Editing Carbanion

{{short description|Organic ion with a negatively charged carbon atom}}

In [[organic chemistry]], a '''carbanion''' is an [[anion]] with a [[lone pair]] attached to a [[tervalent]] [[carbon]] [[atom]].<ref>{{GoldBookRef |title=carbanion |file=C00804 }}</ref> This gives the carbon atom a negative charge.

Formally, a carbanion is the [[conjugate base]] of a '''carbon acid''':
:{{chem2|R3CH + B− → R3C− + HB}}
where B stands for the [[base (chemistry)|base]]. The carbanions formed from [[deprotonation]] of [[alkanes]] (at an [[Orbital hybridisation#sp3|sp<sup>3</sup>]] carbon), [[alkenes]] (at an [[Orbital hybridisation#sp2|sp<sup>2</sup>]] carbon), [[arenes]] (at an sp<sup>2</sup> carbon), and [[alkynes]] (at an [[Orbital hybridisation#sp|sp]] carbon) are known as [[alkyl]], alkenyl ([[Vinyl group|vinyl]]), [[aryl]], and alkynyl ([[acetylide]]) anions, respectively.

Carbanions have a concentration of electron density at the negatively charged carbon, which, in most cases, reacts efficiently with a variety of [[electrophile]]s of varying strengths, including [[carbonyl group]]s, [[Imine|imines]]/[[Iminium|iminium salts]], halogenating reagents (e.g., [[N-Bromosuccinimide|''N''-bromosuccinimide]] and [[Iodine|diiodine]]), and [[Brønsted–Lowry acid–base theory|proton donors]]. A carbanion is one of several [[reactive intermediate]]s in [[organic chemistry]]. In organic synthesis, [[organolithium reagent]]s and [[Grignard reagents]] are commonly treated and referred to as "carbanions." This is a convenient approximation, although these species are generally clusters or complexes containing highly polar, but still covalent bonds metal–carbon bonds (M<sup>δ+</sup>–C<sup>δ−</sup>) rather than true carbanions.

== Geometry ==
Absent [[Conjugated system|π delocalization]], the negative charge of a carbanion is localized in an [[Orbital hybridisation#spx hybridisation|sp<sup>''x''</sup>]] 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. By [[Bent's rule]], placement of the carbanionic lone pair electrons in an orbital with significant s character is favorable, accounting for the pyramidalized and bent geometries of alkyl and alkenyl carbanions, respectively. [[VSEPR theory|Valence shell electron pair repulsion (VSEPR) theory]] makes similar predictions. This contrasts with carbocations, which have a preference for unoccupied nonbonding orbitals of pure atomic p character, leading to planar and linear geometries, respectively, for alkyl and alkenyl carbocations.[[File:Carbanion Structural Formulae V.1.svg|thumb|250x250px|An alkyl carbanion is trigonal pyramidal.]]
[[File:Ez-isomerismofvinylanions.png|thumb|Vinyl anions are bent. 1,2-Disubstituted vinyl anions have ''E'' and ''Z'' isomers that undergo inversion through a linear transition state.|300x300px]]
However, ''delocalized'' carbanions may deviate from these geometries. Instead of residing in a hybrid orbital, the carbanionic lone pair may instead occupy a p orbital (or an orbital of high p character). A p orbital has a more suitable shape and orientation to overlap with the neighboring π system, resulting in more effective charge delocalization. As a consequence, alkyl carbanions with neighboring conjugating groups (e.g., allylic anions, enolates, nitronates, etc.) are generally planar rather than pyramidized. Likewise, delocalized alkenyl carbanions sometimes favor a linear instead of bent geometry. More often, a bent geometry is still preferred for substituted alkenyl anions, though the linear geometry is only ''slightly'' less stable, resulting in facile equilibration between the (''E'') and (''Z'') isomers of the (bent) anion through a linear [[transition state]].<ref>{{Cite journal|last1=Caramella|first1=Pierluigi|last2=Houk|first2=K. N.|date=1981-01-01|title=The influence of electron-withdrawing substituents on the geometries and barriers to inversion of vinyl anions|url=https://dx.doi.org/10.1016/0040-4039%2881%2980005-6|journal=Tetrahedron Letters|language=en|volume=22|issue=9|pages=819–822|doi=10.1016/0040-4039(81)80005-6|issn=0040-4039|url-access=subscription}}</ref> For instance, calculations indicate that the parent vinyl anion or ethylenide, {{chem2|H2C\dCH−}}, has an inversion barrier of {{cvt|27|kcal/mol|kJ/mol}}, while allenyl anion or allenide, {{chem2|H2C\dC\dCH− ↔ H2C−\sC\tCH}}), whose negative charge is stabilized by delocalization, has an inversion barrier of only {{cvt|4|kcal/mol|kJ/mol}}, reflecting stabilization of the linear transition state by better π delocalization.<ref>{{Cite book|last=Alabugin|first=Igor V.|url=http://doi.wiley.com/10.1002/9781118906378|title=Stereoelectronic Effects: A Bridge Between Structure and Reactivity|date=2016-09-19|publisher=John Wiley & Sons, Ltd|isbn=978-1-118-90637-8|location=Chichester, UK|language=en|doi=10.1002/9781118906378}}</ref>

