Carbanion
Definition and meaning of Carbanion in chemistry.
A carbanion is a reactive chemical species in which a carbon atom carries a negative charge and an unshared (lone) pair of electrons. Because carbon is not very electronegative, carbanions are generally strong bases and potent nucleophiles that exist only fleetingly under normal conditions.
In more detail
Most simple carbanions adopt a pyramidal, sp3-hybridized geometry analogous to ammonia, with the lone pair occupying one vertex, though hybridization and stability shift with substituents. Stability increases when adjacent electron-withdrawing groups or π systems delocalize the negative charge (as in enolates and cyclopentadienide) or when the carbanion carbon has greater s-character, which holds the electron pair closer to the nucleus. Carbanions are central intermediates in organic mechanisms, including enolate chemistry in the aldol reaction, organolithium and Grignard reagent reactivity, and E1cb eliminations.
Key facts
| General formula | R3C⁻ |
|---|---|
| Charge | −1, with a lone pair on carbon |
| Typical geometry | Pyramidal (sp3), often rapidly inverting |
| Field | Organic Chemistry |
Treating acetylene (ethyne) with the strong base sodium amide (NaNH2) removes an acidic terminal proton to form the acetylide ion, HC≡C⁻, a carbanion stabilized by the high s-character of its sp-hybridized carbon.
Frequently asked questions
Is an enolate a type of carbanion?
Yes. An enolate is a resonance-stabilized carbanion generated alpha to a carbonyl group; the negative charge is delocalized onto the adjacent oxygen, making it far more stable than an unstabilized alkyl carbanion.
What factors stabilize a carbanion?
Adjacent electron-withdrawing groups, resonance delocalization into pi systems, and increased s-character at the carbanion carbon (sp > sp2 > sp3) all help stabilize the negative charge.