Ion-pair dissociation

MC has a much stronger influence on ion-pair dissociation than PC. The EO units on MC coordinate cations which have been dissociated by the carbonate group, and prevent cation association with the anion. It is thought that, whereas conventional plasticizers like PC create fast ion-... 

Hardy effect.248-249 The internal return part of the ionization equilibrium is particularly hard to detect since it is almost completely independent of the concentration of anything in the bulk of the solution outside of the solvent cage. The extent of internal return will depend on the reactivity of the cage walls and their resistance to the escape of either ion. Unless internal return has been eliminated by the use of an extremely reactive cage wall, the measured rate is not that of the ionization but the lesser rate of ion pair dissociation. In the case of the acetolysis of a, a-dimethylallyl chloride (XXXIX), internal return is detectable by virtue of the fact that the chloride ion can return to either of two allylic carbon atoms.248... 

Again, bromination of 11 in methanol shows (21) an inconsistent behavior with reactions found in all the other solvents, and in particular in TFE. Despite the fact that its polarity should favor ion pair dissociation, the reaction gives at all reagent concentrations examined, practically only the bromomethoxy derivative. This behavior may be attributed to the ability of MeOH to provide nucleophilic assistance a reaction pathway occurring through ion sandwich could account for the absence of the elimination product. 

As far as the polymerisation of heterocyclic monomers is concerned, the situation is qualitatively similar, but quantitatively different. As a model for the active species in oxonium polymerisations, Jones and Plesch [10] took Et30+PF6 and found its K in methylene dichloride at 0 °C to be 8.3 x 10"6 M however, in the presence of an excess of diethyl ether it was approximately doubled, to about 1.7 x 10 5 M. This effect was shown to be due to solvation of the cation by the ether. Therefore, in a polymerising solution of a cyclic ether or formal in methylene dichloride or similar solvents, in which the oxonium ion is solvated by monomer, the ion-pair dissociation equilibrium takes the form... 

The role of ion-pairs is discussed at some length. Conductivity measurements on polymerised solutions of EVE with successive dilutions gave results from which ion-pair dissociation constants KD were calculated conventionally by means of Shedlovsky plots. However, since the conductivity of solutions of the model system EtOCHMe+ SbCl6" can be interpreted much more plausibly in terms of a BIE (Plesch and Stannett, 1982),... 

The regio-, stereo- and chemoselectivities have been mainly interpreted in terms of bridging of the ionic intermediate and/or ion pair dissociation. Solvent-separated ion pairs and free ions have often been considered to explain the product selectivities of these reactions. Nevertheless, the stereochemical outcomes can also be determined by the relative rates of the ion pair dissociation and of the nucleophilic trapping of the intermediate, i.e. by the lifetime of the intermediate . 

As for solvents, liquid ammonia or dimethylsulfoxide are most often used. There are some cases when tert-butanol is used as a solvent. In principle, ion-radical reactions need aprotic solvents of expressed polarity. This facilitates the formation of such polar forms as ion-radicals are. Meanwhile, the polarity of the solvent assists ion-pair dissociation. This enhances reactivity of organic ions and sometimes enhances it to an unnecessary degree. Certainly, a decrease in the permissible limit of the solvent s polarity widens the possibilities for ion-radical synthesis. Interphase catalysis is a useful method to circumvent the solvent restriction. Thus, 18-crown-6-ether assists anion-radical formation in the reaction between benzoquinone and potassium triethylgermyl in benzene (Bravo-Zhivotovskii et al. 1980). In the presence of tri(dodecyl)methylammonium chloride, fluorenylpi-nacoline forms the anion-radical on the action of calcium hydroxide octahydrate in benzene. The cation of the onium salts stabilizes the anion-radical (Cazianis and Screttas 1983). Surprisingly, the fluorenylpinacoline anion-radicals are stable even in the presence of water. 

