Structure and Stability of Carbanions

Carbanions are considered to be derived by the heterolytic fission of the C—X bond in an organic molecule in which carbon is more electronegative than X. 

The shape of simple carbanions as determined on the basis of a number of experiments is found to be pyramidal, similar to that of amines. The central carbon atom is sp hybridized with the unshared pair occupying one apex of the tetrahedron (if the electron pair is viewed as a substituent). These species invert rapidly at room temperature, passing through a higher energy planar form in which the electron pair occupies a p-orbital. However, when the carbanion is stabilized by delocalization, it assumes sp hybridization for effective resonance. 

In practice, any organic compound having a C—H bond (all such compounds can be considered to be an acid in the classical sense) can donate a proton to a suitable base the species obtained as a result is the carbanion. 

Like carbocations, carbanions are also stabilized by resonance. Thus, benzyl carbanion and allyl carbanion are more stable than ethyl carbanion. The stabilization by resonance is due to the delocalization of the negative charge, which is distributed over other carbon atoms. The canonical (resonating) forms of benzyl carbanion and allyl carbanion are given below  

When a functional group X is present at the a-position to the carbon having negative charge, it may increase or decrease the stability of the carbanion. The effect of various groups (X) on the stability of the carbanion is of the order  

The stability of a carbanion (or ion pair) is increased by certain substituents and decreased by others. It is possible to rank the various structures in an order of increasing stability of the carbanion just as was done for carbonium ions. It will be recalled that our information about carbonium ions does not suffice for a prediction of the effect of temperature changes on the relative stabilities, and that it is unknown to what degree an increase in stability actually reflects a decrease in potential energy. The situation is similar in the case of carbanions the precise relationship of the stabilities is an unknown function of the temperature. It is also likely that the effects of structural changes are somewhat dependent on the solvent. Nevertheless it is possible to make valuable qualitative comparisionsof the various structures and to interpret them in terms of resonance and other potential energy quantities. 

The ortho-ring junction that converts the triphenylmethyl structure into that of the ion LX increases the stability of the carbanion but decreases that of the carbonium ion. It will be recalled that this structural modification of the triphenylcarbonium ion had about the same effect as the introduction of one to two nitro groups. 

Many of the reactions of the weak carbon acids are reactions of the carbanion, the rate being the rate of ionization and independent of the concentration or nature of the reagent that determines what the product will be. 

In this respect such reactions are analogous to the S 1 or limiting reactions of compounds producing carbonium ions, although the intermediate is a solvated carbanion rather than a solvated carbonium ion. In the base-catalyzed halogenation of ketones, for example, the rate is independent of the halogen concentration and is the same for the reaction with bromine as for the reaction with chlorine.384 

If a sufficiently wide range of structures is considered, there is a definite parallelism between the effect of structural change on the ionization equilibrium constant and the rate constant. This parallelism is conveniently described as a linear relationship between the logs of the equilibrium and rate constants, a relationship which is equivalent to a linear relationship between the free energy and the free energy of 

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The fundamental aspects of the structure and stability of carbanions were discussed in Chapter 6 of Part A. In the present chapter we relate the properties and reactivity of carbanions stabilized by carbonyl and other EWG substituents to their application as nucleophiles in synthesis. As discussed in Section 6.3 of Part A, there is a fundamental relationship between the stabilizing functional group and the acidity of the C-H groups, as illustrated by the pK data summarized in Table 6.7 in Part A. These pK data provide a basis for assessing the stability and reactivity of carbanions. The acidity of the reactant determines which bases can be used for generation of the anion. Another crucial factor is the distinction between kinetic or thermodynamic control of enolate formation by deprotonation (Part A, Section 6.3), which determines the enolate composition. Fundamental mechanisms of Sw2 alkylation reactions of carbanions are discussed in Section 6.5 of Part A. A review of this material may prove helpful. 

This chapter will begin with a brief overview of the development of carbanion chemistry followed by a section devoted to the structure and stability of carbanions. Methods of measuring carbon acidity and systematic trends in carbanion stability will be key elements in this chapter. Next, processes in which carbanions appear as transient, reactive intermediates will be presented and typical carbanion mechanisms will be outlined. Finally, some new developments in the field will be described. Although the synthetic utility of carbanions will be alluded to many times in this chapter, specific uses of carbanion-like reagents in synthesis will not be explored. This topic is exceptionally broad and well beyond the scope of this chapter. 

