# Carbanion

A carbanion is an anion in which a tervalent carbon atom carries an unshared pair of electrons and a formal negative charge, or, for a mesomeric (resonance-delocalized) ion, has at least one significant contributing structure of that kind; examples include \( \ce{Cl3C^{-}} \) and the acetylide ion \( \ce{HC#C^{-}} \).<sup>[1](https://goldbook.iupac.org/terms/view/C00804/plain)</sup> Carbanions are the carbon-centered counterparts of carbocations: they are strong bases and strong nucleophiles, and in practice they are most often encountered as the carbon component of organometallic compounds such as methyllithium (\( \ce{CH3Li} \)) or methylmagnesium chloride (\( \ce{CH3MgCl} \)), where the electropositive metal stabilizes the negative charge on the more electronegative carbon.<sup>[2](https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_General_Chemistry%3A_Principles_Patterns_and_Applications_%28Averill%29/23%3A_Organic_Compounds/23.04%3A_Reactivity_of_Organic_Molecules)</sup>

| Key fact | Detail |
|---|---|
| Definition | Anion with an even number of electrons and an unshared pair on a tervalent carbon atom, or at least one significant contributing structure with such a pair (IUPAC)<sup>[1](https://goldbook.iupac.org/terms/view/C00804/plain)</sup> |
| Geometry | Simple alkyl carbanions are typically sp³-hybridized with the lone pair in a hybrid orbital; resonance-stabilized carbanions such as enolates place the lone pair in a p orbital (sp²)<sup>[3](https://openstax.org/books/organic-chemistry/pages/22-5-acidity-of-alpha-hydrogen-atoms-enolate-ion-formation)</sup> |
| Hybridization and stability | Stability increases with s character: sp³ alkyl carbanions are least stable, sp² vinylic more stable, and sp acetylide anions most stable, because higher s character holds the charge closer to the nucleus<sup>[4](https://openstax.org/books/organic-chemistry/pages/9-summary)</sup> |
| Acidity scale | Acetone (pK\(_a\) 19.3) is more acidic than ethane (pK\(_a\) about 60) by a factor of 10\(^{40}\) because the enolate conjugate base is resonance-stabilized<sup>[3](https://openstax.org/books/organic-chemistry/pages/22-5-acidity-of-alpha-hydrogen-atoms-enolate-ion-formation)</sup> |
| Common generation | Deprotonation with strong bases; lithium diisopropylamide (LDA), whose conjugate acid has pK\(_a\) 36, converts most carbonyl compounds completely into enolate ions<sup>[3](https://openstax.org/books/organic-chemistry/pages/22-5-acidity-of-alpha-hydrogen-atoms-enolate-ion-formation)</sup> |
| Practical form | Free carbanions are rare; the species used in synthesis are organometallics, and organolithiums are the most widely used organometallics in contemporary organic chemistry<sup>[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC3145419/)</sup><sup> • </sup><sup>[2](https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_General_Chemistry%3A_Principles_Patterns_and_Applications_%28Averill%29/23%3A_Organic_Compounds/23.04%3A_Reactivity_of_Organic_Molecules)</sup> |
| Signature reactions | Alkylation of enolates and acetylides at carbon, and conjugate (Michael) addition to α,β-unsaturated carbonyl compounds<sup>[6](https://openstax.org/books/organic-chemistry/pages/9-8-alkylation-of-acetylide-anions)</sup><sup> • </sup><sup>[7](https://openstax.org/books/organic-chemistry/pages/23-10-conjugate-carbonyl-additions-the-michael-reaction)</sup> |

## Structure and geometry

A simple carbanion has three substituents plus one lone pair, for a total of eight valence electrons on the anionic carbon. In the absence of resonance, the lone pair occupies a hybrid orbital and the carbon is pyramidal, as in an sp³-hybridized alkyl anion. When the negative charge can be delocalized, the geometry changes: the α carbon of an enolate ion is sp²-hybridized, with a p orbital that overlaps the neighboring carbonyl p orbitals, so the charge is shared between the α carbon and the electronegative oxygen.<sup>[3](https://openstax.org/books/organic-chemistry/pages/22-5-acidity-of-alpha-hydrogen-atoms-enolate-ion-formation)</sup>

**Hybridization controls stability.** Because an sp orbital has 50 percent s character, an sp² orbital about 33 percent, and an sp³ orbital 25 percent, the negative charge of an sp-hybridized carbanion is held closest to the nucleus and is the most stabilized. This is why acetylide anions (\( \ce{R-C#C:^{-}} \)), formed by removing a proton from a terminal alkyne, are more stable than either alkyl anions or vinylic anions.<sup>[4](https://openstax.org/books/organic-chemistry/pages/9-summary)</sup> The stability order runs sp³ alkyl carbanion, then sp² vinylic or allylic, then sp acetylide, with carbonyl-stabilized (enolate) anions stabilized further by delocalization onto oxygen.<sup>[3](https://openstax.org/books/organic-chemistry/pages/22-5-acidity-of-alpha-hydrogen-atoms-enolate-ion-formation)</sup>

