2. Carbanions are the anion, i.e., negatively charged species in which a carbon carries three
bonds and a lone pair of electrons, thus making the carbon atom negatively charged.
Every carbanion possesses unshared pair of electrons and three pairs of bonding electrons
around the central carbon atoms, which is sp3 hybridized.
The origins of carbanion chemistry are rooted in synthesis given the great utility of these
species in forming new C-C bonds.
Carbanions provide a rich chemistry and have been the subject of intense study over the past
100 years, partly because of their central role in important synthetic reaction schemes. Given
the low electronegativity of carbon, it is not surprising that carbon-centered anions can be
highly reactive and appear as transient intermediates in reaction mechanisms; however, when
certain conditions are met, they can be prepared and studied as stable species.
DR ABDUL ROOUF BHAT
Carbanions:
3. Structure Of Carbanions Geometries
An sp3 hybridized carbanion will prefer a structure where the central carbon
adopts a trigonal-pyramidal geometry with a lone-pair occupying one of the
typical tetrahedral valences. This result is in direct analogy to a nitrogen
containing species like NH3, a compound that is isoelectronic with CH3. It is
experimentally difficult to explore the structural details of bare carbanions, a great
deal has been discovered by computational work. High-level work on the methyl
anion indicates a pyramidal structure with HCH angles (109.4) very much like
those found in CH4 (i.e., a tetrahedral arrangement).
DR ABDUL ROOUF BHAT
4. DR ABDUL ROOUF BHAT
The geometry of carbanion stabilized by conjugated with
substituents is quite different. The carbanions which are
greatly stabilized by conjugation are planer in so far as the
delocalized electrons system is concerned.
Thus the nitromethane anion has all of its atoms in a
plane, which sp2 hybridization at both carbon and
nitrogen
5. DR ABDUL ROOUF BHAT
Malonic ester anion is planer, only the hydrogens on the ethyl group lies out of the molecular
plane
6. DR ABDUL ROOUF BHAT
The heterolytic cleavage of alkanes is a vary difficult process. the electronegativity difference
between Carbon and Hydrogen (C-H) is very less. pKa values are very high.
7. DR ABDUL ROOUF BHAT
Carbanions are formed only from those compounds which contain a functional group capable
of weakening a nearby C-H bond. Thus hydrogen on the carbon atoms a to -NO2, -CN, -
COOH, -CO, and CCH are acidic and can be removed as proton leaving resonance stabilized
anions.
8. DR ABDUL ROOUF BHAT
The Carbon attached to groups
such as –SO3R, –COOH are not
electronegative enough to be
sufficiently acidic like –CN
group. However, two such groups
attached to the same carbon atom
remarkably enhance the acidity of
C-H bond.
Carbanions are also formed when nucleophile adds to a, b- unsaturated compounds
9. Metal salts of carbanions have been widely used in synthesis for >100 years. The best known
example is the Grignard reagent (RMgX), where bonding of Mg to a carbanion center leads to
a species that is relatively easy to prepare and handle. The initial evidence of
organomagnesium compounds actually predates Grignard and goes back to the middle of the
nineteenth century
Organolithium compounds were investigated later and are more difficult to handle because
the C-Li bond has greater ionic character, yielding a more carbanion-like salt with higher
reactivity.
The modern study of carbanions as reactive intermediates and the investigation of their
physical properties begins with early efforts to gauge their basicity.
DR ABDUL ROOUF BHAT
11. DR ABDUL ROOUF BHAT
Order of substituents with respect to their ability to stabilize carbanion
12. DR ABDUL ROOUF BHAT
1. Conjugative Effect or Delocalization of Negative Charge : Stability of allylic and benzylic
cations)
Stability of Carbanions
(C6H5)3C- > (C6H5)2CH- > (C6H5)2CH2
-
13. DR ABDUL ROOUF BHAT
Carbanions Increase in Stability with an Increase in the Amount of s Character at
the Carbanionic Carbon
Stabilization by Sulfur or Phosphorus.
14. DR ABDUL ROOUF BHAT
Field Effects Carbanions are stabilized by a field effect if there is any heteroatom (O, N, or
S) connected to the carbanionic carbon, provided that the heteroatom bears a positive charge
in at least one important canonical form
15. DR ABDUL ROOUF BHAT
Certain Carbanions are Stable because they are Aromatic
Stabilization by a Nonadjacent p Bond: Ions in which a negatively charged carbon is
stabilized by a carbonyl group two carbons away, are called homoenolate ions.
