Structure of Carbanions

C—Li bond has greater ionic character, yielding a more carbanion-Uke salt with higher reactivity.On the other hand, carbanions derived from highly acidic species such malonate esters are readily formed under mild conditions and have long been used in the generation of new carbon-carbon bonds. 

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Figure 7.8. X-ray structures of carbanion 11 and carbanion 12. ORTEP representations adapted from references 55 and 56.

Closely related to boraethenes is the structure of carbanions derived from boraethene 61 (Eq. 20). 

Nuclear magnetic resonance (NMR) has proven to be a very powerful technique for probing the structures of carbanions in the condensed phase. In particular, much work has been completed on the ion-pairing behavior of carbanions with lithium cations as well as the formation of aggregates of these lithium salts. A full discussion of this topic, particularly the methodology, is beyond the scope of this chapter, but a brief overview is appropriate. 

Changes in the Inter- and intramolecular interaction and in the structure of carbanion pairs on adding cation-binding ligands (ethers, crown ethers, poly-amines) is briefly reviewed. 

To compare the rates of substitution in chlorobenzene itself, a chlorobenzene containing an electron-withdrawing group, and a chlorobenzene containing an electron-releasing group, we compare the structures of carbanions I. 11, and III. 

The covalent character present in many carbanion carbon-metal bonds means that we must use caution in discussing the properties of carbanions based on reactions of organometaUics. One way to study the structures of carbanions is to determine whether chiral carbanions undergo racemization. Studies of noncycHc carbanions indicate that the retention of configuration at a chiral carbanionic center depends on solvent and temperature, with solvents such as diethyl ether decreasing the covalent character of the carbon-metal interaction, and thus facilitating epimerization at the chiral center. 

Carbanions are very useful intermediates in the formation of carbon-carbon bonds. This is true both for unstabilized structures found in organometallic reagents and stabilized structures such as enolates. Carbanions can participate as nucleophiles both in addition and in substitution reactions. At this point, we will discuss aspects of the reactions of carbanions as nucleophiles in reactions that proceed by the 8 2 mechanism. Other synthetic aj lications of carbanions will be discussed more completely in Part B. 

Discuss the relevance of these observations to the structure of sulfur-stabilized carbanions and rationalize your conclusion about the structure of the carbanions in MO terms. 

You should be able to predict the structure of the product by determining which hydrogen in the starting material is most acidic, that is, by assigning the structure of the intermediate carbanion. 

Obtain the energies of the different possible carbanions alleyne-H+). Which one is most stable Does it correspond to removal of the most electron-poor proton Examine the geometry and atomic charges of the favored carbanion. Where is the negative charge Draw the Lewis structure of this ion. Predict the structure of the Sn2 product. 

The solid-state structures of several benzylic carbanion salts have been elucidated by X-ray analysis9 depending on the nature of the benzylic part, the cation, and the additives, the structures range from er-bonded organometallic compounds to delocalized ion pairs, from monomeric to dimeric and polymeric aggregates. Some compounds are listed together with leading references ... 

Normally, reactive derivatives of sulfonic acids serve to transfer electrophilic sulfonyl groups259. The most frequently applied compounds of this type are sulfonyl halides, though they show an ambiguous reaction behavior (cf. Section III.B). This ambiguity is additionally enhanced by the structure of sulfonyl halides and by the reaction conditions in the course of electrophilic sulfonyl transfers. On the one hand, sulfonyl halides can displace halides by an addition-elimination mechanism on the other hand, as a consequence of the possibility of the formation of a carbanion a to the sulfonyl halide function, sulfenes can arise after halide elimination and show electrophilic as well as dipolarophilic properties. 

It is unlikely that free carbanions exist in solution. Like carbocations, they usually exist as either ion pairs or they are solvated. " Among experiments that demonstrated this was the treatment of PhCOCHMe with ethyl iodide, where was Li ", Na", or K" . The half-lives of the reaction were for Li, 31 x 10 Na, 0.39 X 10 and K, 0.0045 x 10 , demonstrating that the species involved were not identical. Similar results were obtained with Li, Na, and Cs triphenylmethides (PhsC M Where ion pairs are unimportant, carbanions are solvated. Cram " demonstrated solvation of carbanions in many solvents. There may be a difference in the structure of a carbanion depending on whether it is free (e.g., in the gas phase) or in solution. The negative charge may be more localized in solution in order to maximize the electrostatic attraction to the counterion. ... 

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. 

Since a carbanion is what remains when a positive species is removed from a carbon atom, the subject of carbanion structure and stability (Chapter 5) is inevitably related to the material in this chapter. So is the subject of very weak acids and very strong bases (Chapter 8), because the weakest acids are those in which the hydrogen is bonded to carbon. 

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