Ion-aggregates

The order of enolate reactivity also depends on the metal cation which is present. The general order is BrMg < Li < Na < K. This order, too, is in the order of greater dissociation of the enolate-cation ion pairs and ion aggregates. Carbon-13 chemical shift data provide an indication of electron density at the nucleophilic caibon in enolates. These shifts have been found to be both cation-dependent and solvent-dependent. Apparent electron density increases in the order > Na > Li and THF/HMPA > DME > THF >ether. There is a good correlation with observed reactivity under the corresponding conditions. 

The behavior of ionic liquids as electrolytes is strongly influenced by the transport properties of their ionic constituents. These transport properties relate to the rate of ion movement and to the manner in which the ions move (as individual ions, ion-pairs, or ion aggregates). Conductivity, for example, depends on the number and mobility of charge carriers. If an ionic liquid is dominated by highly mobile but neutral ion-pairs it will have a small number of available charge carriers and thus a low conductivity. The two quantities often used to evaluate the transport properties of electrolytes are the ion-diffusion coefficients and the ion-transport numbers. The diffusion coefficient is a measure of the rate of movement of an ion in a solution, and the transport number is a measure of the fraction of charge carried by that ion in the presence of an electric field. 

Chemical models of electrolytes take into account local structures of the solution due to the interactions of ions and solvent molecules. The underlying information stems from spectroscopic, kinetic, and electrochemical experiments, as well as from dielectric relaxation spectroscopy. The postulated structures include ion pairs, higher ion aggregates, and solvated and selectively solvated ions. 

The investigation of ion-aggregate formation based on conductivity studies can be extended to quadruple-ion formation [108-111], which is thought to be the reason for the conductivity maximum and the subsequent decrease of conductivity. 

Further, in the case of virtually non-existent ion-solvent interactions (low degree of solvation), so that solute-solute interactions become more important, Kraus and co-workers47 confirmed that in dilute solutions ion pairs and some simple ions occurred, in more concentrated solutions triple ions of type M+ X M+ orX M+X andinhighly concentrated solutions even quadrupoles the expression triple ions was reserved by Fuoss and Kraus48 for non-hydrogen-bonded ion aggregates formed by electrostatic attraction. 

PET chain undergoing folding into crystallites around sodium ion aggregate... 

The formation of solvent separated ion aggregates is largely determined... 

XAGWUX yes 2D slabs parallel to ab water and sulfate ions aggregate in dimers... 

Conductiometric studies showed that ion aggregates exist under these conditions. In dimethylformamide, the association between one dipositive cation and two anions, bromide or thiocyanate, was almost complete when stoichiometric amounts of anion and cation were mixed, even in dilute solution. But there was no tendency for a third anion to be added to the assembly. In acetone, thiocyanate saturated the ion aggregate at the 2 1 composition and did not add further to it. But chloride... 

Since movement within the solvation shell in these complexes is relatively sluggish, it is postulated that a complex remains activated only long enough to react with its immediate environment, the inner solvation shell. In the reaction with anionic species, a situation can be reached in which nearly all of the substrate is in the form of the maximum ion aggregate. Any increase in the anion concentration in the bulk solvent will not change the immediate environment of nearly all the substrate and, therefore, will not effect the reaction rate. In this way a limiting rate can be independent of the concentration of added anionic reagent, irrespective of the actual mechanism of the actual act of substitution. 

The unimolecular reaction of the ion aggregate follows a similar course and the intermediate faces the same three possibilities for reaction. The rate of bond fission will not necessarily be the same as that of the free ion because the solvation environment has changed. We see this effect in the ion pair-catalyzed solvolytic reactions (7). In addition, since the reagent Y is in position before the five-coordinate intermediate is formed, the path by which X re-enters the coordination shell becomes less probable as a result of more effective competition by Y, and the rate is increased. 

The bimolecular process is depicted in Figure 4, where the free ion has little choice in its reaction, being surrounded only by solvent molecules. If it can undergo solvolysis, then one of these molecules will attack if not, there will be no reaction. The ion aggregate contains suitable reagents in the inner solvation shell, and the bimolecular reaction can take place, either by solvolysis or by anion attack. 

All of this suggests that the ion association explanation may be applied here to an essentially bimolecular (or associative) phenomenon. Considering the difference between hydroxide and any other reagent in water, apart from its basicity, one concludes that its mobility must play an important part. Whereas all the other reagents must be in a suitable position within the solvation shell before they can enter the complex, the hydroxide ion, by means of a Grotthus chain proton transfer, can be transmitted to any position where it is needed while the complex becomes activated. It can therefore be looked upon as an unsaturatable ion aggregate with hydroxide fully delocalized about the complex. Consequently, we do not observe any departure from the first-order dependence upon hydroxide concentration. This contribution to the reactivity will appear in the activation entropy rather than in the enthalpy term. 

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