Solvation of anion

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. 

FIG. 32 Top Semilog plot of the time constant t for ionic motion as a function of RH for KF. Bottom Simultaneously measured contact potential. At a critical humidity A, there is a break or a change in slope in these two surface properties. Below A, water solvates preferentially cations at the step edges. Above A, the rates of dissolution (solvation) of anions and cations are similar and water uni-... 

It is apparent that the exclusive application of the donicity-rule may fail to account for the actual reaction which is occurring, since solvation of anions, steric factors, and other specific donor-acceptor interactions as well as the en-tropic effects must also be considered. 

A change from a protic to an aprotic solvent can also affect the acidity or basicity, since there is a difference in solvation of anions by a protic solvent (which can form hydrogen bonds) and an aprotic one.158 The effect can be extreme in DMF, picric acid is stronger than HBr,159 though in water HBr is far stronger. This particular result can be attributed to size. That is, the large ion (C N CsE C)- is better solvated by DMF than the smaller ion Br-.160 The ionic strength of the solvent also influences acidity or basicity, since it has an influence on activity coefficients. 

Although solvents may be classified as donor solvents (Lewis bases) and acceptor solvents (Lewis acids), most of the more widely used nonaqueous solvents are donor solvents. Some acceptor solvents, such as S02, BrF3, AsC13, or the liquid hydrogen halides, have proved to be useful in coordination chemistry.13"16 Ionization is promoted in a donor solvent by solvation of cations and in an acceptor solvent by solvation of anions. For example, arsenic(m) iodide is ionized in a donor solvent D according to the reaction... 

Solvation of anionic nucleophiles is poorer in dipolar aprotic than in dipolar protic solvents. The reason is that dipolar aprotic solvents cannot form a hydrogen bond to such nucleophiles. Consequently, anionic nucleophiles are not stabilized in dipolar aprotic... 

A second limitation of the Hughes-Ingold theory concerns the fact that the solvent is treated as dielectric continuum, characterized by one of the following its relative permittivity, e, the dipole moment, fi, or by its electrostatic factor, EF, defined as the product of and [27]. The term solvent polarity refers then to the ability of a solvent to interact electrostatically with solute molecules. It should be remembered, however, that solvents can also interact with solute molecules through specific inter-molecular forces like hydrogen bonding or EPD/EPA complexation cf. Section 2.2). For example, specific solvation of anionic solutes by pro tic solvents may reduce their nucleophilic reactivity, whereas in dipolar aprotic solvents solvation of anions is less,... 

Solubility equilibrium, 24 Solubility parameter, 415 Solvation, 401 change in, 354 preferential, 403 selective, 403 Solvation and polarity, 399 Solvation energy, 403, 420 Solvation of anions, 360 Solvation shell, 403 Solvatochromic comparison method, 439... 

J.E. Combariza, N.R. Kestner and J. Jortner, Energy-structure relationships for microscopic solvation of anions in water clusters, J. 

D. Solvation of Anions Si 2 Reactions of DiSerent Charge Types... 

Fig. 1. Qualitative representation of the effect of electrostatic, hydrogen-bonding, and mutual polarizability interactions on the free energy of solvation of anions in methanol and in DMF. (cf. Table 26).

In the particular case of sodamide containing complex bases where the base to be activated is insoluble, alcoholates can complex sodamide with help of two ionic sites (ROs and Nas+). This must be more favorable for dissolution of NaNH2 than simple solvation of the cation by dipolar aprotic solvent (solvation of anions being low). However, in solution because of the double complexation the basic power of complex bases must be lower than the basic power of NaNH2 in a dipolar aprotic solvent if this latter was able to dissolve it substantially. 

The shift in the redox potentials for O2 within aprotic solvents [-0.65 V in dimethylformamide (DMF) versus -0.16 V in H2O] is the result of a much weaker solvation of anions by such media relative to water (for O2 -, about 88 kcal mol in DMF versus 100 kcal mol l in H2O). Likewise, the addition of base to hydrogen peroxide results in its spontaneous decomposition. 

Polar solvents are those liquids whose relative permittivity is sufficiently high that electrolytes can be dissolved in them. The best-known example of such a liquid is water. The oxygen end of this simple molecule is electron-rich and can stabilize cations. The hydrogen atoms are electron-poor and thus are involved in the solvation of anions. The structure of pure water is very much influenced by the... 

With its combination of high dielectric constant, good donor ability and hydrogen bonding capability (which contributes to the solvation of anions) water is one of the best solvents for ionic substances. Two other solvents of comparable polarity are HF and H2S04 (see Topic El. Table 71. Solubility... 

In the case of acetonitrile solvent, where the solvation of anions is unlikely to occur, pyrrole can solvate anions itself.44 This may explain why PPy films formed from acetonitrile with perchlorate counterions are less porous with pyrrole solvating the counterion during synthesis, a denser structure would be expected. This also results in polymers that are more conductive and have lower capacitance and greater electrochemical reversibility than those grown from water. Similar differences in conductivity were observed between acetonitrile and water when DS was used as the counterion,45 although the differences in conductivity were not so marked. The presence of DS probably provides some protection from the nucleophilic solvent. In other work,46 we have shown that pyrrole can be reversibly oxidized in surfactant-containing media (i.e., the surfactant stabilizes the free radical produced). This reversibility could not be detected in the absence of surfactants. 

Moreover, addition of the small amount of water required to regenerate the hydration sphere of the anions in the organic phase (wet PhCl) restored the rate constants obtained under PTC conditions. This is a clear demonstration that the specific solvation of anions in the organic phase is responsible for the particular nucleophilicity scale of anions in PTC conditions. On the other hand, it is also a proof that the reaction takes place in the organic phase. 

B. The Solvated Proton The Proton Solvation Energy The Solvation of Anions... 

---