Carbanion
From Free net encyclopedia
A carbanion is an anion in which carbon has an unshared pair of electrons and bears a negative charge usually with three substituents for a total of eight valence electrons. The carbanion exists in a trigonal planar geometry. Formally a carbocation is the conjugate base of a carbon acid.
- R3C-H + B− → R3C− + H-B
where B stands for the base.
A carbanion is a nucleophile. The stability and reactivity of a carbanion is determined by several factors. these include:
1) The inductive effect. Electronegative atoms adjacent to the charge will stabalise the charge. 2) Hybridisation of the charge-bearing atom. The greater the s-character of the charge-bearing atom, the more stable the anion. 3) The extent of conjugation of the anion. Resonance effects (conjugation) can stabalise the anion
A carbanion is a reactive intermediate and is encountered in organic chemistry for instance in the E1cB elimination reaction and in organometallic chemistry in for instance a Grignard reaction or in alkyl lithium chemistry. Stable carbanions do however exist. In 1984 Olmstead presented the lithium crown ether salt of the diphenylmethyl carbanion from diphenylmethane, butyl lithium and 12-crown-4 at low temperatures [1].
References
[1] The isolation and x-ray structures of lithium crown ether salts of the free phenyl carbanions [CHPh2]- and [CPh3]- Marilyn M. Olmstead, Philip P. Power; J. Am. Chem. Soc.; 1985; 107(7); 2174-2175. DOI abstractde:Carbanion fr:Carbanion nl:Carbanion