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Concept
Carbanion
Summary
In organic chemistry, a carbanion is an anion in which carbon is negatively charged.
Formally, a carbanion is the conjugate base of a carbon acid:
where B stands for the base. The carbanions formed from deprotonation of alkanes (at an sp3 carbon), alkenes (at an sp2 carbon), arenes (at an sp2 carbon), and alkynes (at an sp carbon) are known as alkyl, alkenyl (vinyl), aryl, and alkynyl (acetylide) anions, respectively.
Carbanions have a concentration of electron density at the negatively charged carbon, which, in most cases, reacts efficiently with a variety of electrophiles of varying strengths, including carbonyl groups, imines/iminium salts, halogenating reagents (e.g., N-bromosuccinimide and diiodine), and proton donors. A carbanion is one of several reactive intermediates in organic chemistry. In organic synthesis, organolithium reagents and Grignard reagents are commonly treated and referred to as "carbanions." This is a convenient approximation, although these species are generally clusters or complexes containing highly polar, but still covalent bonds metal–carbon bonds (Mδ+–Cδ−) rather than true carbanions.
Absent π delocalization, the negative charge of a carbanion is localized in an spx hybridized orbital on carbon as a lone pair. As a consequence, localized alkyl, alkenyl/aryl, and alkynyl carbanions assume trigonal pyramidal, bent, and linear geometries, respectively. By Bent's rule, placement of the carbanionic lone pair electrons in an orbital with significant s character is favorable, accounting for the pyramidalized and bent geometries of alkyl and alkenyl carbanions, respectively. Valence shell electron pair repulsion (VSEPR) theory makes similar predictions. This contrasts with carbocations, which have a preference for unoccupied nonbonding orbitals of pure atomic p character, leading to planar and linear geometries, respectively, for alkyl and alkenyl carbocations.
However, delocalized carbanions may deviate from these geometries.
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