## Stability and the acidity of carbon acids

A carbanion is the conjugate base of a carbon acid, so the stability of the anion is read directly from the pK\(_a\) of the acid that generates it. Simple alkanes are extremely weak acids; ethane has pK\(_a\) of about 60, and its conjugate base is among the strongest bases encountered in organic chemistry. A carbonyl group flanking the acidic position changes the picture dramatically: acetone, with pK\(_a\) 19.3, is more acidic than ethane by a factor of 10\(^{40}\), because the resulting enolate shares its negative charge between the α carbon and the oxygen atom.<sup>[3](https://openstax.org/books/organic-chemistry/pages/22-5-acidity-of-alpha-hydrogen-atoms-enolate-ion-formation)</sup>

**Dicarbonyl compounds are more acidic still.** When the acidic hydrogen sits between two carbonyl groups, the enolate is stabilized by both neighboring carbonyl oxygens. Approximate pK\(_a\) values place 1,3-diketones at 9, 3-keto esters at 11, and 1,3-diesters at 13, all more acidic than water; the enolate of 2,4-pentanedione has three resonance forms.<sup>[3](https://openstax.org/books/organic-chemistry/pages/22-5-acidity-of-alpha-hydrogen-atoms-enolate-ion-formation)</sup>

**Aromaticity provides an additional stabilizing mechanism.** In cyclic carbanions such as the cyclopentadienyl anion (\( \ce{C5H5^{-}} \)), the lone pair participates in the π system and completes an aromatic sextet. Calculated magnetic properties support this: IGLO magnetic susceptibility exaltations and GIAO nucleus-independent chemical shifts show that the cyclopentadienyl, indenyl (\( \ce{C9H7^{-}} \)), and fluorenyl (\( \ce{C13H9^{-}} \)) anions are highly aromatic, comparable to benzene, naphthalene, and anthracene respectively.<sup>[8](https://pubs.acs.org/doi/10.1021/ja970380x)</sup>

## Generation

Carbanions are generated in three principal ways: deprotonation of a carbon acid, reductive lithiation of sulfides, selenides, tellurides, or halides, and transmetalation, most commonly tin–lithium exchange between an organostannane and butyllithium.<sup>[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC3145419/)</sup> Deprotonation is useful only when the conjugate acid is activated in some way, for example by a carbonyl group, a heteroatom directing group, a double bond, or an aromatic ring.<sup>[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC3145419/)</sup>

**Base strength matters.** Because carbonyl compounds are only weakly acidic, a strong base is needed to form enolate ions completely. With sodium ethoxide (the conjugate acid, ethanol, has pK\(_a\) 16), deprotonation of acetone proceeds only to the extent of about 0.1 percent. [Lithium diisopropylamide](https://www.edgepedia.org/lithium-diisopropylamide), \( \ce{LiN(i-C3H7)2} \), prepared from butyllithium and diisopropylamine, has a conjugate acid with pK\(_a\) 36 and converts aldehydes, ketones, esters, thioesters, acids, and amides into their enolate ions.<sup>[3](https://openstax.org/books/organic-chemistry/pages/22-5-acidity-of-alpha-hydrogen-atoms-enolate-ion-formation)</sup>

## Carbanions in organometallic compounds

A free carbanion in solution is uncommon; the working form is almost always an organometallic reagent. In methyllithium or a [Grignard reagent](https://www.edgepedia.org/grignard-reagent) such as methylmagnesium chloride, the electropositive metal stabilizes the negative charge on carbon.<sup>[2](https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_General_Chemistry%3A_Principles_Patterns_and_Applications_%28Averill%29/23%3A_Organic_Compounds/23.04%3A_Reactivity_of_Organic_Molecules)</sup>

**The carbon–lithium bond is largely ionic.** Collins and Streitwieser calculated methyllithium to be 79.7 percent ionic, and the prevailing theoretical view treats the C–Li bond as best described as ionic, although a review of the question concludes that the nature of the bond varies from compound to compound and that covalent components cannot be neglected. Authors alternatively describe the bond as a contact ion pair, with lithium and carbon in van der Waals contact, or a solvent-separated ion pair; unless the carbanion is strongly stabilized by delocalization and the lithium ion by solvation, the lithium cation does not stray far.<sup>[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC3145419/)</sup>

**Aggregation complicates structure.** Organolithium compounds tend to aggregate, and their aggregation states in solution are influenced by the solvent and the bulkiness of the alkyl residue.<sup>[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC3145419/)</sup><sup> • </sup><sup>[9](https://www.sciencedirect.com/topics/medicine-and-dentistry/organolithium-compound)</sup> [Nuclear magnetic resonance](https://www.edgepedia.org/nuclear-magnetic-resonance) methods, including experiments with the \(^{6}\)Li isotope and diffusion-ordered spectroscopy, are the main tools for characterizing these solution structures.<sup>[10](https://www.thieme-connect.de/products/ejournals/html/10.1055/s-0037-1610162?device=desktop&innerWidth=412&offsetWidth=412)</sup>

