Types of Organic Reactions Lecture

Introduction

• Acknowledgment to a teaching colleague.
• Focus: Understanding and explaining data, not just transmitting information.

SN1 and SN2 Reactions

Solvent Terminology

• Importance of polar solvents: necessary for dissolution in water and reaction occurrence.
• Protic vs. Aprotic solvents:
  • Protic solvents: Release protons, e.g., water, ethanol.
  • Aprotic solvents: Do not release protons, e.g., acetone, propanone.

SN2 Reactions

• Involves a direct attack on carbon, leading to the displacement of a leaving group.
• No steric hindrance from hydrogen atoms.
• Lone pair of electrons repels hydrogen atoms; the oxygen bond is shorter and stronger.
• Transition state: Momentary and not stable.
• Depends on concentrations of nucleophile and substrate.
• Requires polar aprotic solvents to avoid interaction with nucleophiles and forming unwanted species.

SN1 Reactions

• Involves formation of a carbocation intermediate.
• Carbocation stabilization by CH3 groups via positive induction.
• Protic solvents needed for reaction stabilization.
• Mechanism includes: Leaving group departure, carbocation formation, nucleophile attack.

Factors Affecting SN1 and SN2

• Periodic Table Trends:
  • Bond Strength: C-F > C-Cl > C-Br > C-I.
  • Rate of Reaction: Stronger bonds (like C-F) react slower; weaker bonds (lower in the table) react faster.
• Tertiary, Primary, Secondary carbon differences:
  • Tertiary: SN1.
  • Primary: SN2.
  • Secondary: Both SN1 and SN2.

Electrophilic Addition Reactions

• Important to know the mechanism.
• Involves heterolytic fission creating a carbocation and subsequent nucleophilic attack.
• Stabilization by side groups better than by hydrogens.
• Rule of thumb: Carbons with the most hydrogens attract more hydrogen in the reaction.

Electrophilic Substitution Reactions

• Sulfuric Acid vs. Nitric Acid:
  • Sulfuric Acid (KA = 1000) stronger than Nitric Acid (KA = 43).
  • Nitric Acid better at holding hydrogen due to oxygen ratios.
• Reaction mechanism:
  • Formation of nitronium ion as an electrophile.
  • Heating to destabilize electron structure.
  • Nitronium ion replaces hydrogen.

Reduction Reactions

• Remembering catalysts via the periodic table:
  • Lithium Aluminium Hydride (LAH): Used for primary alcohols.
  • Sodium Borohydride (NaBH4): Used for secondary alcohols.
• LAH is the stronger reducing agent, coming first in the periodic table.
• Mechanism understanding recommended but not always required in exams.

Tips for Memorization

• Certain reactions and mechanisms are essential to memorize for exams.
• Focus on key reaction mechanisms and conditions for high-yield studying.