Carbanions: Structure, Stability, Formation and Reactivity

Carbanions

A carbanion is an anion in which a carbon atom bears a negative charge and possesses a lone pair of electrons. Carbanions are important reactive intermediates in many organic reactions.

Structure and Characteristics

Caption: Structure of Carbanion (8 electrons, sp³ hybridized Carbon and Pyramidal geometry)

• Structure: A carbanion is typically represented as R₃C⁻. The carbon forms three covalent bonds and carries a lone pair, fulfilling the octet.
• Geometry: The geometry is generally pyramidal (like ammonia, NH₃). The negatively charged carbon undergoes rapid inversion. Hybridization is usually sp³.
• Reactivity: Carbanions behave as:
  • Nucleophiles: They attack electron-deficient centers, forming new C–C or C–heteroatom bonds.
  • Lewis Bases: They donate their lone electron pair to protons or other electrophiles.

Stability of Carbanions

Carbanion stability depends on the ability to delocalize or disperse the negative charge. Unlike carbocations, electron-donating groups destabilize carbanions.

General stability order (simple alkyl carbanions): Methyl > Primary > Secondary > Tertiary

Stabilizing Factors:

• Resonance (π-delocalization): Delocalization of negative charge over adjacent π-systems (allylic/benzylic carbanions) greatly increases stability.


• Inductive Effects: Nearby electron-withdrawing groups (–NO₂, –CN) stabilize carbanions by pulling electron density away.


• Hybridization (s-character): Stability increases with higher s-character due to the greater electronegativity of s-orbitals, which better accommodate and stabilize the negative charge.
  sp > sp² > sp³.
  sp hybridization: Contains 50% s-character (one s and one p orbital).
  sp² hybridization: Contains 33% s-character (one s and two p orbitals).
  sp³ hybridization: Contains 25% s-character (one s and three p orbitals).
  
• Solvent Effects: Carbanions are more stable in aprotic or non-polar solvents (like hydrocarbons or THF) that are poor at stabilizing or donating electrons to the negatively charged carbon. Protic solvents (like water) destabilize carbanions by donating H-bond protons to the lone pair.

Formation and Reactions

Carbanions are generally formed by the deprotonation of C–H bonds using very strong bases.

General reaction:

R₃C–H + Base → R₃C⁻ + Base–H

• When a group or atom departs from a carbon atom without its bonding pair.


• When a negative ion attacks on multiple bond.

Reactions of Carbocations

• Nucleophilic Substitution: Carbanions, being nucleophilic, can undergo nucleophilic substitution reactions, where they displace a leaving group in a substrate.
  R-LG + Nu⁻ → R-Nu + LG⁻
• Addition Reactions: Carbanions can participate in addition reactions, such as 1,4-addition (Michael addition) to α,β-unsaturated carbonyl compounds, where the carbanion adds to the β-carbon of the carbonyl group.
  Carbanion + Electrophilic Substrate → Product
• Alkylation and Acylation: Carbanions are commonly used in alkylation and acylation reactions, where they act as nucleophiles and react with alkyl or acyl halides to form new carbon-carbon bonds.
  Alkylation: Carbanion + Alkyl Halide → Product
  Acylation: Carbanion + Acyl Halide → Product
• Formation of Organometallic Reagents: Carbanions are involved in the formation of organometallic reagents, such as organolithium and Grignard reagents.
  R-X + Li → RXLi
  
  R-X + Mg → RMgX Alkyl Halide + Lithium → Organolithium Reagent
• Enolate formation: Removal of the α-hydrogen in carbonyl compounds leads to enolates (important in Aldol condensation and Claisen condensation).

Common Carbanion Rearrangements

Carbanion rearrangements are reactions where the carbanion intermediate undergoes an internal structural change to achieve greater stability. These can proceed via radical-pair dissociation-recombination or concerted mechanisms.

• Wittig Rearrangements: These are named [2,3] or [1,2] sigmatropic rearrangements. The [2,3] Wittig rearrangement typically proceeds via a concerted mechanism, while the [1,2]$ Wittig rearrangement can proceed via a radical-pair dissociation-recombination mechanism.
• Brook Rearrangement: In this mechanism, a silyl group migrates toward an alcoholate (alkoxide), which liberates another carbanion. This allows the subsequent carbanion to participate in a new nucleophilic attack, enabling sequential anion-dependent reactions.
• Benzilic Acid Rearrangement: This reaction involves a carbanionic 1,2-shift (a rearrangement where a group moves from one carbon to the adjacent carbon) and is synthetically important for forming new carbon-carbon bonds and introducing functional groups.

Comparison Table: Carbanions vs. Carbocations

Feature	Carbanion	Carbocation
Charge	Negative (C⁻ with lone pair)	Positive (C⁺ electron-deficient)
Hybridization	Usually sp3	Usually sp2
Geometry	Pyramidal (inversion possible)	Planar (trigonal planar)
Stability Order (Alkyl)	Methyl > 1° > 2° > 3°	3° > 2° > 1° > Methyl
Stabilizing Factor	Electron-withdrawing groups, resonance	Electron-donating groups, resonance
Reactivity	Acts as nucleophile/base	Acts as electrophile