Formation of Enolate Anions

Introduction

• Discussion on enolate anion formation in organic chemistry.
• Focus on which base to use for the formation of enolate anions.

Ethoxide as a Base

• Base: Ethoxide anion (OEt⁻) from sodium ethoxide (Na⁺, OEt⁻).
• Reactant: Acetaldehyde (an aldehyde), focus on the alpha carbon (carbon next to the carbonyl carbon).
  • Alpha Carbon: Has three alpha protons.

Mechanism

• Ethoxide acts as a base by taking a proton from the alpha carbon of acetaldehyde.
• Formation of enolate anion:
  • Carb Anion Form: Electrons on carbon form a carb anion.
  • Resonance Structure: Electrons move, creating a double bond and an oxyanion (negative charge on oxygen).

Equilibrium Considerations

• Equilibrium: Exists between aldehyde and enolate anion.
• pKa Values:
  • Acetaldehyde: pKa ~17
  • Ethanol (formed by protonating ethoxide): pKa ~16
  • Calculation: pKaEq = 17 - 16 = 1
  • Keq = 10^(-pKaEq) = 0.1 (favors reactants).
• Acid Strength:
  • Ethanol is more acidic than acetaldehyde.
  • Equilibrium favors formation of the weaker acid, acetaldehyde.

Complete Formation of Enolate Anion

• Alternative Base: Hydride anion (from sodium hydride or potassium hydride).

Mechanism

• Hydride anion takes a proton from alpha carbon of acetaldehyde, similar to ethoxide.
• Results in formation of hydrogen gas (H₂), which bubbles out, driving reaction to completion.

Summary

• Ethoxide Base: Results in equilibrium between aldehyde and enolate anion.
• Hydride Base: Drives reaction to completion, forming enolate anion.
• Upcoming Discussion: Next video will cover LDA, another base for complete enolate anion formation.