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)3C−Na+ + 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
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