Carbanion

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.

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.

A p orbital has a more suitable shape and orientation to overlap with the neighboring π system, resulting in more effective charge delocalization.

Carbanions, especially ones derived from weak carbon acids that do not benefit sufficiently from the two stabilizing factors listed above, are generally oxygen- and water-sensitive to varying degrees.

Organometallic reagents like butyllithium (hexameric cluster, [BuLi]6) or methylmagnesium bromide (ether complex, MeMg(Br)(OEt2)2) are often referred to as "carbanions," at least in a retrosynthetic sense.

[4] However, in 1978, the methanide anion was unambiguously synthesized by subjecting ketene to an electric discharge, and the electron affinity (EA) of •CH3 was determined by photoelectron spectroscopy to be +1.8 kcal/mol, making it a bound species, but just barely so.

[5] Simple primary, secondary and tertiary sp3 carbanions (e.g., ethanide CH3CH−2, isopropanide (CH3)2CH−, and t-butanide (CH3)3C− were subsequently determined to be unbound species (the EAs of CH3CH2•, (CH3)2CH•, (CH3)3C• are −6, −7.4, −3.6 kcal/mol, respectively) indicating that α substitution is destabilizing.

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:[7] Adding n-butyllithium to triphenylmethane (pKa in DMSO of CHPh3 = 30.6) in THF at low temperatures followed by 12-crown-4 results in a red solution and the salt complex [Li(12-crown-4)]+[CPh3]− precipitates at −20 °C.

A crystal structure for the analogous diphenylmethanide anion ([Li(12-crown-4)]+[CHPh2]−), prepared form diphenylmethane (pKa in DMSO of CH2Ph2 = 32.3), was also obtained.

[9] Early in 1904[10] and 1917,[11] Schlenk prepared two red-colored salts, formulated as [NMe4]+[CPh3]− and [NMe4]+[PhCH2]−, respectively, by metathesis of the corresponding organosodium reagent with tetramethylammonium chloride.

The reaction of the putative "[NMe4]+[PhCH2]−" with water was reported to liberate toluene and tetramethylammonium hydroxide and provides indirect evidence for the claimed formulation.

Nevertheless, it is very weak Brønsted acid with an estimated pKa of 49 which may undergo deprotonation in the presence of a superbase like the Lochmann–Schlosser base (n-butyllithium and potassium t-butoxide).

Moreover, aqueous values are often given in introductory organic chemistry textbooks for pedagogical reasons, although the issue of solvent dependence is often glossed over.

[14] In general, pKa values in water and organic solvent diverge significantly when the anion is capable of hydrogen bonding.

The stannyl group is replaced by lithium to intermediate 2 which undergoes a phosphate–phosphorane rearrangement to phosphorane 3 which on reaction with acetic acid gives alcohol 4.

A carbanionic structure first made an appearance in the reaction mechanism for the benzoin condensation as correctly proposed by Clarke and Arthur Lapworth in 1907.