## Reactions

**Alkylation at carbon.** The unshared electron pair makes carbanions strongly nucleophilic. An acetylide anion attacks the electrophilic carbon of an alkyl halide in an S\(_{N}\)2 process, displacing halide and forming a new carbon–carbon bond; this alkylation of acetylide anions with primary alkyl halides is one of the standard methods for building larger alkynes.<sup>[6](https://openstax.org/books/organic-chemistry/pages/9-8-alkylation-of-acetylide-anions)</sup><sup> • </sup><sup>[4](https://openstax.org/books/organic-chemistry/pages/9-summary)</sup> [Enolate](https://www.edgepedia.org/enolate) ions behave similarly: because they are resonance hybrids of a vinylic alkoxide form and an α-keto carbanion form, they can react with electrophiles either on oxygen or on carbon, and reaction on carbon, giving an α-substituted carbonyl compound, is the more common outcome.<sup>[11](https://openstax.org/books/organic-chemistry/pages/22-6-reactivity-of-enolate-ions)</sup>

**Conjugate addition.** Enolate ions also add to the β carbon of α,β-unsaturated carbonyl compounds, the Michael reaction, named for Arthur Michael of Tufts College and Harvard University. The best Michael reactions pair a particularly stable enolate donor, such as one derived from a β-keto ester or other 1,3-dicarbonyl compound, with an unhindered α,β-unsaturated ketone acceptor; unsaturated aldehydes, esters, thioesters, nitriles, amides, and nitro compounds can all serve as acceptors.<sup>[7](https://openstax.org/books/organic-chemistry/pages/23-10-conjugate-carbonyl-additions-the-michael-reaction)</sup>

**Electrophilic substitution at the metal-bearing carbon.** In chiral organolithium compounds, replacement of lithium by an electrophile can proceed with inversion (S\(_{E}\)2inv), with retention (S\(_{E}\)2ret), or by single electron transfer, in which the carbanion is oxidized to a rapidly inverting radical and coupling affords racemic products. Which pathway operates depends on the electrophile and on whether the carbanion inverts faster or slower than it reacts; some organolithiums are configurationally stable for hours, others for only seconds.<sup>[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC3145419/)</sup>

## References

1. [Carbanion, IUPAC Compendium of Chemical Terminology (Gold Book), C00804](https://goldbook.iupac.org/terms/view/C00804/plain)
2. [Reactivity of Organic Molecules, Chemistry LibreTexts (Averill, General Chemistry)](https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_General_Chemistry%3A_Principles_Patterns_and_Applications_%28Averill%29/23%3A_Organic_Compounds/23.04%3A_Reactivity_of_Organic_Molecules)
3. [22.5 Acidity of Alpha Hydrogen Atoms: Enolate Ion Formation, OpenStax Organic Chemistry](https://openstax.org/books/organic-chemistry/pages/22-5-acidity-of-alpha-hydrogen-atoms-enolate-ion-formation)
4. [Ch. 9 Summary, OpenStax Organic Chemistry](https://openstax.org/books/organic-chemistry/pages/9-summary)
5. [Robert E. Gawley (2010). Overview of Carbanion Dynamics and Electrophilic Substitutions in Chiral Organolithium Compounds. Topics in Stereochemistry 26, 93–133. PMC3145419](https://pmc.ncbi.nlm.nih.gov/articles/PMC3145419/)
6. [9.8 Alkylation of Acetylide Anions, OpenStax Organic Chemistry](https://openstax.org/books/organic-chemistry/pages/9-8-alkylation-of-acetylide-anions)
7. [23.10 Conjugate Carbonyl Additions: The Michael Reaction, OpenStax Organic Chemistry](https://openstax.org/books/organic-chemistry/pages/23-10-conjugate-carbonyl-additions-the-michael-reaction)
8. [Magnetic Evidence for the Aromaticity and Antiaromaticity of Charged Fluorenyl, Indenyl, and Cyclopentadienyl Systems, Journal of the American Chemical Society](https://pubs.acs.org/doi/10.1021/ja970380x)
9. [Organolithium Compound, ScienceDirect Topics overview](https://www.sciencedirect.com/topics/medicine-and-dentistry/organolithium-compound)
10. [Recent Advances in NMR: Aggregation, Solvation, and Dynamics of Selected Organolithium Compounds, Thieme](https://www.thieme-connect.de/products/ejournals/html/10.1055/s-0037-1610162?device=desktop&innerWidth=412&offsetWidth=412)
11. [22.6 Reactivity of Enolate Ions, OpenStax Organic Chemistry](https://openstax.org/books/organic-chemistry/pages/22-6-reactivity-of-enolate-ions)

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*Topic: Encyclopedia › Physical world and mathematics › Chemistry › Organic substances › Organic reactions, structure and reference › Organic reactions and synthetic methods › Physical organic chemistry and reaction mechanisms › Reactive intermediates › Carbanions and enolates*

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