Carbanion 515683 205134859 2008-04-12T15:09:44Z DHN-bot 575307 robot Adding: [[pt:Carbânion]] A '''carbanion''' is an [[anion]] in which [[carbon]] has an unshared pair of [[electron]]s 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 pyramid (chemistry)|trigonal pyramidal]] geometry. Formally a carbanion is the [[conjugate base]] of a '''carbon acid'''. :R<sub>3</sub>C-H + B<sup>−</sup> → R<sub>3</sub>C<sup>−</sup> + H-B where B stands for the base. A carbanion is one of several [[reactive intermediate]]s in [[organic chemistry]]. ==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 stabilize the charge; #[[Orbital hybridisation|Hybridisation]] of the charge-bearing atom. The greater the s-character of the charge-bearing atom, the more stable the anion; #The extent of [[Conjugated system|conjugation]] of the anion. [[Resonance (chemistry)|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 [[crown ether|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. [http://pubs3.acs.org/acs/journals/doilookup?in_doi=10.1021/ja00293a059 DOI abstract]</ref>: [[Image:TriphenylmethaneAnion.png|center|500px|Formation of the triphenylmethane anion]] Adding [[n-butyllithium]] to [[triphenylmethane]] in [[THF]] at low temperatures followed by [[crown ether|12-crown-4]] results in a red solution and the salt complex precipitates at -20°C. The central C-C [[bond length]]s 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]] 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 [[DMSO|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. {|align="center" class="wikitable" |[[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 |- | colspan=2 align=left style="background: #ccccff;"| ''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 [[carboxylic acid|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 reaction]]s 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 (chemistry)|trigonal pyramid]] the question is whether or not carbanions can display [[chirality (chemistry)|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 [[butyric acid|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 <ref>''FORMATION OF OPTICALLY ACTIVE 1-METHYLHEPTYLLITHIUM'' Robert L. Letsinger [[J. Am. Chem. Soc.]]; '''1950'''; 72(10) pp 4842 - 4842; {{DOI|10.1021/ja01166a538}}</ref>: [[Image:ChiralcarbanionsI.png|center|400px|optically active organolithium]] 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 <ref>''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}}</ref>. [[Image:ChiralcarbanionsII.png|center|500px|stereochemistry of organolithiums]] 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]] <ref>''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}}</ref>: [[Image:ChiralcarbanionsIII.png|center|500px|Optical Stability of 1-Methyl-2,2-diphenylcyclopropyllithium]] Of recent date are chiral methyllithium compounds <ref>''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}}</ref>: [[Image:PhosphatePhosphonateRearrangement.png|center|500px|Chiral -Oxy-[2H1]methyllithiums, Bu stands for butyl, i-Pr stands for isopropyl]] The [[organophosphorus chemistry|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 <ref>[[Enantioselectivity]] determined by [[NMR spectroscopy]] after derivatization with [[Mosher's acid]]</ref>. ==External links== * 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] ==See also== *[[Carbocation]] ==References== {{Reflist}} [[Category:Anions]] [[Category:Reactive intermediates]] [[de:Carbanion]] [[es:Carbanión]] [[fr:Carbanion]] [[it:Carbanione]] [[he:קרבאניון]] [[nl:Carbanion]] [[ja:カルバニオン]] [[pt:Carbânion]] [[sv:Karbanjon]] [[vi:Cacbanion]] [[zh:碳负离子]]