Specific cation effect

We have seen this situation before m the reaction of alcohols with hydrogen halides (8ection 4 11) m the acid catalyzed dehydration of alcohols (8ection 5 12) and m the conversion of alkyl halides to alkenes by the El mechanism (8ection 5 17) As m these other reactions an electronic effect specifically the stabilization of the carbocation intermediate by alkyl substituents is the decisive factor The more stable the carbo cation the faster it is formed... 

Essentially identical results were obtained by a second group with fcj = 1.4+0.3 l.mole . sec at 25 °C (/i = 0.02 M). Specific cation effects were also found with rate sequences K > Na" N(C2H40H)4. > Li" " and positive deviations of the reaction orders above unity were also noted. 

R.M.McCready et al.[l] demonstrate that the adding of O.IM NaCl to 0.33 M ammonia hydroxide enhances the deesterification more than twice. The adding of other chlorides, such as KCl, MgCb and CaCb also enhances the reaction. The effectiveness of the ions in relation to the deesterification and rate constant is arranged in the following order Ca>Mg>Na, K. The equal values of pH and ion strength prove that only the specific cation effect is responsible for the enhanced velocity. 

Recent GGA first principles pseudopotential calculations conclude that Aun clusters adopt planar structures up to larger sizes than silver and copper, particularly the anionic species, due to relativistic effects. Specifically, Fernandez and coworkers obtain planar structures for the ground state of anionic v=l), neutral v=0), and cationic (z/=+l) species of Au(( clusters... 

Accordingly, correlations are often found between activity, selectivity and the concentration of cations in specific oxidation states, e.g. V4+ in V205. The improvement of selective catalytic qualities of metal oxides by addition of modifiers, or by combination in mixed oxides may hence be explained by stabilization of the essential cation in the proper oxidation state. In some cases, stabilization of a (partially) reduced cation appears to yield the most effective catalyst however, more often it is the higher oxidation state that should be maintained, and accordingly the role attributed to a second cation often concerns facilitating the reoxidation, for instance, by direct electron transfer between the cations or, in general, by increasing electron conductivity. 

Tphe force-area (tt-A), surface potential-area (AV-A), and surface vis-cosity-area (-q-A) characteristics of long-chain fatty acid monolayers at the air-water interface have been extensively investigated over acid and neutral aqueous solutions (17). Studies of fatty acid monolayer isotherms at high pH and the specific cation effect on the isotherms are less numerous. The difficulties inherent in working at high pH are ... 

Surface pressures, potentials, and viscosities of stearic acid films on hydroxides of Li, Na, K, Rb, Cs, and NH4 have been determined to characterize the specific cation effect. The first observation was that of expansion of the isotherms at high pH related to the nature of the cation. In addition, unexpected results were evident with NH4OH substrates, notably high viscosity and reversal in the sign of the surface potential with compression. 

Specific Cation Effect. Adam and Miller (2) noted a specific expansion in the tt-A isotherms of behenic acid on 2N solutions of NaOH and KOH. They attributed the expansion of the ionized monolayer to cou-... 

Sears and Schulman (51) measured surface pressures and potentials vs. molecular area (20 to 110 sq. A. per molecule) for the alkali metal stearates over 0.5N solutions of LiOH, NaOH, and KOH. Like Adam and Miller, they detected a specific cation effect on the 7r-A and AV-A isotherms of the ionized monolayer at high pH the ir-A curves were expanded in the order of the crystalline sizes of the alkali metal cations K > Na > Li. The sequence is the reverse observed for the long-chain... 

Reversal of Specific Cation Effect in ir-A Isotherms. The reversal of the specific cation effect on the tt-A curves for the long-chain carboxylic acids (51) and the alkyl sulfates (45, 58) can be related to a model sug-... 

Similarities between reversal of charge spectra for colloids with solubility sequences of salts in bulk solution have been indicated (7, 37)— for example, for salts possessing different cations but the same anion, the solubility sequence of the phosphates is K+ > Na+ > Li+, while it is the reverse for the sulfates (7, 30). In view of this similarity, it is natural to introduce this fact as another possible link to explain the reversal of the specific cation effect in tt-A isotherms, though the precise relationship to monolayers is not clear at this time. 

