Structure and Geometry of Carbanions

Radicals, Ion Radicals, and Triplets The Spin-Bearing Intermediates of Organic Chemistry, Wiley-VCH New York, 1997. 

The term carbanion was proposed by Wallis, E. S. Adams, F. H. /. Am. Chem. Soc. 1933,55,3838. Pale, P. Vogel, P. in Katritzky, A. R. Taylor, R. J. K., Eds. Comprehensive Organic Functional Group Transformations IT, Elsevier Oxford, 2005 p. 889. 

Buncel, E. Dust,]. M. Carbanion Chemistry Structures and Mechanisms,Oxford University Press New York, 2003. 

The application of molecular orbital theory to carbanions was discussed by Nobes, R. H. Poppinger, D. Li, W.-K. Radom, L. in Buncel, E. Durst, T., Eds. Comprehensive Carbanion Chemistry. Part C. Ground and Excited State Reactivity Elsevier Amsterdam, 1987 pp. 1-92. 

Often carbanions are shown without explicit depiction of the nonbonded pair of electrons. Thus, the methyl anion is also indicated by HjC . 

Only intermediates that are formed before the rate-determining step can accumulate. Reactions where intermediates can be isolated in a normal work up are rather rare. More often, intermediates might be observable by spectroscopic techniques. The existence of short-lived intermediates or of intermediates occurring after the rate-determining step can still be demonstrated by trapping reactions or by special techniques such as matrix isolation. This chapter provides detailed explanations of reaction mechanisms involving carbanions, possible transition states, and the scope of the reactions. The selected experimental procedures can be implemented quickly and easily in the laboratory. 

Carbanions are trivalent spedes with sp hybridization. The lone pair of electrons occupies one of the sp orbitals, and the geometry is thus tetrahedral. The tetrahedron can undergo inversion or retain its stereochemistry depending upon the attached substituents. However, the geometry of a carbanion stabiUzed by conjugation with substituents is different. A methyl carbanion has a barrier to inversion of about 2 kcal mol , whereas the trifluoromethyl carbanion has a 

Reactive Intermediates in Organic Chemistry Structure, Mechanism, and Reactions, First Edition. 

By similar arguments, we would predict that vinyl carbanions exhibit sp hybridization and acetylenic carbanions show sp hybridization. Based on the idea that the greater the s character in an orbital the more easily it can accommodate 

NM R studies of n-butyllithium and tert-butyllithium in hydrocarbon solvent suggest a polar covalent C-Li bond with only a small negative charge on the carbon atom. Moreover, the species present appears to be an associated, not a monomeric species. n-Butyllithium is reported to be hexameric in hydrocarbon solutions but to exist as an equilibrium mixture of tetramers and dimers in THF (tetrahydrofuran) solution. 

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