Carbanion
A carbanion is a species containing unshared pair on central carbon atom and three groups
attached to the Carbon. As there are three groups on carbon and unshared pair on carbon,
carboanion is negatively charged . Carboanion has eight electrons in its outermost orbital, sp3
hybridized, pyramidal shaped and act as nucleophile.
Carboanion has eight electrons in its outermost orbital, sp3 hybridized, pyramidal shape and act as nucleophile . Formally carboanion is a conjugated base of carbon acid.
Stability of Carboanion
Stability of carboanion depends on many factors.
1. Electronegativity of Carboanionic carbon .When elctronegativity of carboanionic carbon increases, stability of carboanion increases. Electronegativity of carboanion depends on % s-character of carboanionic carbon. % s character depends on hybridization of carbon and increase in following order;
Therefore, stability of carboanion is high when carboanionic carbon is sp hybridized.
2. Inductive effect of substituents attached to the carboanionic carbon There are two inductive effects operate in organic compounds , +I and –I. When groups with +I effect is attached to the carboanionic carbon, stability of carboanion decreases. Therefore, tertiary carboanion is the least stable and methyl carboanion is the highest stable as shown below.
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| Carbaions |
When group or atom with – I effect is attached to the carboanionic carbon, the stability of caroanion increases. Electronegative atoms or groups with electronegative atoms are connected to carboanionic carbon, stability of carboanion increases. Therefore, following carboanions are very stable.
3. Delocalization or Resonance.
When carboanion is in conjugation with double bond those carboanins are stable due
to resonance as shown in following figure.
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| Benzylic Carboanion |
4. Stabilization by aromaticity
Carboanions are stabilized by aromaticity. Following carboanion is stable as it is aromatic.
When carboanion is conjugated with carbonyl group (enolate ion) or nitrile group, these carboanions are stable due to resonance and –I effect of electronegative atoms.
Generation of Carboanions
There are many of ways of generating carboanions
1. From Grignard reagent(RMgX). Grignard reagent is good source for carboanion.
2. Metal Halogen interchange reaction
Organo metallics are good sources for carboanions. Metal-halogens exchange reactions
are frequently used to prepare vinyl and aryllithium.
3. H+ abstraction by a base
Deprotonation using a base is often used method for the generation of stable carbanions.
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4. Removing of proton at the alpha position of alpha-carbonyl compounds
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