropane: Example of beta carbon variations.
  • Position Variability: Beta positions can vary based on carbon type (primary vs secondary).

Stability and Substitution

  • Substitution Types:
    • Mono-Substituted: One carbon linked to the double bond.
    • Di, Tri, Tetra-Substituted: More carbons linked, affecting stability.
  • Stability: Influenced by types of carbons linked (primary, secondary, etc.).

Cis-Trans Isomerism

  • Cis and Trans Definitions:
    • Cis: Same side of the double bond.
    • Trans: Opposite sides of the double bond.
  • Interconversion: Normally stable; can shift under heat (e.g., trans fats formation).

Learning Checks

  • Alpha and Beta Carbons: Identify and label in compounds.
  • Double Bonds and Stability: Classify based on mono, di, tri, tetra substitution.
  • Stereoisomers: Determine possibility in compounds.
  • Constitutional vs. Stereoisomers: Differentiate between isomers.

Environmental Implications

  • DDT Example: Shows environmental impact leading to thin eggshells in birds.
  • Degradation Reactions: Initial step involves elimination reaction forming DDE.

Conclusion

  • Study and Preparation: Emphasized reviewing key concepts and preparing for class discussions on problem-solving related to elimination reactions.

Ensure to review these concepts and practice identifying and classifying elimination reactions as discussed in this lecture.

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