Rogers and Schulman (45) report the existence of both a specific cation and a specific anion effect on ir-A isotherms for the anionic octa-decyl sulfates. The anionic fatty acids have indicated a specific cation effect, but no demonstration of an anion effect has been attempted. However, the long-chain cationic surfactants STA+ exhibit a specific anion effect only. 

The distinction between electrophilic and electron-transfer mechanisms of addition reactions to vinyl double bonds of ArX—CH=CH2 (X = S, O, Se) has been achieved by studying substituent effects. Specifically, the effects of meta and para substituents on the rates of electrophilic additions correlated with Hammett radical cations correlates with statistical tests. The ofclcctrophilicj/o-1 (FT) dichotomy is in accord with the conventional paradigm for cr/cr+ correlations and further support has been found by ah initio calculations. Interestingly, the application of this criterion to the reactions of aryl vinyl sulfides and ethers with tetracyanoethylene indicates that cyclobutanes are formed via direct electrophilic addition to the electron-rich alkene rather than via an electron-transfer mechanism.12... 

The site preferences shown by cations in the spinel structure demonstrate that transition metal ions prefer coordination sites that bestow on them greatest electronic stability. In addition, certain cations deform their surrounding in order to attain enhanced stability by the Jahn-Teller effect. These two features suggest that similar factors may operate and cause enrichments of cations in specific sites in silicate structures, leading to cation ordering or intersite (intracrystalline ) partitioning within individual minerals which, in turn, may influence distribution coefficients of cations between coexisting phases. 

Electrolytes affect dispersed polysaccharides through water inactivation, specific ion binding, and polyanion neutralization. Each effect is valence-dependent, but is less on neutral polysaccharides than on ionic polysaccharides. Di- and polyvalent cations gel or precipitate a constant amount of polysacchride at much lower concentrations than do monovalent cations. The precipitation reaction is used to advantage in isolating pectin with alkaline Al3+, because this cation and polymeric forms of Al(OH)3 readily precipitate and entrain pectinic acid from apple tissue homogenates. Other di- and polyvalent cation effects are crosslinking (Prud homme et al., 1989) and an increased rate of (3 elimination over monovalent cations (Sajjaanan-... 

A completely different situation occurs for mica surfaces, for which both charges and dipoles are present on the surface. For these surfaces, strong specific cation effects have been observed experimentally at relatively low electrolyte concentrations [17]. These specific ion effects cannot be explained by the additional interactions between ions (such as excluded volume effects) or ions and surfaces (such as ion-dispersion [9] or ion-hydration [10] forces), because these interactions are in general negligible for electrolyte concentrations smaller than about 0.05 M. 

In Fig. 6c,d, the calculations were repeated for large pH values, for which most of the basic sites of the surface are associated with OH-, and consequently the association of anions is negligible. In this case, only specific cation effects can be predicted by changes in the effective dipole moment on the surface. As already noted, the interactions between... 

In some cases, secondary forces may exert a sizable influence on the coordination environment as well. For example, it was seen earlier how the second-order JT effect frequently manifests itself as a displacement of transition metals from the center of an octahedron. The phenomenon is only observed for metals with low d electron counts. This could be used advantageously in synthetic strategies where the goal is to selectively place cations in specific sites within a stmcture. 

H. C. Hornig et al., 7hid,y 72, 3808 (1950), and J. C. Sheppard and A. C. Wahl, ihid.y 79, 1020 (1957), suggest on the basis of specific cation effects that cations act as bridges between the two reacting anions. In such case the exchange could take place via metal atom transfer. 

The oxidation of [W(CN)g] by periodate in neutral and weak basic media is of the first order with respect to both reagents, and a specific cation effect of Li < Na < < Rb Cs < Mg was observed (97). 

Solubility practically insoluble in cold ethanol (95%) and in cold water. Starch swells instantaneously in water by about 5-10% at 37°C. Polyvalent cations produce more swelling than monovalent ions, but pH has little effect. Specific surface area ... 

The possibility of the existence of specific, cationic effects, as opposed to pH effects, in the saccharinic acid rearrangement requires further study. 

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