Substitution and Elimination Chemistry Basics

Feb 23, 2025

Organic Chemistry Lecture: Substitution and Elimination Chemistry

Introduction

  • Focus on substitution chemistry basics
    • Not covering solvent effects, temperature, dielectric constants, etc.
  • Importance in biological systems.
  • Emphasis on primary, secondary, tertiary carbons
  • Key factors: stability of carbocations, strength of nucleophile

Substitution Reactions

SN2 Reactions

  • Definition: Bimolecular nucleophilic substitution
  • Mechanism: Nucleophile attacks carbon, pushes out leaving group (X)
    • Two things happen simultaneously
  • Characteristics:
    • No carbocations formed
    • Involves backside attack (Walden inversion)

SN1 Reactions

  • Definition: Unimolecular nucleophilic substitution
  • Mechanism: Leaving group departs, forming carbocation
    • Nucleophile attacks the carbocation
  • Characteristics:
    • Carbocations are formed
    • Occurs in two steps

Carbon Types and Pathways

  • Primary Carbons: Mainly undergo SN2
  • Secondary Carbons: Can undergo either SN1 or SN2 (depends on nucleophile strength)
  • Tertiary Carbons: Mainly undergo SN1
  • Zero-order Carbons: Rarely form carbocations

Nucleophile Strength

  • Strong nucleophile favors SN2
  • Weak nucleophile favors SN1
  • Stability influences nucleophilicity

Elimination Reactions

Introduction to Elimination Chemistry

  • Formation of carbon-carbon double bond
  • Base removes a proton from adjacent carbon

Types of Elimination

  • E1 Reaction
    • Similar to SN1, forms carbocation intermediate
    • Requires weak base
  • E2 Reaction
    • Proton removal facilitates leaving group departure
    • Requires strong base

Carbon Types and Elimination

  • Primary Carbons: Favor E2
  • Secondary Carbons: Can undergo E1 or E2
  • Tertiary Carbons: Mainly undergo E2, but E1 is possible

Base Strength

  • Weak Base: Favors E1 pathway
  • Strong Base: Favors E2 pathway

Anti-Periplanar Effect

  • Importance: Hydrogen has to be opposite the leaving group
  • E2 Mechanism: Requires anti-periplanar geometry

Combining Substitution and Elimination

Reaction Pathway Decisions

  • SN1 vs SN2 and E1 vs E2 depends on:
    • Type of carbon
    • Strength and type of nucleophile/base
    • Other factors like temperature

General Trends

  • Oxygen-based bases used in class
  • Primary: SN2/E2
  • Secondary: Based on nucleophile/base strength
  • Tertiary: SN1/E2

Temperature Effects

  • High temperatures favor elimination
  • Low temperatures favor substitution

Conclusion

  • Substitution and elimination often occur together; must determine predominant pathway
  • Review videos and notes for better understanding
  • Questions encouraged in discussion sections

These notes cover the essentials of substitution and elimination chemistry as discussed in the lecture. They highlight the main concepts, mechanisms, and conditions influencing the pathways of these reactions.

Full transcript