Lecture Notes: Class 3 Carbonyl Chemistry

Introduction to Class 3 Carbonyls

• Class 1 and Class 2 Carbonyls:
  • Class 1: Have good leaving groups.
  • Class 2: No good leaving groups.
• Class 3 Carbonyls:
  • Not a separate class but can be present in Class 1 and 2.
  • Focuses on the alpha carbon attached to the carbonyl group.

Alpha Carbon and Acidity

• Alpha Carbon:
  • The carbon directly attached to the carbonyl group.
  • Hydrogens on alpha carbons are acidic.
  • Removal of protons by a base creates a carbanion stabilized by resonance with the carbonyl.

Enolates

• Enolate Formation:
  • Enolates are formed when a base removes an alpha proton.
  • The nucleophilic form has a negative charge on carbon.
• Base Types Used:
  • Common bases: Hydroxide, sodium hydride, sodium amide, LDA (Lithium Diisopropylamide).

Enolate Reactions

• Nucleophilic Attack:
  • Enolates can attack carbon atoms in SN2-like reactions to form new carbon-carbon bonds.

Aldol Reactions

• Aldol Chemistry:
  • Involves aldehydes or ketones.
  • Similar chemistry for aldehydes and ketones due to identical mechanisms.
  • Aldol addition results in beta-hydroxy carbonyl compounds.
• Aldol Addition vs. Aldol Condensations:
  • Focus on aldol additions for this lecture.

Class 1 Carbonyl Reaction (Claisen-like Reactions)

• Esters and Aldol-like Reactions:
  • Esters can undergo aldol-like reactions, forming beta-keto esters.
  • Important for decarboxylation processes.

Reaction Conditions and Mechanisms

• Controlling Reactions:
  • Use of half-equivalents of base to control aldol reactions with identical aldehydes or ketones.

Decarboxylation Process

• Decarboxylation Explained:
  • Conversion of esters to carboxylic acids using H+ and water.
  • Heating facilitates decarboxylation, releasing CO2 gas.
  • Requires a beta-carbonyl for resonance and cleavage of carbon-carbon bonds.

Biological Relevance

• Significance in Biology:
  • Decarboxylation contributes to the Krebs cycle in cellular respiration,
  • Involves formation of electron carriers and ATP.

Conclusion

• Study Recommendations:
  • Review video lectures and notes.
  • Reach out for clarification in discussion sections or office hours.