Carbanion

A carbanion is an anion in which carbon has an unshared pair of electrons and bears a negative charge usually with three substituents for a total of eight valence electrons ^[1]. The carbanion exists in a trigonal pyramidal geometry. Formally a carbanion is the conjugate base of a carbon acid.

R₃C-H + B⁻ → R₃C⁻ + H-B

where B stands for the base. A carbanion is one of several reactive intermediates in organic chemistry.

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Theory

A carbanion is a nucleophile. The stability and reactivity of a carbanion is determined by several factors. These include

1. The inductive effect. Electronegative atoms adjacent to the charge will stabilize the charge;
2. Hybridisation of the charge-bearing atom. The greater the s-character of the charge-bearing atom, the more stable the anion;
3. The extent of conjugation of the anion. Resonance effects can stabilise the anion. This is especially true when the anion is stabilized as a result of aromaticity.

A carbanion is a reactive intermediate and is encountered in organic chemistry for instance in the E1cB elimination reaction and in organometallic chemistry in for instance a Grignard reaction or in alkyl lithium chemistry. Stable carbanions do however exist. In 1984 Olmstead presented the lithium crown ether salt of the diphenylmethyl carbanion from diphenylmethane, butyl lithium and 12-crown-4 at low temperatures^[2]:

Adding n-butyllithium to triphenylmethane in THF at low temperatures followed by 12-crown-4 results in a red solution and the salt complex precipitates at -20°C. The central C-C bond lengths are 145 ppm with the phenyl ring propelled at an average angle of 31.2°.

One tool for the detection of carbanions in solution is proton NMR ^[3]. A spectrum of cyclopentadiene in DMSO shows four vinylic protons at 6.5 ppm and 2 methylene proton at 3 ppm whereas the cyclopentadienyl anion has a single absorption at 5.50 ppm.

Carbon acids

Any molecule containing a C-H can lose a proton forming the carbanion. Hence any hydrocarbon containing C-H bonds can be considered an acid with a corresponding pKa value. Methane is certainly not an acid in its classical meaning yet its estimated pKa is 56. Compare this to acetic acid with pKa 12. The same factors that determine the stability of the carbanion also determine the order in pKa in carbon acids. These values are determined for the compounds either in water in order to compare them to ordinary acids, in dimethyl sulfoxide in which the majority of carbon acid and their anions are soluble or in the gase phase. With DMSO the acidity window solutes is limited to its own pKa of 35.5.

Starting from methane in table 1, the acidity increases when the anion is stabilized by aromaticity such as in indene and cyclopentadiene, or when the negative charge on carbon can be delocalized in one of three phenyl rings in triphenylmethane. The stabilization can be purely inductive for instance in malononitrile. The α-protons of carbonyl groups are acidic because the negative charge in the enolate can be partially distributed in the oxygen atom. One compound called meldrum's acid, even more acidic than acetic acid and historically named an acid, in fact is a lactone but its acidic carbon protons make it acidic. The acidity of carbonyl compound is an important driving force in many organic reactions such as the Aldol reaction.

The champion carbon acid is carborane superacid with an acidity one million times stronger than that of sulfuric acid.

Chiral carbanions

With the molecular geometry for a carbanion described as a trigonal pyramid the question is whether or not carbanions can display chirality. After all when the activation barrier for inversion of this geometry is too low any attempt at introducing chirality will end in racemization. However, solid evidence exists that carbanions can indeed be chiral for example in research carried out with certain organolithium compounds.

The first ever evidence for the existence of chiral organolithium compounds was obtained in 1950. Reaction of chiral 2-iodooctane with sec-butyllithium in petroleum-ether at -70°C followed by reaction with dry ice yielded mostly recemic 2-methylbutyric acid but also an amount of optically active 2-methyloctanoic acid which could only have formed from likewise optical active 2-methylheptyllithium with the carbon atom linked to lithium the carbanion ^[5]:

On heating the reaction to 0°C the optical activity is lost. More evidence followed in the 1960s. A reaction of the cis isomer of 2-methylcyclopropyl bromide with sec-butyllithium again followed by carboxylation with dry ice yielded cis-2-methylcyclopropylcarboxylic acid. The formation of the trans isomer would have indicated that the intermediate carbanion was unstable ^[6].

In the same manner the reaction of (+)-(S)-l-bromo-l-methyl-2,2-diphenylcyclopropane with n-butyllithium followed by quench with methanol resulted in product with retention of configuration ^[7]:

Of recent date are chiral methyllithium compounds ^[8]:

The phosphate 1 contains a chiral group with a hydrogen and a deuterium substituent. 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. Once again in the range of -78°C to 0°C the chirality is preserved in this reaction sequence ^[9].

See also

• Carbocation

References