Carbanions and Aldol Condensation

Carbanion

• A carbanion is a negatively charged ion with a carbon atom that has three bonds and a formal negative charge of at least -1.
• Carbanions are the conjugate bases of carbon acids.
• Carbanions can have bent, linear, or trigonal pyramidal molecular geometries.

Condensation Reaction of Carbanions

• Aldol Condensation: An aldol condensation occurs when an aldehyde with an α-hydrogen reacts with a dilute base to form a β-hydroxy aldehyde, also known as an aldol.
• Crossed Aldol Condensation: A crossed aldol condensation occurs when two different carbonyl compounds react.
• Aldol Condensation Mechanism:
  • Step I: An enolate ion is formed by removing a proton from the α-carbon of one molecule of the aldehyde using a base.
  • Step II: The enolate ion attacks the carbonyl carbon of the second molecule of the aldehyde in a nucleophilic addition reaction, forming an alkoxide intermediate.
  • Step III: The alkoxide ion accepts a proton from water to form a β-hydroxy aldehyde (aldol).
  • Step IV: The aldol product can undergo dehydration to produce α, β-unsaturated aldehydes or ketones.

Favorskii Rearrangement

• The Favorskii rearrangement involves the rearrangement of cyclopropanones or α-halo ketones to form carboxylic acid derivatives.
• This reaction is base-catalyzed and can form acids, esters, or amides depending on the base used.
• Mechanism:
  • Step I: The base removes a proton from the carbon adjacent to the carbonyl group.
  • Step II: The enolate ion attacks the carbonyl carbon, forming a cyclic intermediate.
  • Step III: The cyclic intermediate opens up to form a carboxylic acid derivative.

Wittig Reaction

• The Wittig reaction involves the reaction of an aldehyde or ketone with a triphenylphosphonium ylide (Wittig reagent) to form an alkene and triphenylphosphine oxide.
•  This reaction is widely used in organic synthesis to prepare alkenes.
• Preparation of Wittig Reagents:
  • Step I: Triphenylphosphine reacts with an alkyl halide via an SN2 reaction to form phosphonium salt.
  • Step II: The phosphonium salt is deprotonated with a strong base such as n-butyllithium to form the ylide.

Stability of Carbanions

• Induction: Highly electronegative substituents stabilize carbanions by drawing electrons towards themselves. Electropositive substituents destabilize carbanions.
  • Hybridization: Carbanions with more s character are more stable.
  • Resonance: Resonance delocalizes the negative charge, increasing stability. Aromatic systems are particularly stable.

Significance of Carbanions

• Reaction Intermediates: Carbanions are intermediates in many important organic reactions.
• Base Catalysts: Carbanions are strong bases that catalyze many reactions, especially in the formation of carbon-carbon bonds.
• Synthetic Applications: Carbanions are used in the synthesis of complex organic compounds.