==Trends and occurrence==
Carbanions are typically [[nucleophilic]] and basic. The basicity and nucleophilicity of carbanions are determined by the substituents on carbon. These include
*the [[inductive effect]]. Electronegative atoms adjacent to the charge will stabilize the charge;
*the extent of [[Conjugated system|conjugation]] of the anion. [[Resonance (chemistry)|Resonance effects]] can stabilize the anion. This is especially true when the anion is stabilized as a result of [[aromaticity]].

Geometry also affects the [[orbital hybridization]] of the charge-bearing carbanion. The greater the s-character of the charge-bearing atom, the more stable the anion.

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. While some merely degrade and decompose over several weeks or months upon exposure to air, others may react vigorously and exothermically with air almost immediately to spontaneously ignite ([[pyrophoricity]]). Among commonly encountered carbanionic reagents in the laboratory, ionic salts of hydrogen cyanide ([[Cyanide|cyanides]]) are unusual in being indefinitely stable under dry air and hydrolyzing only very slowly in the presence of moisture.

Organometallic reagents like [[butyllithium]] (hexameric cluster, {{chem2|[BuLi]6}}) or [[methylmagnesium bromide]] (ether complex, {{chem2|MeMg(Br)(OEt2)2}}) are often referred to as "carbanions," at least in a [[retrosynthetic analysis|retrosynthetic]] sense. However, they are really clusters or complexes containing a polar covalent bond, though with electron density heavily polarized toward the carbon atom. The more electropositive the attached metal atom, the closer the behavior of the reagent is to that of a true carbanion.

In fact, true carbanions (i.e., a species not attached to a stabilizing covalently bound metal) without electron-withdrawing and/or conjugating substituents are not available in the condensed phase, and these species must be studied in the gas phase. For some time, it was not known whether simple alkyl anions could exist as free species; many theoretical studies predicted that even the [[methanide]] anion {{chem2|CH3–}} should be an unbound species (i.e., the [[electron affinity]] of {{chem2|•CH3}} was predicted to be negative). Such a species would decompose immediately by spontaneous ejection of an electron and would therefore be too fleeting to observe directly by mass spectrometry.<ref>{{Cite journal|last1=Marynick|first1=Dennis S.|last2=Dixon|first2=David A.|date=1977|title=Electron Affinity of the Methyl Radical: Structures of CH<sub>3</sub> and {{chem|CH|3|-}}|journal=Proceedings of the National Academy of Sciences of the United States of America|volume=74|issue=2|pages=410–413|bibcode=1977PNAS...74..410M|doi=10.1073/pnas.74.2.410|jstor=66197|pmc=392297|pmid=16592384|doi-access=free}}</ref> However, in 1978, the methanide anion was unambiguously synthesized by subjecting [[ketene]] to an electric discharge, and the electron affinity (EA) of {{chem2|•CH3}} was determined by photoelectron spectroscopy to be +1.8&nbsp;kcal/mol, making it a bound species, but just barely so. The structure of {{chem2|CH3–}} was found to be pyramidal (C<sub>3v</sub>) with an H−C−H angle of 108° and [[Pyramidal inversion|inversion]] barrier of 1.3&nbsp;kcal/mol, while {{chem2|•CH3}} was determined to be planar (D<sub>3h</sub> point group).<ref>{{Cite journal|last1=Ellison|first1=G. Barney|last2=Engelking|first2=P. C.|last3=Lineberger|first3=W. C.|date=April 1978|title=An experimental determination of the geometry and electron affinity of methyl radical|journal=Journal of the American Chemical Society|language=EN|volume=100|issue=8|pages=2556–2558|doi=10.1021/ja00476a054|issn=0002-7863}}</ref>

