Everything You Need to Know About Mechanisms

Introduction to Organic Reaction Mechanisms

• Understanding organic reaction mechanisms is crucial for success in organic chemistry.
• Arrow Pushing Technique: Used to depict electron flow during chemical reactions.
  • Helps track electron redistribution as bonds form and break.

Rules of Arrow Pushing

First Rule: Movement of Electrons

• Regular arrow (double-sided arrowhead) for movement of two electrons.
• Single-sided arrowhead (fish hook) for single electron movement (radical reactions).
• Most reactions involve pairs of electrons.

Second Rule: Not for Atom Movement

• Arrows indicate electron movement, not atom movement.
• Atom movement is implied as a result of electron movement during bond formation/breaking.

Third Rule: Electron Sources and Sinks

• Arrows start at an electron source (bond or lone pair) and end at an electron sink (atom capable of accepting electrons).
• Identifying sources and sinks in functional groups is key.

Fourth Rule: Avoid Hypervalence

• Bond breaking may occur to prevent overfilling valence at electron sink atoms.
• Electron source could be a bond breaking while a sink is an atom capable of lone pair accommodation.

Important Concepts

• Nucleophiles and Electrophiles:
  • Nucleophiles: Electron-rich molecules (akin to Lewis bases).
  • Electrophiles: Electron-poor molecules (akin to Lewis acids).

Common Mistakes in Arrow Pushing

Backwards Arrows

• Mistake: Reversing arrow direction, indicating atom instead of electron movement.
• Solution: Ensure arrows depict electron flow.

Not Enough Arrows

• Mistake: Missing arrows lead to incomplete electron depiction.
• Solution: Analyze electron positions post-reaction.

Hypervalency

• Mistake: Atoms depicted with excess electrons (beyond valence capacity).
• Solution: Adhere to Octet Rule; ensure correct depiction of bonds/lone pairs.

Mixed Media Errors

• Do not show strong acid/base creation in opposite media conditions.
  • Acidic conditions: Intermediates/products neutral or positive.
  • Basic conditions: Intermediates/products neutral or negative.

Charge Conservation

• Overall charge must be conserved; errors often arise from missing arrows or hypervalency.

Mechanism Prediction

• Understand overall transformation first (e.g., addition, elimination).
• Recognize individual steps within the mechanism.
• Make use of common mechanism patterns:
  1. Form new bonds between nucleophiles and electrophiles.
  2. Break bonds to form stable ions/molecules.
  3. Add or remove protons as necessary.
• Mastery involves learning mechanism elements and how to apply them.

These notes cover the foundational rules and concepts for understanding and predicting organic reaction mechanisms with a focus on the correct use of arrow pushing.