Carbanion

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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 <ref>Organic Chemistry - Robert Thornton Morrison, Robert Neilson Boyd</ref>. 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.

1 Theory
2 Carbon acids
3 External links
4 See also
5 References

[edit] Theory

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

The inductive effect. Electronegative atoms adjacent to the charge will stabilise the charge;
Hybridisation of the charge-bearing atom. The greater the s-character of the charge-bearing atom, the more stable the anion;
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<ref> The isolation and x-ray structures of lithium crown ether salts of the free phenyl carbanions [CHPh2]- and [CPh3]- Marilyn M. Olmstead, Philip P. Power; J. Am. Chem. Soc.; 1985; 107(7); 2174-2175. DOI abstract</ref>:

Image:TriphenylmethaneAnion.png

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 <ref>A Simple and Convenient Method for Generation and NMR Observation of Stable Carbanions. Hamid S. Kasmai Journal of Chemical Education • Vol. 76 No. 6 June 1999</ref>. A spectrum of cyclopentadiene in DMSO shows four vinylic protons at 6.5 ppm and 2 methylene protone at 3 ppm whereas the cyclopentadienyl anion has a single absorbtion at 5.50 ppm.

[edit] 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 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.

cyclopentane	~ 59
methane	~ 56
anisole	~ 49
propene	~ 44
toluene	~ 43
diphenylmethane	32.3
aniline	30.6
triphenylmethane	30.6
xanthene	30
ethanol	29.8
phenylacetylene	28.8
thioxanthene	28.6
acetone	26.5
benzoxazole	24.4
fluorene	22.6
indene	20.1
phenylacetylene	28.8
cyclopentadiene	18
acetylacetone	13.3
acetic acid	12.6
malononitrile	11.2
meldrum's acid	7.3
Table 1. Carbon acid acidities in pKa in DMSO <ref>Equilibrium acidities in dimethyl sulfoxide solution Frederick G. Bordwell Acc. Chem. Res.; 1988; 21(12) pp 456 - 463; DOI:10.1021/ar00156a004</ref>. For reference regular acids in bold

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.