Counterion cesium

At this point the hygroscopic potassium salt may be isolated and dried, or, more conveniently, the potassium salt may be dissolved in water and the carborane anion precipitated with one of a variety of large cations, such as the rubidium, cesium, tetramethylammonium, or trimethylammonium ions. The tri-methylammonium salt of the carborane anion is useful because it is readily purified by recrystallization from water and may be easily converted in solution to salts containing other counterions. ... 

The reactivity of the solvent-separated ion pair is hardly affected by the counterion. This can be observed from the kj values in THF (Table 5-11). Most of the observed propagation in THF is due to solvent-separated ion pairs and kj is a good indication of ks. The variation in kj from Li+ to Cs+ is relatively small and is probably due more to differences in the fractions of solvent-separated ion pairs than to differences in ks. The reactivity of contact ion pairs is more sensitive to the counterion. The variation of kj in dioxane is by a factor of 25 between the different counterions. Since the fraction of solvent-separated ion pairs is extremely small in dioxane, kj is indicative of kc. The larger, more loosely held cesium counterion results in a higher reactivity for the contact ion pair. The variation of kc and ks with solvating power of the reaction medium is not established. Some data indicate that ks is insensitive to solvent while kc increases with increasing solvating power, but these results are limited to ether solvents. 

Propagation of two-ended (bifunctional) propagating species often proceeds at a rate lower than that of the corresponding monoanion species as a result of triple-ion formation [Bhattacharyya et al., 1964 Smid, 2002]. For example, ionic dissociation of the counterion from one end of the cesium salt of a two-ended propagating species XXX yields XXXI in which the newly dissociated anionic center remains near the ion pair at the other end of... 

Tertiary carbanions have been isolated, but secondary and primary structures do not exhibit sufficient kinetic stability. The lifetimes of the former is highly dependent on structure, counterion, solvent and other factors. One of the simpler tertiary carbanions is present in (perfluoro-fer/-butyl)cesium obtained as a solid from the reaction of perfluoro(2-methyl-propene) and cesium fluoride in tetraglyme.36 The structure of the salt was analyzed by 19F and l3C NMR spectroscopy. 

The surfactant is cesium perfluorooctanoate, with cesium being the counterion. The CsPFO-H20 system has been well studied experimentally and is regraded as a typical binary system to exhibit micellization. As the details of the simulation have been discussed elsewhere [8], we directly proceed to the discussion of the significant results. 

The cesium thlontle structure. Cesium chloride crystallizes in the cubic arrangement shown in Fig. 4.1b. The cesium or chloride ions occupy the eight comers of the cube and the counterion occupies the center of the cube.1 Again,... 

When triethanolamine H3L13 (35) was reacted with sodium hydride and iron(III) chloride, the hexanuclear centrosymmetric ferric wheel [Nac Fe6(L13)6 )Cl (36) was isolated. Amidst a set of possibilities in the template-mediated self-assembly of a supramolecular system, the one combination of building blocks is realized that leads to the best receptor for the substrate [112]. Therefore, the six-membered cyclic structure 36 is exclusively selected from all the possible iron triethoxyamine oligomers, when sodium ions are present. The iron(III) complex 36 is present as an Sg-symmetric wheel, with an encapsulated sodium ion in the center and a chloride counterion. Consequently, the trianion (L13)3- acts as a tripodal, tetradentate, tetratopic ligand, which each links three iron(III) ions and one sodium ion. In the presence of cations with different ionic radii, different structures are expected. Therefore, when triethanolamine H3L13 (35) was reacted with cesium carbonate and iron(III) chloride, the octanuclear centrosymmetric ferric wheel [Csc Fe8(L13)8 ]Cl (37) was isolated (Scheme 13) [113]. 

An alternative suggestion is that the coordinated monomer dissociates to give an oxyanion stabilized by the nucleophile s counterion, most commonly, tetrabutylammonium or cesium ions,69 by analogy to the mechanism proposed for activation of tributylstannyl ethers by nucleophiles.57 This proposal is summarized in Fig. 14, which also shows the product mixture obtained for methyl (2S,3R)-2,3-dihydroxybutanoate. For this compound, reaction on the oxygen atom of the inherently less acidic hydroxyl is favored. Both anions, E and F, are in equilibrium with the coordinated monomer, and the less populated (but more reactive) anion E reacts to a greater extent, or in other words, the difference in the rate constants for trapping is greater than the difference in equilibrium constants. 

Octaethyl-l,l -biarsole/stibole 19 (E = As, Sb) can be reduced by alkali metals such as sodium, potassium, rubidium, and cesium in DME or TMEDA yielding metal arsolides 69 (E = As) or stibolides 69 (E = Sb) with corresponding counterion and solvents being bidentate co-ligands (Equation 11) <20040M3417>. 

Figure 2.18 Effect of radius of hydration on distance between counterion and fixed charge of ion-exchange stationary phase. Cesium, with smaller radius of hydration, is shown with one water molecule (small circle) between it and fixed charge of the bead. Lithium is shown with three water molecules.

Another effect, called the cesium effect [843, 844], is also connected with the observation that salts of anions with large counterions are highly dissociated in dipolar aprotic solvents, and consequently more anion-reactive. For example, the higher solubility of cesium carbonate in dipolar aprotic solvents and the fact that this salt is far more dissociated than the corresponding Li+, Na" ", or K" " salts, makes this carbonate a superior base in organic synthesis. Amongst the alkali metal cations, the ionic radius of Cs+ r = 334 pm) is more than twice that of Li+ (r = 152 pm). 

The mechanism of the Kolbe-Schmitt reaction was investigated since the late 1800s, but the mechanism of the carboxylation could not be elucidated for more than 100 years. For a long time, the accepted mechanism was that the carbon dioxide initially forms an alkali metal phenoxide-C02 complex, which is then converted to the aromatic carboxylate at elevated temperature. The detailed mechanistic study conducted by Y. Kosugi et al. revealed that this complex is actually not an intermediate in the reaction, since the carefully prepared phenoxide-C02 complex started to decompose to afford phenoxide above 90 °C. They also demonstrated that the carboxylated products were thermally stable even at around 200 °C. The CO2 electrophile attacks the ring directly to afford the corresponding ortho- or para-substituted products. (When the counterion is large (e.g., cesium) the attack of CO2 at the ortho-position is hindered therefore, the para-substituted product is the major product.)... 

The results for the isotopic separation of Na/ Na which were obtained by Knochel and Wilken in the system Dowex 50/aqueous or methanolic solution of cryptands are summarized in Table 13 (explanation for Krl and Kr see Chap. 4.3.1.2). To reach a high total enrichment compared with one equilibrium stage, the batch experiments were carried out as a cascade (Chap. 2.5.2). Then Eq. (20) was used for the calculation of a-values. To determine the isotopic separation factor Mr for the complex formation as well, the Kr-vuIucs were analyzed in the same system without cryptands " .iss) see Chap. 4.3.1.2). In all experiments 30 mg cation exchanger resin (Li - or Cs -form) were equilibrated with a 10M Na -solution where a lithium or cesium salt, which corresponds to the counterion of the resin, was added up to a total cation concentration of 10 M. If one has used a complexing ageftt, the initial cryptand concentration has been established to be 10 M (pH = 8). For most of the systems, the standard deviations given in Table 13 correspond to seven parallel experiments. The measurement of the radionuclides Na and Na was carried out as described in Chap. 4.2.4. 

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