Carbanionen

Carbanionen are organoelectronic species bearing a negative charge on carbon. They are typically described as carbon-centered anions and often function as reactive intermediates in organic synthesis. Simple carbanions have the charge localized on an sp3-hybridized carbon, but many are stabilized by resonance or conjugation, giving enolates, allylic, benzyl, or propargyl carbanions.

Stability comes from delocalizing the negative charge. Resonance with carbonyl groups yields enolates; conjugation with adjacent double bonds or aromatic rings stabilizes allyl or benzyl carbanions. Electron-withdrawing substituents and metal coordination to counterions (such as Li+ or Na+) also help. More stabilized carbanions are less basic but remain good nucleophiles.

Preparation and handling: Carbanions are typically generated by deprotonation of relatively acidic C–H bonds with strong bases, such as LDA or organolithium reagents (butyllithium, phenyllithium), often in aprotic solvents at low temperature. They are highly reactive toward moisture and oxygen and require strictly anhydrous conditions.

Reactivity and applications: They act as bases and nucleophiles, enabling C–C bond formation via alkylation or acylation and serving as key intermediates in enolate chemistry. Enolates, benzyl, and allyl carbanions illustrate how resonance-stabilized species participate in a broad range of synthetic transformations.

Examples: The methanide anion CH3− is the simplest carbanion. The acetylide anion HC≡C−, and stabilized forms such as enolates of ketones or aldehydes and benzylic or allylic carbanions are widely encountered in laboratory synthesis.