Simple primary, secondary and tertiary sp<sup>3</sup> carbanions (e.g., ethanide {{chem2|CH3CH2–}}, isopropanide {{chem2|(CH3)2CH−}}, and ''t''-butanide {{chem2|(CH3)3C−}}) were subsequently determined to be unbound species (the EAs of {{chem2|CH3CH2•}}, {{chem2|(CH3)2CH•}}, {{chem2|(CH3)3C•}} are −6, −7.4, −3.6&nbsp;kcal/mol, respectively) indicating that α substitution is destabilizing. However, relatively modest stabilizing effects can render them bound. For example, [[cyclopropyl]] and [[cubane|cubyl]] anions are bound due to increased s character of the lone pair orbital, while [[neopentyl]] and [[phenethyl]] anions are also bound, as a result of negative hyperconjugation of the lone pair with the β-substituent (n<sub>C</sub> → σ*<sub>C–C</sub>). The same holds true for anions with [[benzyl]]ic and [[allyl]]ic stabilization. Gas-phase carbanions that are sp<sup>2</sup> and sp hybridized are much more strongly stabilized and are often prepared directly by gas-phase deprotonation.<ref>{{Cite book|title=The encyclopedia of mass spectrometry|last1=Blanksby|first1=S. J.|last2=Bowie|first2=J. H.|date=2005|publisher=Elsevier|others=Gross, Michael L., Caprioli, R. M.|isbn=9780080438504|edition= 1st|location=Amsterdam|chapter=Carbanions: formation, structure and thermochemistry|oclc=55939535}}</ref>

In the condensed phase only carbanions that are sufficiently stabilized by delocalization have been isolated as truly ionic species. In 1984, Olmstead and [[Philip Power|Power]] presented the lithium [[crown ether]] [[salt (chemistry)|salt]] of the triphenylmethanide carbanion from [[triphenylmethane]], [[n-butyllithium|''n''-butyllithium]] and [[crown ether|12-crown-4]] (which forms a stable complex with lithium cations) at low temperatures:<ref>{{cite journal | doi = 10.1021/ja00293a059 | volume=107 | title=The isolation and X-ray structures of lithium crown ether salts of the free phenyl carbanions [CHPh<sub>2</sub>]<sup>−</sup> and [CPh<sub>3</sub>]<sup>−</sup> | year=1985 | journal=Journal of the American Chemical Society | pages=2174–2175 | last1 = Olmstead | first1 = Marilyn M.| issue=7 }}</ref>

:[[File:TriphenylmethaneAnion.png|none|500px|Formation of the triphenylmethane anion]]

Adding [[N-Butyllithium|''n''-butyllithium]] to [[triphenylmethane]] (p''K''<sub>a</sub> in DMSO of {{chem2|CHPh3}} = 30.6) in [[THF]] at low temperatures followed by [[crown ether|12-crown-4]] results in a red solution and the salt complex [Li(12-crown-4)]{{chem2|+[CPh3]−}} precipitates at −20&nbsp;°C. The central C–C [[bond length]]s are 145&nbsp;pm with the phenyl ring propellered at an average angle of 31.2°. This propeller shape is less pronounced with a tetramethylammonium counterion. A crystal structure for the analogous diphenylmethanide anion ([Li(12-crown-4)]{{chem2|+[CHPh2]−}}), prepared form diphenylmethane (p''K''<sub>a</sub> in DMSO of {{chem2|CH2Ph2}} = 32.3), was also obtained. However, the attempted isolation of a complex of the benzyl anion {{chem2|PhCH2−}} from toluene (p''K''<sub>a</sub> in DMSO of {{chem2|CH3Ph}} ≈ 43) was unsuccessful, due to rapid reaction of the formed anion with the THF solvent.<ref>{{Cite journal| doi = 10.1002/1521-3765(20020715)8:14<3229::AID-CHEM3229>3.0.CO;2-3| title = Schlenk's Early "Free" Carbanions| year = 2002| last1 = Harder | first1 = S.| journal = Chemistry: A European Journal| volume = 8| issue = 14| pages = 3229–3232| pmid = 12203352}}</ref> The free benzyl anion has also been generated in the solution phase by [[Radiolysis|pulse radiolysis]] of dibenzylmercury.<ref>{{Cite journal|title=Submicrosecond formation and observation of reactive carbanions|journal = Journal of the American Chemical Society|volume = 96|issue = 18|pages = 5708–5715|last1=Bockrath|first1=Bradley|last2=Dorfman|first2=Leon M.|date=2002-05-01|language=EN|doi=10.1021/ja00825a005}}</ref>

