Carbanions

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Kerala KEAM Chemistry Syllabus December 11th, 2009 Goto comments Leave a comment Kerala KEAM Chemistry Syllabus CHEMISTRY UNIT 1: BASIC CONCEPTS AND ATOMIC

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CARBANIONS

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.

R

CR

R

a carbanion

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.

Carbanion Structure

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.

inversion

C

R

R

R

C

R

R

R = H; barrier = 2 kcal/mole

R = F; barrier = 120 kcal/mole

tetrahedral carbanion

R

The rate of inversion in hydrocarbon system is slowed by incorporation of the

carbanion into a three membered ring.

N

SO2Ph

SO2Ph

5.8 kcal

10 kcal

17 kcal

NH3

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...

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