Carbanions


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Related to Carbanions: Carbenes, Free radicals

Carbanions

 

molecular particles that contain a negatively charged tricovalent carbon atom:

Carbanions are highly reactive and are therefore unstable; they occur as intermediate particles in many organic reactions. For example, they form upon action of strong bases B: (the dots indicate an unshared electron pair) on hydrocarbons, from which any hydrogen atom is readily liberated as a proton:

Thus, sodium trityl, a bright red compound of ionic structure that contains a trityl anion, is formed upon the action of sodium amide on triphenyl methane:

(C6H5)3CH + Na+NH2→(C6H5)3CNa+ + NH3

This carbanion is stable because of the distribution of the negative charge among several carbon atoms.

Most organometallic compounds have a covalent structure; however, because of polarization of the metal-carbon bond, excess electron density appears in the carbon atom bonded to the metal:

This atom is partially carbanionic; therefore, organometallic compounds serve as carbanion donors.

Ylides, which are bipolar ions that contain a positively charged heteroatom (N, P, As, O, or S) and a negatively charged carbon atom, are a special type of carbanion. An example is pyridine fluorenylylide:

The existence of carbanions as kinetically independent particles has been rigorously proved in a few cases, but the concept of their intermediate formation is frequently used to interpret the mechanism of organic reactions, many of which have great theoretical and practical value (for example, anionic polymerization).

REFERENCES

Cram, D. Osnovy khimii karbanionov. Moscow, 1967. (Translated from English.)
Breslow, R. Mekhanizmy organicheskikh reaktsii. Moscow, 1968. (Translated from English.)

B. L. DIATKIN

References in periodicals archive ?
This result is consistent with the concept that the stabilization of aliphatic and alicyclic carbanions is largely dependent on anionic hyperconjugation, involving the interaction between the electronic "lone pair" and antibonding C-H and C-C [[sigma].
This includes enantioselective synthesis of [Alpha]-hetereoatom stabilized carbanions, additions of chirally-modified organometallics to carbonyl compounds, and enantioselective synthesis of natural products, particularly insect pheromones.
In the case of the sequential anionic living polymerization of (meth)acrylic monomers, it is generally known that the nucleophilic attack of polystyryl carbanions on the caronyl group of (meth)acrylic monomer is the most usual side reaction, leading to the limited monomer conversion and branch formation.
Chapters summarize findings in the areas of reactions of aldehydes and ketones and their derivatives; reactions of carboxylic, phosphoric, and sulfonic acids and their derivatives; radical reactions; oxidation and reduction; carbenes and nitrenes; nucleophilic aromatic substitution; electrophilic aromatic substitution; carbocations; nucleophilic aliphatic substitution; carbanions and electrophilic aliphatic substitutions; elimination reactions; polar addition reactions; cycloaddition reactions; and molecular rearrangements.
For example, studies of the superbases and carbanions are often very sensitive to oxidation.