Early in 1904<ref name=":1" /> and 1917,<ref>{{Cite journal|last1=Schlenk|first1=W.|last2=Holtz|first2=Johanna|date=1917|title=Über Benzyl-tetramethyl-ammonium|trans-title=On benzyl tetramethyl ammonium|journal=Berichte der Deutschen Chemischen Gesellschaft|language=en|volume=50|issue=1|pages=274–275|doi=10.1002/cber.19170500143|issn=1099-0682|url=https://zenodo.org/record/1426617}}</ref> [[Wilhelm Schlenk|Schlenk]] prepared two red-colored salts, formulated as {{chem2|[NMe4]+[CPh3]−}} and {{chem2|[NMe4]+[PhCH2]−}}, respectively, by metathesis of the corresponding organosodium reagent with tetramethylammonium chloride. Since tetramethylammonium cations cannot form a chemical bond to the carbanionic center, these species are believed to contain free carbanions. While the structure of the former was verified by X-ray crystallography almost a century later,<ref>{{Cite journal|last=Harder|first=Sjoerd|date=2002-07-15|title=Schlenk's Early "Free" Carbanions|journal=Chemistry – A European Journal|language=en|volume=8|issue=14|pages=3229–3232|doi=10.1002/1521-3765(20020715)8:14<3229::AID-CHEM3229>3.0.CO;2-3 |pmid=12203352 }}</ref> the instability of the latter has so far precluded structural verification. The reaction of the putative "{{chem2|[NMe4]+[PhCH2]−}}" with water was reported to liberate toluene and tetramethylammonium hydroxide and provides indirect evidence for the claimed formulation.

One tool for the detection of carbanions in solution is [[proton NMR]].<ref>{{cite journal|title=A Simple and Convenient Method for Generation and NMR Observation of Stable Carbanions|first1=Hamid S.|last1=Kasmai|journal=Journal of Chemical Education|volume=76|issue=6|date=June 1999|doi=10.1021/ed076p830 }}</ref> A spectrum of [[cyclopentadiene]] in DMSO shows four vinylic protons at 6.5&nbsp;ppm and two [[methylene bridge]] protons at 3 ppm whereas the [[cyclopentadienyl]] anion has a single resonance at 5.50&nbsp;ppm. The use of {{chem2|^{6}Li}} and {{chem2|^{7}Li}} NMR has provided structural and reactivity data for a variety of [[organolithium]] species.

==Carbon acids==
Any compound containing hydrogen can, in principle, undergo deprotonation to form its conjugate base. A compound is a '''carbon acid''' if deprotonation results in loss of a proton from a carbon atom. Compared to compounds typically considered to be acids (e.g., [[mineral acid]]s like [[nitric acid]], or [[carboxylic acid]]s like [[acetic acid]]), carbon acids are typically many orders of magnitude weaker, although exceptions exist (see below). For example, [[benzene]] is not an acid in the classical [[Arrhenius acid|Arrhenius]] sense, since its aqueous solutions are neutral. Nevertheless, it is very weak [[Brønsted–Lowry acid–base theory|Brønsted acid]] with an estimated [[Acid dissociation constant|p''K''<sub>a</sub>]] of 49 which may undergo deprotonation in the presence of a superbase like the [[Schlosser's base|Lochmann–Schlosser base]] ([[n-Butyllithium|''n''-butyllithium]] and [[Potassium t-butoxide|potassium ''t''-butoxide]]). As conjugate acid–base pairs, the factors that determine the relative stability of carbanions also determine the ordering of the p''K''<sub>a</sub> values of the corresponding carbon acids. Furthermore, p''K''<sub>a</sub> values allow the prediction of whether a proton transfer process will be thermodynamically favorable: In order for the deprotonation of an acidic species HA with base {{chem2|B−}} to be thermodynamically favorable (''K'' > 1), the relationship p''K''<sub>a</sub>(BH) > p''K''<sub>a</sub>(AH) must hold.

