carbanion

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 [1]. 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. A carbanion is one of several reactive intermediates in organic chemistry.

Theory

A carbanion is a nucleophile. The stability and reactivity of a carbanion is determined by several factors. These include
  1. The inductive effect. Electronegative atoms adjacent to the charge will stabilize the charge;
  2. Hybridisation of the charge-bearing atom. The greater the s-character of the charge-bearing atom, the more stable the anion;
  3. 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[2]:

Enlarge picture
Formation of the triphenylmethane anion


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 [3]. 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 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
diphenylmethane32.3
aniline30.6
triphenylmethane30.6
xanthene30
ethanol29.8
phenylacetylene28.8
thioxanthene28.6
acetone26.5
benzoxazole24.4
fluorene22.6
indene20.1
phenylacetylene28.8
cyclopentadiene18
acetylacetone13.3
acetic acid12.6
malononitrile11.2
meldrum's acid7.3
Table 1. Carbon acid acidities in pKa in DMSO [4]. 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.

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 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 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 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 [5]:

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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 [6].

Enlarge picture
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 [7]:

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Optical Stability of 1-Methyl-2,2-diphenylcyclopropyllithium


Of recent date are chiral methyllithium compounds [8]:

Enlarge picture
Chiral -Oxy-[2H1]methyllithiums, Bu stands for butyl, i-Pr stands for isopropyl


The 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 [9].

External links

  • Large database of Bordwell pKa values at www.chem.wisc.edu Link
  • Large database of Bordwell pKa values at daecr1.harvard.edu Link

See also

References

1. ^ Organic Chemistry - Robert Thornton Morrison, Robert Neilson Boyd
2. ^ 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
3. ^ 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
4. ^ Equilibrium acidities in dimethyl sulfoxide solution Frederick G. Bordwell Acc. Chem. Res.; 1988; 21(12) pp 456 - 463; doi:10.1021/ar00156a004
5. ^ FORMATION OF OPTICALLY ACTIVE 1-METHYLHEPTYLLITHIUM Robert L. Letsinger J. Am. Chem. Soc.; 1950; 72(10) pp 4842 - 4842; doi:10.1021/ja01166a538
6. ^ 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
7. ^ 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
8. ^ 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
9. ^ Enantioselectivity determined by NMR spectroscopy after derivatization with Mosher's acid
4, 2
(mildly acidic oxide)
Electronegativity 2.55 (Pauling scale)
Ionization energies
(more) 1st: 1086.5 kJmol−1
2nd: 2352.6 kJmol−1
3rd: 4620.5 kJmol−1

Atomic radius 70 pm
Atomic radius (calc.
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Electron

