Carbanion:
A carbanion may be defined as a species containing a carbon atom carrying a negative charge. These are generated by the heterolytic fission of covalent bond involving carbon atom in which the atom linked to carbon is less electronegative than carbon. As a result, carbon acquires a negative charge. For example, methyl carbanion.
Like carbocations, carbanions are also classified as primary (1°), secondary (2°) and tertiary (30) depending upon whether one, two or three carbon atoms are attached to the carbon bearing negative charge as:
The order of stability of carbanions is reverse of that of carbocations and free radicals.
–CH3> 1o > 2°> 3°