Active hydrogen compound

A carbanion is an anion in which carbon is threevalent (forms three bonds) and bears a formal negative charge in at least one significant mesomeric contributor (resonance form). Absent π delocalization, carbanions assume a trigonal pyramidal, bent, or linear geometry when the carbanionic carbon is bound to three (e.g., methyl anion), two (e.g., phenyl anion), or one (e.g., acetylide anion) substituents, respectively. Formally, a carbanion is the conjugate base of a carbon acid:

where B stands for the base. A carbanion is one of several reactive intermediates in organic chemistry. In organic synthesis, organolithium reagents and Grignard reagents are commonly regarded as carbanions. This is a convenient approximation, although these species are almost always multinuclear clusters containing polar covalent bonds rather than true carbanions.

Carbanions are typically nucleophile and basic. The basicity and nucleophilicity of carbanions are determined by the substituents on carbon. These include

Geometry also affects the orbital hybridization of the charge-bearing carbanion. The greater the s-character of the charge-bearing atom, the more stable the anion.

Organometallic reagents like butyllithium (hexameric cluster, [BuLi]₆) or methylmagnesium bromide (ether complex, MeMgBr(OEt)₂) are often referred to as "carbanions," at least in a retrosynthetic sense. However, they are really clusters or complexes containing a polar covalent bond, though with electron density heavily polarized toward the carbon atom. Methyl anion and its chemistry have been observed in the gas phase. However, the electron affinity of methyl radical is –2 to –8 kcal/mol, so that gas-phase methyl anion spontaneously decomposes by ejection of an electron, and a true "salt" of CH₃^– is unlikely to be isolable. (Even the highly ionic and "salt-like" methylcesium has non-negligible covalent character.) In the condensed phase only carbanions that are sufficiently stabilized by delocalization have been isolated as truly ionic species. In 1984, Olmstead and Power presented the lithium crown ether salt of the triphenylmethanide carbanion from triphenylmethane, n-butyllithium and 12-crown-4 (which forms a stable complex with lithium cations) at low temperatures:

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