1. Carbanions

2. INTRODUCTION
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.

3. DEVELOPMENT OF CARBANION CHEMISTRY
The origins of carbanion chemistry are rooted in synthesis given the great utility of these species in forming new C-C bonds.
Carbanions are highly nucleophilic and rarely undergo rearrangement reactions so they are very attractive reactive intermediates from a synthetic point of view.
Metal salts of carbanions have been widely used in synthesis for >100 years.
The best known example is the Grignard reagent 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

4. DEVELOPMENT OF CARBANION CHEMISTRY
The modern study of carbanions as reactive intermediates and the investigation of their physical properties begins with early efforts to gauge their basicity.
In their classic paper, ‘‘The Study of Extremely Weak Acids,’’ Conant and Wheland qualitatively explored the basicity of a range of stabilized carbanions such as polyarylmethides by allowing their sodium or potassium salts to react with a carbon acid and using color changes to indicate whether a proton transfer had occurred or not

5. 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 valencies.
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)

6. Stereochemistry and Racemization
For a carbanion derived from a tetrahedral carbon to exhibit chirality, it must either remain pyramidal or if inversion occurs, one face must remain distinct from the other.
The latter requirement can be attained by the presence of chiral auxiliaries apart from the carbanion center, or from a barrier to rotation in the carbanion that locks the system into a single, asymmetric conformation.
The inversion barrier in the bare methyl anion is exceedingly small, ~2 kcal/mol, so no stereochemical integrity is expected.
Most studies have shown that simple sp3 hybridized carbanions generally are unable to retain their asymmetry even under conditions where they react soon after being formed.

7. Stereochemistry and Racemization
As the C–M bond becomes more covalent, this effect becomes more important and chiral mercury salts of carbanions are easily prepared and retain their asymmetry under most conditions.
For lithium salts, a few structural motifs have been identified that lead to sp3 hybridized carbanions with appreciable stereochemical integrity.
One of the earliest to be identified was the cyclopropyl carbanion. In this case, inversion is hindered by ring strain (in the inversion, the carbanion must become sp2 hybridized) and a large inversion barrier, 16.3 kcal/mol, has been predicted by high-level ab initio calculations

8. Magnetic Properties and Nuclear Magnetic Resonance
The carbanion center is naturally electron-rich so NMR signals associated with it will appear unusually far upfield.
Isopropyllithium gives an hydrogen NMR (1H NMR) around 0.7 ppm for the proton on the secondary carbon.
The 13C NMR signal for a carbanion is also upfield from TMS and methyllithium gives a carbon resonance in the range of -10 to -20 ppm depending on the solvent and temperature.

9. Comparison of Carbocation and Carbanion

10. Stabilization of Carbanions
Conjugation of the Unshared Pair with an Unsaturated Bond.
Carbanions Increase in Stability with an Increase in the Amount of s Character at the Carbanionic Carbon
Stabilization by Sulfur or Phosphorus.
Field Effects.
Certain Carbanions are Stable because they are Aromatic
Stabilization by a Nonadjacent p Bond

11. 1. Conjugation of the Unshared Pair with an Unsaturated Bond

12. 3. Stabilization by Sulfur or Phosphorus.
2. Carbanions Increase in Stability with an Increase in the Amount of s Character at the Carbanionic Carbon
3. Stabilization by Sulfur or Phosphorus.

13. 4. 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

14. 5. Certain Carbanions are Stable because they are Aromatic
6. 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.

15. 6.1. Acidity of Hydrocarbons
The basicity of a base-solvent system can be specified by a basicity function H−.
The value of H− corresponds essentially to the pH of strongly basic nonaqueous solutions.
The larger the value of H−, the greater the proton-abstracting ability of the medium.

22. Comparison of Carbocation and Carbanion Stability

23. GENERATION OF CARBANIONS
1. A group attached to a carbon leaves without its electron pair 2. A negative ion adds to a carbon–carbon double or triple bond