Aldol Condensation and Addition Reactions

Importance in Organic Chemistry

• Aldol condensations are fundamental reactions in organic chemistry.

Aldol Addition

• Example: Acetaldehyde with sodium hydroxide forms an "aldol" product.
  • Term Explanation: "Aldol" comes from "aldehyde" and "ol" (alcohol).
• Mechanism: Formation of Aldol
  • Starting Material: Acetaldehyde
  • Key Steps:
    • Identify alpha carbon (next to carbonyl).
    • Sodium hydroxide acts as a base, removing an alpha proton.
    • Formation of carbanion, which can act as a nucleophile.
    • Can attack another molecule of acetaldehyde, forming a new C-C bond.
    • Resultant alkoxide intermediate is protonated by water.
  • Result: Aldol product (carbon-carbon bond formation).

Aldol Condensation

• Reaction Conditions: Acetaldehyde with sodium hydroxide, water, and heat.
• Product: Enal (alpha-beta unsaturated aldehyde).
  • Characteristics: Double bond ("en") and aldehyde ("al").
• Mechanism: Formation of Enal
  • Start with aldol product.
  • Sodium hydroxide creates another enolate anion.
  • Formation of enal involves removing hydroxide as a leaving group, forming a double bond.
  • Major product is the trans product due to lower steric hindrance.

Example: Enone Formation

• Setup: Starting with a ketone, using sodium hydroxide, and heat.
• Formation of Enone
  • Ketone has symmetrical alpha carbons; proton removal forms enolate anion.
  • Enolate anion attacks another molecule (cyclohexanone).
  • New C-C bond forms, creating an alkoxide intermediate.
  • Protonation by water leads to aldol intermediate.
  • Hydroxide removes alpha proton, promoting formation of enone via elimination of hydroxide.
  • Product: Conjugated enone (alternating single-double bonds).
• Practice Tip: Understand various conditions to predict product formation.

Key Points for Study

• Understand the mechanisms for aldol reactions.
• Focus on the role of resonance structures (carbanion vs. oxyanion).
• Practice drawing mechanisms to solidify understanding.
• Analyze reaction conditions to predict product types.

This lecture emphasizes the significance of aldol reactions in forming carbon-carbon bonds and their variations under different conditions. Understanding these will aid in mastering organic chemistry reactions.