The major factor in determining which mechanism is followed is the stability of the carbocation intermediate. Alkenes that can give rise to a particularly stable carbocation are likely to react via the ion-pair mechanism. The ion-pair mechanism would not be expected to be stereospecific, because the carbocation intermediate permits loss of stereochemistry relative to the reactant alkene. It might be expected that the ion-pair mechanism would lead to a preference for syn addition, since at the instant of formation of the ion pair, the halide is on the same side of the alkene as the proton being added. Rapid collapse of the ion-pair intermediate leads to syn addition. If the lifetime of the ion pair is longer and the ion pair dissociates, a mixture of syn and anti addition products is formed. The termolecular mechanism is expected to give anti addition. Attack by the nucleophile occurs at the opposite side of the double bond from proton addition. 

If ion-pair dissociation constants, K and are roughly the same, which tends to be true for hydrocarbons, then the ion-pair Ki and true, free-ion values will be... 

Whenever equilibrium (a), which is coneentration-independent, is more Important than the conventional dissociation of ion-pairs, then its effect, in conjunction with the ion-pairs dissociation, decreases the anticipated concentration of anions. The reverse is true when equilibrium (b) prevails on (a). 

Since the design of the measuring cell for field modulation studies is not very critical it was relatively easy to study also the pressure-dependence of ion-pair dissociation and ionic recombination (1 4). Here also the values (Table II) for the activation volumes show the essentially diffusion controlled aspects of the association-dissociation phenomena, since the calculated values, essentially the pressure dependence of the viscosity, and the experimentally determined values agree rather well. 

III. In this way we obtain, with the ion-pair dissociation constant determined from conductance data, a value of 5.3 x 10-5 for the average stability constant of the triple ions (K3 from eqn. 13), This should be compared with 1.24 x lCT M as derived from conductance indicates an important fraction of simple ions over the largest part of the concentration range studied. 

In conclusion, it has been shown that use of cryptates for the anionic polymerization of heterocyclic monomers leatis to a tremendous increase of the rates of polymerization. There are two main causes to the higher reaction rates observed with cryptates. The first one is a suppression of the association between ion pairs in the non polar media, and the second one is the possibility of ion pairs dissociation into free ions in ethereal solvents like THP or THF. By this way, it has been possible to make detailed studies of the propagation reaction for propylene sulfide, ethylene oxide, and cycloslloxanes. 

Typically, low temperatures are necessary to suppress these reactions. Additionally, other considerations, such as stabilization of the propagating centers, use of additives to suppress ion-pair dissociation and undesirable protic initiation, and the use of high purity reagents to prevent the deactivation of the carbenium by heteroatomic nucleophiles are often required. However, by careful... 

In order to determine the ion pair dissociation constant Kd, of a salt it is necessary therefore to measure X as a function of C and obtain a roughly extrapolated value for X0. Calculation of the variables F(z)/X and f 2 F(z)CX is usually accomplished with a small computer program, and hence a more accurate value for X0 and a first value for Kd obtained from a straight line plot of these functions. It is, however, more convenient to carry out the whole process by computer with iteration accompanied by a least mean square calculation to obtained the most accurate value for X0 and Kd. For solvents of low dielectric constant, and if sufficiently dilute solutions are not examined, Fuoss plots deviate downward at higher concentrations, because of triple ion formation. This can lead to an excessively low estimate for X0 and too high a value for Kd. 

In D/8 T)P is always negative and its absolute value nearly always exceeds unity. Hence AHd is nearly always negative and dissociation therefore, more often than not, exothermic. The entropy term arises solely from changes in the degree of physical solvation. It does not account for the fact that one particle, the ion pair, dissociates into two free ions. Formation of two species from one increases the entropy of the system by an extra term A S, whose value is determined mainly by the greater translational freedom of the two free ions relative to the ion pair. Thus A Gd is given by the modified equation... 

Table 1. Ion pair dissociation constants, Kd, of sulphonium salts at 20° C (52)...

Table 2. Ion pair dissociation data of triphenylmethyl (Ph3C+) cation salts...

Ion pair dissociation constants for a wide variety of hexachloroantimonate salts in methylene chloride are shown in Table 4, together with that for the highly symmetric salt tri-isoamyl-n-butyl>ammonium tetraphenylborate. 

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