These values below are p''K''<sub>a</sub> values determined in [[dimethylsulfoxide]] (DMSO), which has a broader useful range (~0 to ~35) than values determined in water (~0 to ~14) and better reflect the basicity of the carbanions in typical organic solvents. Values below less than 0 or greater than 35 are indirectly estimated; hence, the numerical accuracy of these values is limited. Aqueous p''K''<sub>a</sub> values are also commonly encountered in the literature, particularly in the context of biochemistry and enzymology. Moreover, aqueous values are often given in introductory organic chemistry textbooks for pedagogical reasons, although the issue of solvent dependence is often glossed over.<ref>{{Cite journal |last=Heller |first=Stephen T. |last2=Silverstein |first2=Todd P. |date=2020-04-23 |title=pKa values in the undergraduate curriculum: introducing pKa values measured in DMSO to illustrate solvent effects |url=https://doi.org/10.1007/s40828-020-00112-z |journal=ChemTexts |language=en |volume=6 |issue=2 |pages=15 |doi=10.1007/s40828-020-00112-z |issn=2199-3793|url-access=subscription }}</ref> In general, p''K''<sub>a</sub> values in water and organic solvent diverge significantly when the anion is capable of hydrogen bonding. For instance, in the case of water, the values differ dramatically: the p''K''<sub>a</sub> in water of water is 14.0,<ref>{{Cite journal|last1=Silverstein|first1=Todd P.|last2=Heller|first2=Stephen T.|date=2017-04-17|title=p''K''<sub>a</sub> Values in the Undergraduate Curriculum: What is the Real p''K''<sub>a</sub> of Water?|journal=Journal of Chemical Education|language=EN|volume=94|issue=6|pages=690–695|doi=10.1021/acs.jchemed.6b00623|bibcode=2017JChEd..94..690S}}</ref> while the p''K''<sub>a</sub> in DMSO of water is 31.4,<ref name=":0">{{Cite web|url=http://evans.rc.fas.harvard.edu/pdf/evans_pKa_table.pdf|title=Chem 206 p''K''<sub>a</sub> Table|last1=Evans|first1=D. A.|last2=Ripin|first2=D. H.|date=2005|archive-url=https://web.archive.org/web/20190702225424/http://evans.rc.fas.harvard.edu/pdf/evans_pKa_table.pdf|archive-date=2019-07-02|url-status=dead}}</ref> reflecting the differing ability of water and DMSO to stabilize the [[hydroxide]] anion. On the other hand, for [[cyclopentadiene]], the numerical values are comparable: the p''K''<sub>a</sub> in water is 15, while the p''K''<sub>a</sub> in DMSO is 18.<ref name=":0" />
:{|align="center" class="wikitable collapsible" style="background: #ffffff; text-align: center;"
|+Carbon acid acidities by [[pKa|p''K''<sub>a</sub>]] in [[Dimethyl sulfoxide|DMSO]].<ref>{{cite journal | doi = 10.1021/ar00156a004 | volume=21 | title=Equilibrium acidities in dimethyl sulfoxide solution | year=1988 | journal=Accounts of Chemical Research | pages=456–463 | last1 = Bordwell | first1 = Frederick G.| issue=12 }}</ref><br><small>These values may differ significantly from aqueous p''K''<sub>a</sub> values.</small>
|-
!Name
!Formula
!Structural formula
!p''K''<sub>a</sub> in DMSO
|-
|[[Cyclohexane]]
|{{chem2|C6H12}}
|[[File:Cyclohexane simple.svg|frameless|45x45px]]
|~60
|-
|[[Methane]]
|{{chem2|CH4}}
|[[File:Methane-2D-dimensions.svg|90px]]
|~56
|-
|[[Benzene]]
|{{chem2|C6H6}}
|[[File:Benzol.svg|40px]]
|~49<ref>{{Cite journal|last1=Bordwell|first1=G. F.|last2=Matthews|first2=Walter S.|date=2002-05-01|title=Equilibrium acidities of carbon acids. III. Carbon acids in the membrane series|journal=Journal of the American Chemical Society|language=EN|volume=96|issue=4|pages=1216–1217|doi=10.1021/ja00811a041}}</ref>
|-
|[[Propene]]
|{{chem2|C3H6}}
|[[File:Propylene skeletal.svg|75px]]
|~44
|-
|[[Toluene]]
|{{chem2|C6H5CH3}}
|[[File:Toluol.svg|40px]]
|~43
|- style="background: lightgray;"
|[[Ammonia]] (N–H)
|{{chem2|NH3}}
|[[File:Ammonia dimensions.svg|frameless|100x100px]]
|~41
|-
|[[Dithiane]]
|{{chem2|C4H8S2}}
|[[File:1,3-dithiane structure.svg|frameless|60x60px]]
|~39
|-
|[[Dimethyl sulfoxide]]
|{{chem2|(CH3)2SO}}
|[[File:DMSO-2D-dimensions.png|90px]]
|35.1
|-
|[[Diphenylmethane]]
|{{chem2|C13H12}}
|[[File:Diphenylmethane.png|130px]]
|32.3
|-
|[[Acetonitrile]]
|{{chem2|CH3CN}}
|[[File:Structural formula of acetonitrile.svg|frameless|120x120px]]
|31.3
