A carbanion is an
anionin which carbonhas an unshared pair of electrons and bears a negative charge usually with three substituents for a total of eight valence electrons [Organic Chemistry - Robert Thornton Morrison, Robert Neilson Boyd] . The carbanion exists in a trigonal pyramidal geometry. Formally a carbanion is the conjugate baseof 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.
A carbanion is a
nucleophile. The stability and reactivity of a carbanion is determined by several factors. These include
inductive effect. Electronegative atoms adjacent to the charge will stabilize 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
A carbanion is a
reactive intermediateand is encountered in organic chemistryfor instance in the E1cB elimination reactionand in organometallic chemistryin for instance a Grignard reactionor in alkyl lithium chemistry. Stable carbanions do however exist. In 1984 Olmstead presented the lithium crown ether saltof the diphenylmethyl carbanion from diphenylmethane, butyl lithium and 12-crown-4 at low temperatures [ "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. [http://pubs3.acs.org/acs/journals/doilookup?in_doi=10.1021/ja00293a059 DOI abstract] ] :
Adding "n"-butyllithium to
triphenylmethanein THFat 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["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] . A spectrum of cyclopentadienein DMSO shows four vinylic protons at 6.5 ppm and 2 methyleneproton at 3 ppm whereas the cyclopentadienylanion has a single absorption at 5.50 ppm.
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
pKavalue. Methaneis certainly not an acid in its classical meaning yet its estimated pKa is 56. Compare this to acetic acidwith 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 gas 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
aromaticitysuch as in indeneand 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 carbonylgroups are acidic because the negative charge in the enolatecan be partially distributed in the oxygen atom. One compound called meldrum's acid, even more acidic than acetic acidand historically named an acid, in fact is a lactonebut 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 superacidwith an acidity one million times stronger than that of sulfuric acid.
molecular geometryfor 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 organolithiumcompounds.
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-etherat -70°C followed by reaction with dry iceyielded mostly recemic 2-methylbutyric acid but also an amount of optically active2-methyloctanoic acid which could only have formed from likewise optical active 2-methylheptyllithium with the carbon atom linked to lithium the carbanion ["FORMATION OF OPTICALLY ACTIVE 1-METHYLHEPTYLLITHIUM" Robert L. Letsinger J. Am. Chem. Soc.; 1950; 72(10) pp 4842 - 4842; DOI|10.1021/ja01166a538] :
On heating the reaction to 0°C the optical activity is lost. More evidence followed in the 1960s. A reaction of the
cis isomerof 2-methylcyclopropyl bromide with sec-butyllithium again followed by carboxylationwith dry ice yielded cis-2-methylcyclopropylcarboxylic acid. The formation of the trans isomer would have indicated that the intermediate carbanion was unstable ["The Configurational Stability of cis- and trans-2-Methylcyclopropyllithium and Some Observations on the Stereochemistry of their Reactions with Bromine and Carbon Dioxide" Douglas E. Applequist and Alan H. Peterson J. Am. Chem. Soc.; 1961; 83(4) pp 862 - 865; DOI|10.1021/ja01465a030] .
In the same manner the reaction of (+)-(S)-l-bromo-l-methyl-2,2-diphenylcyclopropane with n-butyllithium followed by quench with
methanolresulted in product with retention of configuration["Cyclopropanes. XV. The Optical Stability of 1-Methyl-2,2-diphenylcyclopropyllithium" H. M. Walborsky, F. J. Impastato, and A. E. Young J. Am. Chem. Soc.; 1964; 86(16) pp 3283 - 3288; DOI|10.1021/ja01070a017] :
Of recent date are chiral methyllithium compounds ["Preparation of Chiral -Oxy- [2H1] methyllithiums of 99% ee and Determination of Their Configurational Stability" Dagmar Kapeller, Roland Barth, Kurt Mereiter, and Friedrich Hammerschmidt
J. Am. Chem. Soc.; 2007; 129(4) pp 914 - 923; (Article) DOI|10.1021/ja066183s] :
The phosphate 1 contains a chiral group with a hydrogen and a
deuteriumsubstituent. The stannylgroup is replaced by lithium to intermediate 2 which undergoes a phosphate-phosphorane rearrangementto phosphorane3 which on reaction with acetic acid gives alcohol4. Once again in the range of -78°C to 0°C the chirality is preserved in this reaction sequence [ Enantioselectivitydetermined by NMR spectroscopyafter derivatization with Mosher's acid] .
* Large database of Bordwell pKa values at www.chem.wisc.edu [http://www.chem.wisc.edu/areas/reich/pkatable/ Link]
* Large database of Bordwell pKa values at daecr1.harvard.edu [http://daecr1.harvard.edu/pdf/evans_pKa_table.pdf Link]
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Carbanión — Saltar a navegación, búsqueda Un carbanión es un anión de un compuesto orgánico donde la carga negativa recae sobre un átomo de carbono. RnC H + B → RnC– + H B+ n=1,2,3 Donde B es una base. Contenido 1 Estabilidad 2 Propiedades … Wikipedia Español
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carbanion — CARBANIÓN s. m. ion care conţine o pereche de electroni (cu sarcină negativă) neparticipanţi la un atom de carbon, având trei covalenţe. (< fr. carbanion) Trimis de raduborza, 15.09.2007. Sursa: MDN … Dicționar Român
carbanion — ● carbanion nom masculin Anion dont la charge négative est portée par un atome de carbone … Encyclopédie Universelle
carbânion — s. m. [Física, Química] Ânion orgânico em que a carga negativa está num átomo de carbono. • [Brasil] Plural: carbaníones e carbânions. ‣ Etimologia: carbo + ânion ♦ Grafia em Portugal: carbanião … Dicionário da Língua Portuguesa
carbanion — [kär ban′ī än΄, kär ban′ī΄ən] n. [ CARB + ANION] a transient, negatively charged organic ion, as H3C , R3C , that has one more electron than the corresponding free radical … English World dictionary
carbanion — /kahr ban uy euhn, on/, n. an organic ion containing a negatively charged carbon atom (opposed to carbonium ion). [1933; CARB + ANION] * * * ▪ chemistry Introduction any member of a class of organic compounds in which a negative electrical… … Universalium
carbanion — kärˈbaˌnīən also īˌän noun ( s) Etymology: carb + anion : an organic ion carrying a negative charge at a carbon location (as methyl carbanion or butenyl carbanion) compare carbonium … Useful english dictionary
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