Everything about Carbanion totally explained
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 . The carbanion exists in a
trigonal pyramidal geometry. Formally a carbanion is the
conjugate base of a
carbon acid.
» R
3C-H + B
− → R
3C
− + 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
- The 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 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:
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 . 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 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
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 :
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 .
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 :
Of recent date are chiral methyllithium compounds :
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 .
Further Information
Get more info on 'Carbanion'.
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