|- style="background: lightgray;"
|[[Aniline]] (N–H)
|{{chem2|C6H5NH2}}
|[[File:Aniline.svg|50px]]
|30.6
|-
|[[Triphenylmethane]]
|{{chem2|C19H16}}
|[[File:Triphenylmethane.png|100px]]
|30.6
|-
|[[Fluoroform]]
|{{chem2|CHF3}}
|[[File:Fluoroform.svg|frameless|82x82px]]
|30.5<ref name=":2">{{Cite journal|date=1998-11-05|title=Effective nucleophilic trifluoromethylation with fluoroform and common base|journal=Tetrahedron|language=en|volume=54|issue=45|pages=13771–13782|doi=10.1016/S0040-4020(98)00846-1|issn=0040-4020|last1=Russell|first1=Jamie|last2=Roques|first2=Nicolas}}</ref>
|-
|[[Xanthene]]
|{{chem2|C13H10O}}
|[[File:Xanthen.svg|120px]]
|30.0
|- style="background: lightgray;"
|[[Ethanol]] (O–H)
|{{chem2|C2H5OH}}
|[[File:Ethanol-2D-skeletal.svg|75px]]
|29.8
|-
|[[Phenylacetylene]]
|{{chem2|C8H6}}
|[[File:Phenylacetylene.svg|75px]]
|28.8
|-
|[[Thioxanthene]]
|{{chem2|C13H10S}}
|[[File:Thioxanthene.png|100px]]
|28.6
|-
|[[Acetone]]
|{{chem2|C3H6O}}
|[[File:Aceton.svg|90px]]
|26.5
|-
|[[Chloroform]]
|{{chem2|CHCl3}}
|[[File:Chloroform displayed.svg|frameless|70x70px]]
|24.4<ref name=":2" />
|-
|[[Benzoxazole]]
|{{chem2|C7H5NO}}
|[[File:1,3-benzoxazole numbering.svg|90px]]
|24.4
|-
|[[Fluorene]]
|{{chem2|C13H10}}
|[[File:Fluorene.png|100px]]
|22.6
|-
|[[Indene]]
|{{chem2|C9H8}}
|[[File:Indene.png|75px]]
|20.1
|-
|[[Cyclopentadiene]]
|{{chem2|C5H6}}
|[[File:Cyclopentadiene.png|50px]]
|18.0
|-
|[[Nitromethane]]
|{{chem2|CH3NO2}}
|[[File:Nitromethane.svg|frameless|60x60px]]
|17.2
|-
|[[Diethyl malonate]]
|{{chem2|C7H12O4}}
|[[File:Diethyl-malonate.png|frameless]]
|16.4
|-
|[[Acetylacetone]]
|{{chem2|(H3CCO)2CH2}}
|[[File:Acetylacetone.png|frameless|120x120px]]
|13.3
|-
|[[Hydrogen cyanide]]
|HCN
|[[File:Hydrogen-cyanide-2D.svg|80px]]
|12.9
|- style="background: lightgray;"
|[[Acetic acid]] (O–H)
|{{chem2|CH3COOH}}
|[[File:Acetic-acid-2D-skeletal.svg|70px]]
|12.6
|-
|[[Malononitrile]]
|{{chem2|C3H2N2}}
|[[File:Malononitrile.png|100px]]
|11.1
|-
|[[Dimedone]]
|{{chem2|C8H12O2}}
|[[File:Dimedone.png|100px]]
|10.3
|-
|[[Meldrum's acid]]
|{{chem2|C6H8O4}}
|[[File:meldrum's acid.png|80px]]
|7.3
|-
|[[Hexafluoroacetylacetone]]
|{{chem2|(F3CCO)2CH2}}
|[[File:Hexafluoroacetylaceton.svg|frameless|120x120px]]
|2.3
|- style="background: lightgray;"
|[[Hydrogen chloride]] (Cl–H)
|HCl
|HCl (g)
|−2.0<ref>{{Cite journal|last1=Trummal|first1=Aleksander|last2=Lipping|first2=Lauri|last3=Kaljurand|first3=Ivari|last4=Koppel|first4=Ilmar A.|last5=Leito|first5=Ivo|date=2016-05-06|title=Acidity of Strong Acids in Water and Dimethyl Sulfoxide|journal=The Journal of Physical Chemistry A|language=EN|volume=120|issue=20|pages=3663–3669|doi=10.1021/acs.jpca.6b02253|pmid=27115918|bibcode=2016JPCA..120.3663T|s2cid=29697201 }}</ref>
|-
|[[Triflidic acid]]
|{{chem2|HC(SO2CF3)3}}
|[[File:Triflidic acid.svg|frameless|120x120px]]
|~ −16{{efn|The reported p''K''<sub>a</sub> in [[acetonitrile]] (MeCN) is −3.7.<ref>{{cite journal|title=Equilibrium Acidities of Superacids|first1=Agnes|last1=Kütt|first2=Toomas|last2=Rodima|first3=Jaan|last3=Saame|first4=Elin|last4=Raamat|first5=Vahur|last5=Mäemets|first6=Ivari|last6=Kaljurand|first7=Ilmar A.|last7=Koppel|first8=Romute Yu.|last8=Garlyauskayte|first9=Yurii L.|last9=Yagupolskii|first10=Lev M.|last10=Yagupolskii|first11=Eduard|last11=Bernhardt|first12=Helge|last12=Willner|first13=Ivo|last13=Leito|journal=The Journal of Organic Chemistry|date=2011|volume=76|issue=2|pages=391–395|doi=10.1021/jo101409p|pmid=21166439 }}</ref> The p''K''<sub>a</sub> in DMSO was estimated by the correlation {{nowrap|1=p''K''<sub>a</sub><sup>MeCN</sup> = 0.98 × p''K''<sub>a</sub><sup>DMSO</sup> + 11.6}}.<ref>{{cite journal|title=First-Principles Calculation of p''K''<sub>a</sub> Values for Organic Acids in Nonaqueous Solution|first1=Feizhi|last1=Ding|first2=Jeremy M.|last2=Smith|first3=Haobin|last3=Wang|journal=The Journal of Organic Chemistry|date=2009|volume=74|issue=7|pages=2679–2691|doi=10.1021/jo802641r|pmid=19275192 }}</ref>}}
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::<small>Note that acetic acid, ammonia, aniline, ethanol, and hydrogen chloride are not carbon acids, but are common acids shown for comparison.</small>
{{Notelist}}

