Formation of Enolate Anions from Ketones

Overview

• Focus on how to form enolate anions specifically from ketones.
• Example used: Acetone.

Identifying Alpha Carbons and Protons

• Alpha carbon: Carbon adjacent to the carbonyl carbon.
• Acetone has two alpha carbons, each with three alpha protons (total of six).

Deprotonation Process

• Strong base used: Lithium Diisopropyl Amide (LDA).
  • LDA: Strong, bulky, sterically hindered.
• Deprotonation results in a carbanion (conjugate base) with electrons on the carbon.
• Resonance structures formed:
  • Carbanion with negative charge on carbon.
  • Oxyanion with negative charge on oxygen.
  • Oxyanion contributes more to the hybrid due to oxygen's electronegativity.

Reaction Equilibrium

• Formation of amine as byproduct.
• Equilibrium lies to the right, favoring enolate anion formation.
  • Calculated using pKa values.
  • Acetone pKa ~19, Amine pKa ~36.
  • Equilibrium constant (Keq) is 10^17, indicating strong favoring of enolate anion.

Acidity Comparisons

• Acetone vs. Aldehydes:
  • Acetone is less acidic due to electron-donating methyl group, which destabilizes the negative charge.
• Beta-Diketone:
  • Much more acidic than acetone or aldehydes.
  • Central alpha carbon has the most acidic protons (pKa ~9).
  • Weaker base like sodium ethoxide can deprotonate.

Resonance and Stability

• Multiple resonance structures stabilize the enolate anion:
  • Delocalization of negative charge over carbon and oxygens.
  • Conjugation with alternating double and single bonds.
• Stability of conjugate base increases acidity, facilitating proton donation.

Overall Reaction Dynamics

• Beta-diketone deprotonation favors enolate anion formation.
• Ethanol formed as byproduct.
  • Equilibrium constant (Keq) for reaction favors enolate anion formation (Keq = 10^7).