No Marshmallows, Just Term Papers
Carbanions are units that contain a negative charge on a carbon atom. The negative
charge gives good nucleophilic properties to the unit that can be used in the formation of
new carbon carbon bonds. Carbanions thus act as nucleophiles in substitution reactions, in
carbonyl addition and substitution reactions, and in 1,4- addition (Michael) reactions.
Carbanions bear many substituents that can affect the structure and reactivity of the
carbanion, and can affect the acidity of a parent C-H precursor. Halogens stabilize
carbanions in the order of Br > Cl > F. A prominent I-π repulsion between the F and
Carbanionic center causes some destabilization in alpha-fluorinated carbanions. The
magnitude of the destabilization depends on the carbanion structure. The destabiziation
maximizes as the carbanion structure approaces a planar configuration. Thus, fluorinated
carbanions possess pyramidal structues with high barriers to inversion.
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 methy
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.
R = H; barrier = 2 kcal/mole
R = F; barrier = 120 kcal/mole
The rate of inversion in hydrocarbon system is slowed by incorporation of the
carbanion into a three membered ring.
Experiments involving the use of chiral substrates aids the study of carbanion
stereochemistry as inversion causes loss of optical activity. Cleavage of the methyl ketone
below with amide ion give a carbanion with a slow rate of...