As indicated by the examples above, acidity increases (p''K''<sub>a</sub> decreases) when the negative charge is delocalized. This effect occurs when the substituents on the carbanion are unsaturated and/or electronegative. Although carbon acids are generally thought of as acids that are much weaker than "classical" Brønsted acids like acetic acid or phenol, the cumulative (additive) effect of several electron accepting substituents can lead to acids that are as strong or stronger than the inorganic mineral acids. For example, [[trinitromethane]] {{chem2|HC(NO2)3}}, [[tricyanomethane]] {{chem2|HC(CN)3}}, pentacyanocyclopentadiene {{chem2|C5(CN)5H}}, and [[fulminic acid]] HCNO, are all strong acids with aqueous p''K''<sub>a</sub> values that indicate complete or nearly complete proton transfer to water. [[Triflidic acid]], with three strongly electron-withdrawing [[triflyl]] groups, has an estimated p''K''<sub>a</sub> well below −10. On the other end of the scale, hydrocarbons bearing only alkyl groups are thought to have p''K''<sub>a</sub> values in the range of 55 to 65. The range of acid dissociation constants for carbon acids thus spans over 70 orders of magnitude.

The acidity of the α-hydrogen in [[carbonyl]] compounds enables these compounds to participate in synthetically important C–C bond-forming reactions including the [[aldol reaction]] and [[Michael reaction|Michael addition]].

==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]], because if the activation barrier for inversion of this geometry is too low any attempt at introducing chirality will end in [[racemization]], similar to the [[nitrogen inversion]]. 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 [[Butyllithium|''s''-butyllithium]] in [[petroleum ether]] at −70&nbsp;°C followed by reaction with [[dry ice]] yielded mostly racemic [[butyric acid|2-methylbutyric acid]] but also an amount of [[optically active]] 2-methyloctanoic acid, which could only have formed from likewise optically active 2-methylheptyllithium with the carbon atom linked to lithium the carbanion:<ref>{{cite journal | doi = 10.1021/ja01166a538 | volume=72 | year=1950 | journal=Journal of the American Chemical Society | page=4842 | last1 = Letsinger | first1 = Robert L.| title=Formation of Optically Active 1-Methylheptyllithium | issue=10 }}</ref>

:[[File:ChiralcarbanionsI.png|none|400px|Optically active organolithium]]

On heating the reaction to 0&nbsp;°C the optical activity is lost. More evidence followed in the 1960s. A reaction of the [[cis–trans isomerism|''cis'' isomer]] of 2-methylcyclopropyl bromide with ''s''-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>{{cite journal | doi = 10.1021/ja01465a030 | volume=83 | title=The Configurational Stability of ''cis''- and ''trans''-2-Methylcyclopropyllithium and Some Observations on the Stereochemistry of their Reactions with Bromine and Carbon Dioxide | year=1961 | journal=Journal of the American Chemical Society | pages=862–865 | last1 = Applequist | first1 = Douglas E.| issue=4 }}</ref>

:[[File:ChiralcarbanionsII.png|none|500px|Stereochemistry of organolithiums]]

In the same manner the reaction of (+)-(''S'')-''l''-bromo-''l''-methyl-2,2-diphenylcyclopropane with ''n''-butyllithium followed by quenching with [[methanol]] resulted in product with [[retention of configuration]]:<ref>{{cite journal | doi = 10.1021/ja01070a017 | volume=86 | title=Cyclopropanes. XV. The Optical Stability of 1-Methyl-2,2-diphenylcyclopropyllithium | year=1964 | journal=Journal of the American Chemical Society | pages=3283–3288 | last1 = Walborsky | first1 = H. M.| issue=16 }}</ref>