Various aspects of proton transfer to and from carbon have been reviewed previously. Bell s classic work [1] covers the whole field of proton transfers and discusses in detail the mechanisms of acid and base catalysed reactions. The structure and stabilization of carbanions has been described [2] and a general review of the ionization of carbon acids has been published [3], The role of proton transfers in the mechanisms of chemical and biological reactions has also been described [4]. 

The stereoselectivity of anti-Markovnikov adducts (161) and (162) produced through photo-induced electron-transfer reaction of (160) with MeOH in MeCN depends on the optimum structures and stabilities of the corresponding radical and carbanion intermediates (163) and (164). In PhH, steric hindrance in an exciplex, comprising an excited singlet sensitizer and (160), forced cis addition of MeOH to (160) to give trans-isomer (161) as the major addition product. 

The major carbon centered reaction intermediates in multistep reactions are carboca-tions (carbenium ions), carbanions, free radicals, and carbenes. Formation of most of these from common reactants is an endothermic process and is often rate determining. By the Hammond principle, the transition state for such a process should resemble the reactive intermediate. Thus, although it is usually difficult to assess the bonding in transition states, factors which affect the structure and stability of reactive intermediates will also be operative to a parallel extent in transition states. We examine the effect of substituents of the three kinds discussed above on the four different reactive intermediates, taking as our reference the parent systems [ ]+, [ ]-, [ ], and [ CI I21-... 

The closely related research on polyether chelates by Michal Szwarc and his co-workers led to a detailed determination of the structure and properties of carbanions in ion pairs and free ions. The fundamental principles which were developed and clarified in their numerous publications contribute to an understanding and interpretation of much of the polyamine chelate work as well. More recently the crown ether chelates, pioneered by Pederson and co-workers at the Dupont Laboratories, have given additional impetus to research on chelated alkali metal compounds. Crown ethers and amines are cyclic variations which can provide greater stability and specificity in complexation of cations, particularly the heavier alkali metal ions. 

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. 

Except for the most highly stabilized carbanions, carbanion chemistry in solution is always complicated by the presence of the counterion, usually a metal, which is a Lewis acid and almost invariably is involved in the course of the reaction. Relative stabilities of carbanions in solution are difficult to establish for the same reason. In recent years, much information has been gathered about carbanion stabilities, structures, and reactiv-... 

Mechanistic Aspects of Cationic Copolymerizations The relative reactivities of monomers can be estimated from copolymerization reactivity ratios using the same reference active center. However, because the position of the equilibria between active and dormant species depends on solvent, temperature, activator, and structure of the active species, the reactivity ratios obtained from carbocationic copolymerizations are not very reproducible [280]. In general, it is much more difficult to randomly copolymerize a variety of monomers by an ionic mechanism than by a radical. This is because of the very strong substituent effects on the stability of carbanions and carbenium ions, and therefore on the reactivities of monomers substituents have little effect on the reactivities of relatively nonpolar propagating radicals and their corresponding monomers. The theoretical fundamentals of random carbocationic copolymerizations are discussed in detail and the available data are critically evaluated in Ref. 280. This review and additional references [281,282] indicate that only a few of the over 600 reactivity ratios reported are reliable. 

However, this reaction requires the prior formation of the less probable radical ending, M HR-CH2. On these grounds, therefore, it is to be predicted that head-to-tail linkages will predominate over head-to-head and tail-to-tail linkages in the final polymer. Since the stabilization of carbanions and carbonium ions follows similar principles to those discussed above, it is to be expected that the head-to-tail structure will also be favoured in ionic vinyl polymerizations. 

The structures of l,8-bis(trimethylgermyl and stannyl)naphthalene show the substituents twisted out of the plane of the rings, giving Cg symmetry. The p.m.r. spectrum of both compounds support the structures, with one Me group of each MesM pointing towards the naphthalene plane, and 2 away from it, in contrast to the Bu derivative. Enantiomerization barriers decrease with the size of M in The stabilization of carbanions a to silicon induces 9,9-bis-... 

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. 

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