Carbanions hybridization

CjHsOOC)—CH—COOCjHj (I) + HOC Hj The carbanion (I) is a resonance hybrid (mesomeric anion) to which there are contributions carrying the negative charge on either carbon or oxygen ... 

It provides electrostatic stabilization of the carbanion formed upon removal of the C-2 proton. (The sf hybridization and the availability of vacant d orbitals on the adjacent sulfur probably also facilitate proton removal at C-2.)... 

Potassium or lithium derivatives of ethyl acetate, dimethyl acetamide, acetonitrile, acetophenone, pinacolone and (trimethylsilyl)acetylene are known to undergo conjugate addition to 3-(t-butyldimethylsiloxy)-1 -cyclohexenyl t-butyl sulfone 328. The resulting a-sulfonyl carbanions 329 can be trapped stereospecifically by electrophiles such as water and methyl iodide417. When the nucleophile was an sp3-hybridized primary anion (Nu = CH2Y), the resulting product was mainly 330, while in the reaction with (trimethylsilyl)acetylide anion the main product was 331. 

Acetylene, where the carbon is sp hybridized with 50% s character, is much more acidic than ethylene (sp, 33% s), which in turn is more acidic than ethane, with 25% 5 character. Increased s character means that the electrons are closer to the nucleus and hence of lower energy. As previously mentioned, cyclopropyl carbanions are more stable than methyl, owing to the larger amount of s character as a result of strain (see p. 181). 

The structure of simple unsubstituted carbanions is not known with certainty since they have not been isolated, but it seems likely that the central carbon is sp hybridized, with the unshared pair occupying one apex of the tetrahedron. Carbanions would thus have pyramidal structines similar to those of amines. 

The a carbon of mandelic acid is sp hybridized. The corresponding carbons of both a-phenylglycidic acid, 49, and the carbanion intermediate 48 are neither sp hybridized nor sp hybridized, but presumably between these two extremes. It is therefore possible that the a-phenylglycidic acid is restricted to a conformation which resembles a transition state in the racemization process, a transition state which would have much of the character of the intermediate 48, and for which the enzyme would presumably have a high affinity (1). 

Kuhn s carbanion, the all-hydrocarbon anion tris(7//-dibenzo[c,g]-fluorenylidenemethyl)methanide ion [2 ] (Kuhn and Rewicki, 1967a,b), is a stabilized system with tt electrons widely spread over the sp hybridized carbon framework, and was isolated as the potassium salt. It also appears in DMSO solution by dissolving the parent hydrocarbon, resulting in a deep green colour. The pKg value of the precursor hydrocarbon [2]-H is 5.9 in aqueous HCl-DMSO (Kuhn and Rewicki, 1967a,b), and its enormous stability, as compared with cyclopentadiene [9]-H, pKa 18 in DMSO... 

H/D exchange of H and Hg protons of sulfone 86 and estimated the difference in the free energies of activation for 79a and 79b to be < 1.2 kcal mol , based on the kjk value of 3 0.5. In the base-catalyzed H/D exchange of 87, kjk = 1.6, where k and k are the rate constants of H/D exchange of H, and H, respectively. Based on the small kjk value. Brown and colleagues suggested that if the carbanion is pyramidal, the steric stabilities of 79a and 79b are almost identical. Meanwhile, based on their C-NMR study Chassaing and Marquet proposed that the hybridization of the carbon atom of the sulfonyl carbanion, PhSOjCHj , would be between sp and sp . 

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