:[[File:ChiralcarbanionsIII.png|none|500px|Optical stability of 1-methyl-2,2-diphenylcyclopropyllithium]]

Of recent date are chiral methyllithium compounds:<ref>{{cite journal | doi = 10.1021/ja066183s | volume=129 | title=Preparation of Chiral α-Oxy-[<sup>2</sup>''H''<sub>1</sub>]methyllithiums of 99% ee and Determination of Their Configurational Stability | year=2007 | journal=Journal of the American Chemical Society | pages=914–923 | last1 = Kapeller | first1 = Dagmar| issue=4 | pmid=17243828 }}</ref>

:[[File:PhosphatePhosphonateRearrangement.png|none|500px|Chiral oxy[<sup>2</sup>''H''<sub>1</sub>]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 (chemistry)|alcohol]] '''4'''. Once again in the range of −78&nbsp;°C to 0&nbsp;°C the chirality is preserved in this reaction sequence. ([[Enantioselectivity]] was determined by [[NMR spectroscopy]] after derivatization with [[Mosher's acid]].)

==History==
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.<ref>{{Cite journal| doi = 10.1039/CT9079100694| title = LXV. An extension of the benzoin synthesis| year = 1907| last1 = Clarke | first1 = R. W. L.| last2 = Lapworth | first2 = A.| journal = Journal of the Chemical Society, Transactions| volume = 91| pages = 694–705| url = https://zenodo.org/record/1429713}}</ref> In 1904 [[Wilhelm Schlenk]] prepared {{chem2|[Ph3C]−[NMe4]+}} in a quest for tetramethylammonium (from [[tetramethylammonium chloride]] and 
[[Organosodium chemistry|{{chem2|Ph3CNa}}]])<ref name=":1">{{Cite journal| doi = 10.1002/jlac.19103720102| title = Ueber Triphenylmethyl und Analoga des Triphenylmethyls in der Biphenylreihe|trans-title=On triphenylmethyl and analogues of triphenylmethyl in the biphenyl series| year = 1910| last1 = Schlenk | first1 = W.| last2 = Weickel | first2 = T.| last3 = Herzenstein | first3 = A.| journal = Justus Liebig's Annalen der Chemie| volume = 372| pages = 1–20| url = https://zenodo.org/record/1427585}}</ref> and in 1914 he demonstrated how triarylmethyl radicals could be reduced to carbanions by alkali metals <ref>{{Cite journal| doi = 10.1002/cber.19140470256| title = Über Metalladditionen an freie organische Radikale. XII. Über Triarylmethyle |trans-title=On metal addition to free organic radicals. XII. Tryarylmethyls| year = 1914| last1 = Schlenk | first1 = W.| last2 = Marcus | first2 = E.| journal = Berichte der Deutschen Chemischen Gesellschaft| volume = 47| issue = 2| pages = 1664| url = https://zenodo.org/record/1426557}}</ref> The phrase carbanion was introduced by Wallis and Adams in 1933 as the negatively charged counterpart of the [[carbonium ion]]<ref>{{Cite journal| doi = 10.1021/ja01336a068| year = 1933| last1 = Wallis | first1 = E. S.| title = The Spatial Configuration of the Valences in Tricovalent Carbon Compounds1| last2 = Adams | first2 = F. H.| journal = Journal of the American Chemical Society| volume = 55| issue = 9| pages = 3838}}</ref><ref>{{Cite journal| doi = 10.1351/pac199769020211| title = The first century of physical organic chemistry: A prologue| year = 1997| last1 = Tidwell | first1 = T. T.| journal = Pure and Applied Chemistry| volume = 69| issue = 2| pages = 211–214| s2cid = 98171271| doi-access = free}}</ref>

==See also==
*[[Carbocation]]
*[[Enol#Enolates|Enolate]]s
*[[Nitrile anion]]

==References==
{{Reflist}}

==External links==
* Large database of Bordwell p''K''<sub>a</sub> values at www.chem.wisc.edu [https://web.archive.org/web/20081009060809/http://www.chem.wisc.edu/areas/reich/pkatable/ Link]
* Large database of Bordwell p''K''<sub>a</sub> values at daecr1.harvard.edu [https://web.archive.org/web/20061212181531/http://daecr1.harvard.edu/pdf/evans_pKa_table.pdf Link]
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[[Category:Anions]]
[[Category:Reactive intermediates]]