Carbanions pyramidal

What is the preferred geometry about the radical center in free radicals Carbocation centers are characterized by a vacant orbital and are known to be planar, while carbanion centers incorporate a nonbonded electron pair and are typically pyramidal (see Chapter 1, Problem 9). 

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 methyl anion (CH3) has been observed in the gas phase and reported to have a pyramidal structure. If this is a general structure for carbanions, then any... 

The SnI reactions do not proceed at bridgehead carbons in [2.2.1] bicyclic systems (p. 397) because planar carbocations cannot form at these carbons. However, carbanions not stabilized by resonance are probably not planar SeI reactions should readily occur with this type of substrate. This is the case. Indeed, the question of carbanion stracture is intimately tied into the problem of the stereochemistry of the SeI reaction. If a carbanion is planar, racemization should occur. If it is pyramidal and can hold its structure, the result should be retention of configuration. On the other hand, even a pyramidal carbanion will give racemization if it cannot hold its structure, that is, if there is pyramidal inversion as with amines (p. 129). Unfortunately, the only carbanions that can be studied easily are those stabilized by resonance, which makes them planar, as expected (p. 233). For simple alkyl carbanions, the main approach to determining structure has been to study the stereochemistry of SeI reactions rather than the other way around. What is found is almost always racemization. Whether this is caused by planar carbanions or by oscillating pyramidal carbanions is not known. In either case, racemization occurs whenever a carbanion is completely free or is symmetrically solvated. 

The inductive and electrostatic effects, steric constraints and conjugative interactions are the major factors that determine the configurational stability of a-sulfonyl carbanions. These are thought to be pyramidal with appreciable electrostatic inhibition to racemization by way of inversion. LCAO-MO-SCF calculations have indicated the conformer 195 in which the lone pair is directed along the bisector of the OSO angle to be the most stable in acyclic sulfones. ... 

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 . 

It is important that this process results in the preferential formation of a thermodynamically stable alcohol diastereomer. The anion-radicals contain an almost undoubtedly planar C-0 and give rise to pyramidal hydroxy carboradicals. The hydroxy carboradicals form pyramidal hydroxy carbanions, which cannot exist in the presence of ammonium cation for a long time. Therefore, the equilibrium including pyramidal inversion, probably, takes place at the step of carboradical formation, rather than carbanion formation. Transformation of a carboradical into a carbanion obviously proceeds faster than its dimerization or disproportionation. As a consequence, the reduction of an optically active ketone into an alcohol goes without racemization (Rautenstrauch et al. 1981). 

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