Theoretical estimates of the electron density for the first few hydrogen atom electron orbitals shown as cross-sections with color-coded probability density
Composition: Elementary particle
Family: Fermion
Group: Lepton
Generation: First
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trigonal pyramid is a molecular geometry with one atom at the apex and three atoms at the corners of a trigonal base. When all three atoms at the corners are identical the molecule belongs to point group C3V.
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conjugate acid is the acid member, HX, of a pair of two compounds that transform into each other by gain or loss of a proton. A conjugate acid can also be seen as the chemical substance that releases a proton in the backward chemical reaction. Thus, the term acid.
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The IUPAC Gold Book[1] defines a reaction intermediate or an intermediate as a molecular entity (atom, ion, molecule...) with a lifetime appreciably longer than a molecular vibration (corresponding to a local potential energy minimum of depth greater than RT; R
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Organic chemistry is a specific discipline within chemistry which involves the scientific study of the structure, properties, composition, reactions, and preparation (by synthesis or by other means) of chemical compounds consisting primarily of carbon and hydrogen, which may
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In chemistry, a nucleophile (literally nucleus lover as in nucleus and phile) is a reagent that forms a chemical bond to its reaction partner (the electrophile) by donating both bonding electrons.
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The inductive effect in chemistry is an experimentally observable effect of the transmission of charge through a chain of atoms in a molecule by electrostatic induction (IUPAC definition).
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hybridisation or hybridization (see also spelling differences) is the concept of mixing atomic orbitals to form new hybrid orbitals suitable for the qualitative description of atomic bonding properties.
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conjugated system is a system of atoms covalently bonded with alternating single and multiple (e.g. double) bonds (e.g., C=C-C=C-C) in a molecule of an organic compound. This system results in a general delocalization of the electrons across all of the adjacent parallel aligned
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Resonance in chemistry is a tool used to represent and model certain types of non-classical molecular structures.
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Aromaticity is a chemical property in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibit a stabilization stronger than would be expected by the stabilization of conjugation alone.
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The IUPAC Gold Book[1] defines a reaction intermediate or an intermediate as a molecular entity (atom, ion, molecule...) with a lifetime appreciably longer than a molecular vibration (corresponding to a local potential energy minimum of depth greater than RT; R
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Organic chemistry is a specific discipline within chemistry which involves the scientific study of the structure, properties, composition, reactions, and preparation (by synthesis or by other means) of chemical compounds consisting primarily of carbon and hydrogen, which may
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The E1cB elimination reaction is a special type of elimination reaction in organic chemistry. This reaction mechanism explains the formation of alkenes from (mostly) alkyl halides through a carbanion intermediate given specified reaction conditions and specified substrates.
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Organometallic chemistry is the study of chemical compounds containing bonds between carbon and a metal.[1] Since many compounds without such bonds are chemically similar, an alternative may be compounds containing metal-element bonds of a largely covalent character.
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The Grignard reaction, named for the French chemist François Auguste Victor Grignard, is an organometallic chemical reaction involving alkyl- or aryl-magnesium halides, also called Grignard reagents, with electrophiles.
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Crown ethers are heterocyclic chemical compounds that, in their simplest form, are cyclic oligomers of ethylene oxide. The essential repeating unit of any simple crown ether is ethyleneoxy, i.e.
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Salt is a mineral essential for animal life, composed primarily of sodium chloride. Salt for human consumption is produced in different forms: unrefined salt (such as sea salt), refined salt (table salt), and iodized salt.
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Crown ethers are heterocyclic chemical compounds that, in their simplest form, are cyclic oligomers of ethylene oxide. The essential repeating unit of any simple crown ether is ethyleneoxy, i.e.
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The chemical compound n-butyllithium (abbreviated BuLi) is the most prominent organolithium reagent. It enjoys wide use as a polymerisation initiator in the production of elastomers such as polybutadiene or styrene-butadiene-styrene (SBS).
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Triphenylmethane, or triphenyl methane, is a hydrocarbon. It is a colorless to light brown powder. It is the basic skeleton of many synthetic dyes called triarylmethane dyes, many of them are pH indicators, and some display fluorescence.
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Crown ethers are heterocyclic chemical compounds that, in their simplest form, are cyclic oligomers of ethylene oxide. The essential repeating unit of any simple crown ether is ethyleneoxy, i.e.
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In molecular geometry, bond length or bond distance is the distance between two bonded atoms in a molecule.

Explanation

Bond length is directly related to bond order, when more electrons participate in bond formation the bond will get shorter.
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Proton NMR (also Hydrogen-1 NMR, or 1HNMR) is the application of nuclear magnetic resonance in NMR spectroscopy with respect to hydrogen.

Simple NMR spectra are recorded in solution, and solvent protons must not be allowed to interfere.
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Cyclopentadiene is the chemical compound with the formula C5H6. This colorless liquid organic chemical has a strong and upleasant odor. At room temperature, this cyclic diene dimerizes over the course of hours to give dicyclopentadiene via a Diels-Alder
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methylene is a divalent functional group CH2 derived formally from methane. The name "methylene" is used for the following:
  • −CH2− group (bound by two single bonds) present e.g. in dichloromethane (alternative name is methylene chloride).

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A cyclopentadienyl complex is a metal complex with one or more cyclopentadienyl groups (C5H5-, abbreviated as Cp-). Based on the type of bonding between the metals and the cyclopentadienyl moieties, cyclopentadienyl complexes are
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PKA or pKa may be:
  • PKA, an acronym for Previously Known As, or Professionally Known AS
  • PKA, an acronym for protein kinase A
  • pKa, the symbol for Acid dissociation constant
  • Pi Kappa Alpha
  • PKA, an acronym for Public Key Authentication

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Methane is a chemical compound with the molecular formula CH4. It is the simplest alkane, and the principal component of natural gas. Methane's bond angles are 109.
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