Naked anion’ effect example

Since X-ray crystallography cannot observe the lone electron directly (Box 2.1), it is questionable whether it is really situated at such a distance from the Cs+ cation. If true, this would represent a very extreme example of the naked anion effect (Section 3.8.2). An alternative explanation localises the electron on the Cs+ cation, which would also account for the observed low conductivity. However, convincing evidence for the separation of cation and electron comes from the nearly isostructural sodide (Na ) and kalide (K ) analogues of [Cs ([18] crown-6) 2] + -e-. In these, species the alkali metal anions are situated in the same localised cavities as their electride analogues. 

In 1987, Pedersen and Lehn shared the Nobel Prize for the development of Supramolecular Chemistry with a third recipient, Donald Cram, also very well-known for his work in carbanion chemistry. Cram realised that the chemical reactivity of a carbanion could be increased by the separation of a counter-cation from its anion. This separation was achieved by fully encapsulating the cation with cyclic polyethers and represents an example of the naked anion effect. This is where the anion has been stripped away from its counter-ion, such that there is little or no association between the anion and cation. Cram and his team went about synthesising cyclic polyethers in which the oxygen atoms comprise a small cavity that is rigidly preorganised in an octahedral array. These macrocycles are termed spherands, of which the Li -selective spherand-6 (2.15) is one of the best-known examples. 

Other measures of nucleophilicity have been proposed. Brauman et al. studied Sn2 reactions in the gas phase and applied Marcus theory to obtain the intrinsic barriers of identity reactions. These quantities were interpreted as intrinsic nucleo-philicities. Streitwieser has shown that the reactivity of anionic nucleophiles toward methyl iodide in dimethylformamide (DMF) is correlated with the overall heat of reaction in the gas phase he concludes that bond strength and electron affinity are the important factors controlling nucleophilicity. The dominant role of the solvent in controlling nucleophilicity was shown by Parker, who found solvent effects on nucleophilic reactivity of many orders of magnitude. For example, most anions are more nucleophilic in DMF than in methanol by factors as large as 10, because they are less effectively shielded by solvation in the aprotic solvent. Liotta et al. have measured rates of substitution by anionic nucleophiles in acetonitrile solution containing a crown ether, which forms an inclusion complex with the cation (K ) of the nucleophile. These rates correlate with gas phase rates of the same nucleophiles, which, in this crown ether-acetonitrile system, are considered to be naked anions. The solvation of anionic nucleophiles is treated in Section 8.3. 

However, because they cannot form hydrogen bonds and because their positive centers are well shielded by steric effects from any interaction with anions, aprotic solvents do not solvate anions to any appreciable extent. In these solvents anions are unencumbered by a layer of solvent molecules and they are therefore poorly stabilized by solvation. These naked anions are highly reactive both as bases and nucleophiles. In DMSO, for example, the relative order of reactivity of halide ions is opposite to that in protic solvents, and it follows the same trend as their relative basicity ... 

Cation Effects.—The ability of macrocyclic complexing agents to surround and solvate cations in homogeneous solution is known to provide separated ion pairs. Thus, for example, recent work shows that the linked bis-crown (41) and the polyvinyl benzo-crown (42) convert the tight ion pairs of metal picrates to crown-separated ion pairs. This property, although not strictly involving phase transfers, may be utilized to good effect, and for example, contributes to the enhanced reactivity of the so-called naked anions discussed above. 

For example, these solvents bind the cation of KCN by orienting their negative ends around it. Because there are no electropositive hydrogen atoms in aprotic solvents, the CN" ion cannot be effectively solvated it is called a naked anion. Thus, the nucleophihcity of CN is greater in dimethyl sulfoxide than in ethanol. An aprotic solvent such as dimethyl sulfoxidefavors an reaction. 

The crown ethers and cryptates are able to complex the alkaU metals very strongly (38). AppHcations of these agents depend on the appreciable solubihty of the chelates in a wide range of solvents and the increase in activity of the co-anion in nonaqueous systems. For example, potassium hydroxide or permanganate can be solubiHzed in benzene [71 -43-2] hy dicyclohexano-[18]-crown-6 [16069-36-6]. In nonpolar solvents the anions are neither extensively solvated nor strongly paired with the complexed cation, and they behave as naked or bare anions with enhanced activity. Small amounts of the macrocycHc compounds can serve as phase-transfer agents, and they may be more effective than tetrabutylammonium ion for the purpose. The cost of these macrocycHc agents limits industrial use. 

The first step of the Sfjl mechanism involves the formation of ions. Since polar solvents can solvate ions, the rate of Sjj 1 processes is enhanced by polar solvents. On the other hand, solvation of nucleophiles ties up their unshared electron pairs. Therefore, Sfj2 reactions, whose rates depend on nucleophile effectiveness, are usually retarded by polar protic solvents. Polar but aprotic solvents [examples are acetone, dimethyl sulfoxide, (CH3)2S=0, or dimethylformamide, (CH3)2NCHO] solvate cations preferentially. These solvents accelerate 8 2 reactions because, by solvating the cation (say, K in K CN), they leave the anion more naked or unsolvated, thus improving its nucleophilicity. 

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