Anion charge of carbon8/10/2023 More often, a bent geometry is still preferred for substituted alkenyl anions, though the linear geometry is only slightly less stable, resulting in facile equilibration between the ( E) and ( Z) isomers of the (bent) anion through a linear transition state. Likewise, delocalized alkenyl carbanions sometimes favor a linear instead of bent geometry. As a consequence, alkyl carbanions with neighboring conjugating groups (e.g., allylic anions, enolates, nitronates, etc.) are generally planar rather than pyramidized. A p orbital has a more suitable shape and orientation to overlap with the neighboring π system, resulting in more effective charge delocalization. Instead of residing in a hybrid orbital, the carbanionic lone pair may instead occupy a p orbital (or an orbital of high p character). ![]() However, delocalized carbanions may deviate from these geometries. 1,2-Disubstituted vinyl anions have E and Z isomers that undergo inversion through a linear transition state. 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. Valence shell electron pair repulsion (VSEPR) theory makes similar predictions. 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. As a consequence, localized alkyl, alkenyl/aryl, and alkynyl carbanions assume trigonal pyramidal, bent, and linear geometries, respectively. 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 (M δ+–C δ−) rather than true carbanions.Ībsent π delocalization, the negative charge of a carbanion is localized in an sp x hybridized orbital on carbon as a lone pair. ![]() A carbanion is one of several reactive intermediates in organic chemistry. The carbanions formed from deprotonation of alkanes (at an sp 3 carbon), alkenes (at an sp 2 carbon), arenes (at an sp 2 carbon), and alkynes (at an sp carbon) are known as alkyl, alkenyl ( vinyl), aryl, and alkynyl ( acetylide) anions, respectively.Ĭarbanions 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. įormally, a carbanion is the conjugate base of a carbon acid: In organic chemistry, a carbanion is an anion in which carbon is trivalent (forms three bonds) and bears a formal negative charge (in at least one significant resonance form). Ion with a negatively charged carbon atom
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