Carbanion

Acetonitrile IUPAC

Structural formula of acetonitrile

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Carbanion Structural Formulae V.1

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Ez-isomerismofvinylanions

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TriphenylmethaneAnion

Carbanion is an organic chemical species characterized by a carbon atom bearing a negative electric charge. This charge results from the carbon atom having an excess of electrons, typically three bonds and one lone pair. Carbanions are important intermediates in various chemical reactions and are widely studied in the field of organic chemistry for their reactivity and structure.

Structure and Stability

The structure of a carbanion is determined by the electronic configuration of the carbon atom, which in turn influences its stability. The stability of carbanions is affected by several factors, including:

• Inductive Effects: Electron-withdrawing groups attached to the carbanion center can stabilize the negative charge through inductive effects.
• Resonance Stabilization: Carbanions that can delocalize their charge through resonance are generally more stable.
• Hybridization: The hybridization of the carbon atom bearing the negative charge also affects the stability of carbanions. Sp3-hybridized carbanions are less stable than sp2 or sp-hybridized ones due to the increased s-character of the orbital holding the lone pair, which holds the negative charge closer to the carbon nucleus, increasing its energy.

Reactivity

Carbanions are highly reactive species, primarily due to their negative charge. They act as nucleophiles, meaning they are attracted to positively charged or electron-deficient centers. This makes them key intermediates in various types of organic reactions, including alkylation, condensation reactions, and nucleophilic substitution.

Generation of Carbanions

Carbanions can be generated through several methods, most commonly by the deprotonation of a carbon-hydrogen (C-H) bond. This is typically achieved using a strong base, such as lithium diisopropylamide (LDA) or sodium hydride (NaH). Other methods include the reduction of carbon-halogen bonds or the use of organometallic compounds.

Applications

In organic synthesis, carbanions are utilized to form new carbon-carbon (C-C) bonds, making them invaluable tools for building complex molecular structures. They are also involved in the synthesis of pharmaceuticals, agrochemicals, and polymers.

Examples

Some common examples of reactions involving carbanions include the Malonic ester synthesis, the Michael reaction, and the Wittig reaction. Each of these reactions employs carbanions in different ways to construct carbon-carbon bonds or to introduce functional groups into organic molecules.

See Also

• Organometallic chemistry
• Electrophile
• Nucleophile
• Base (chemistry)