Answer to Question #33059 in Organic Chemistry for naeim

Question

why carbanion shows sp3 hybridisation when it is non-conjugated?
why carbanion shows sp2 hybridisation when it is conjugated?

Answer 1

Carbanions are trivalent with sp³ hybridization. The lone pare of electrons occupies one of the sp³ orbitals. The geometery is thus
tetrahedral. The tetrahedron can undergo inversion or retain its stereochemistry depending on the attached substitutents. A methyl carbanion has a barrier to inversion of about 2 kcal/ mole. The trifluoromethyl carbanion has a barrier of 120 kcal/mole. A fluorine atom is however more stabilizing than a hydrogen atom because of the fluorine electronegativity. Carbanion have sp² hybridisation when attached to conjugative groups like olefine, ketone.