Chemistry CARBOCATIONS AND CARBANIONS

Reactive Intermediates :

Synthetic intermediate are stable products which are prepared, isolated and purified and subsequently used as starting materials in a synthetic sequence. Reactive intermediate, on the other hand, are short lived and their importance lies in the assignment of reaction mechanisms on the pathway from the starting substrate to stable products. These reactive intermediates are not isolated, but are detected by spectroscopic methods, or trapped chemically or their presence is confirmed by indirect evidence.

Carbocations :

Carbocations are the key intermediates in several reactions and particularly in nucleophilic substitution reactions.

Structure: Generally, in the carbocations the positively charged carbon atom is bonded to three other atoms and has no nonbonding electrons. It is `sp^2` hybridized with a planar structure and bond angles are of about `120^(o)`. There is a vacant unhybridized p orbital which (e.g. in the case of `CH_3^+`) lies perpendicular to the plane of `C-H` bonds.

Carbanions :

Structure: A carbanion posses an unshared pair of electron and thus represents a base. The best likely description is that the central carbon atom is `sp^3` hybridized with the unshared pair occupying one apex of the tetrahedron. Carbanions would thus have pyramidal structures similar to those of amines. It is believed that carbanions undergo a rapid interconversion between two pyramidal forms. There is evidence for the `sp^3` nature of the central carbon and for its tetrahedral structure. At bridgehead a carbon does not undergo reactions in which it must be converted to a carbocation. However, the reactions which involve carbanions at such centers take place with ease, and stable bridgehead carbanions are known. In case this structure is correct and if all three `R` groups on a carbanions are different, the carbanion should be chiral. All reactions therefore, which involve the formation of chiral carbanion should give retention of configuration. However, this never happens and has been explained due to an umbrella effect as in amines. Thus the unshared pair and the central carbon rapidly oscillate from one side of the plane to the other.