Organic Chemistry Mechanisms Lecture Notes

Introduction

• Focus on four basic types of organic chemistry mechanisms.
• Emphasis on reaction arrow patterns.
• Study mechanisms to understand every step of a reaction.
• Key question: How do molecules interact to form products?

Types of Mechanism Patterns

1. Nucleophilic Attack
2. Loss of Leaving Group
3. Proton Transfer
4. Rearrangement

Reaction Arrows

• Represent electron movement, not protons or other particles.
• Double Headed Arrow: Indicates movement of a pair of electrons (common in >90% of reactions).
• Single Headed Arrow (Fish Hook): Indicates movement of a single electron (used in radical reactions).
• Arrow Tail: Origin of electrons.
• Arrow Head: Destination of electrons.

Nucleophilic Attack

• Nucleophile: Electron-rich species seeking positive charges (nucleus-loving).
  • Represented as NU with two electrons.
  • Example: Iodide ions attacking carbocations.
• Electrophile: Electron-poor species (electron-loving).
  • Represented as E+.
• Example Pattern: Iodide (nucleophile) attacks carbocation (electrophile), forming a new bond.
• Carbonyl Example:
  • Carbonyl carbon acts as electrophile due to resonance.
  • Resonance shifts pi electrons towards oxygen, leading to partial positive charge on carbon.
  • Methyl Grignard reacts with carbonyl carbon (nucleophilic attack).

Loss of a Leaving Group

• Definition: Leaving group detaches from the molecule.
• Example: Chlorine in 2-chloropropane pulls on its bond until it breaks, forming chloride ion with lone pairs.
• Charges:
  • Neutral leaving group results in a -1 charge.
  • Positive leaving group results in charge reduction by one unit.
• Advanced Reaction Example: Nucleophilic substitution on aromatic rings with resonance effects.
  • Domino electron movement leads to ejection of leaving group.

Summary

• Understanding electron flow is key to mechanisms.
• Next video will cover proton transfer and rearrangement.
• Series available on Leah4Sci website.