Karbanioni

Karbanioni, or carbanions in English, are negatively charged species in which a carbon atom bears the extra electron pair. They function as strong bases and potent nucleophiles in organic synthesis. They are typically generated by deprotonation of hydrocarbons or related substrates under strongly basic conditions, or by metallation of suitable precursors.

Carbanions can be primary, secondary, or tertiary depending on the substituents on the carbanionic center. Their stability is influenced by resonance and solvation; stabilization occurs when the negative charge is delocalized into adjacent π systems (for example, enolates, benzyl, allyl carbanions) or via electron-withdrawing groups (carbonyls, nitriles, sulfonyls). Solvation with polar solvents and coordination to metal cations (such as Li+, Na+) also enhances stability.

Common methods for generating carbanions include deprotonation with strong bases such as butyllithium, lithium diisopropylamide (LDA), potassium tert-butoxide, or sodium hydride in appropriate solvents. Carbanions participate in nucleophilic additions to electrophiles like carbonyl compounds to form alcohols after workup, and in alkylation or acylation reactions to form new C–C or C–heteroatom bonds. Enolate chemistry is a major subset, where deprotonation α to carbonyls yields stabilized enolate carbanions.

Examples include the methanide CH3−, and benzyl or allyl carbanions, which are stabilized by resonance. The reactivity of carbanions depends on substitution pattern, resonance opportunities, and the solvent and counterions present. As reactive intermediates, they play a central role in many carbon–carbon bond-forming strategies.