Associated ion-pairs

A criterion for the presence of associated ion pairs was suggested by Bjerrum. This at first appeared to be somewhat arbitrary. An investigation by Fuoss,2 however, threw light on the details of the problem and set up a criterion that was the same as that suggested by Bjerrum. According to this criterion, atomic ions and small molecular ions will not behave as strong electrolytes in any solvent that has a dielectric constant less than about 40. Furthermore, di-divalent solutes will not behave as strong electrolytes even in aqueous solution.2 Both these predictions are borne out by the experimental data. 

In aqueous electrolyte solutions the molar conductivities of the electrolyte. A, and of individual ions, Xj, always increase with decreasing solute concentration [cf. Eq. (7.11) for solutions of weak electrolytes, and Eq. (7.14) for solutions of strong electrolytes]. In nonaqueous solutions even this rule fails, and in some cases maxima and minima appear in the plots of A vs. c (Eig. 8.1). This tendency becomes stronger in solvents with low permittivity. This anomalons behavior of the nonaqueous solutions can be explained in terms of the various equilibria for ionic association (ion pairs or triplets) and complex formation. It is for the same reason that concentration changes often cause a drastic change in transport numbers of individual ions, which in some cases even assume values less than zero or more than unity. 

As already mentioned, the criterion of complete ionization is the fulfilment of the Kohlrausch and Onsager equations (2.4.15) and (2.4.26) stating that the molar conductivity of the solution has to decrease linearly with the square root of its concentration. However, these relationships are valid at moderate concentrations only. At high concentrations, distinct deviations are observed which can partly be ascribed to non-bonding electrostatic and other interaction of more complicated nature (cf. p. 38) and partly to ionic bond formation between ions of opposite charge, i.e. to ion association (ion-pair formation). The separation of these two effects is indeed rather difficult. 

Neither the electronic absorption nor the emission spectrum of Re2Cl8 changes in the presence of the quenchers, and no evidence for the formation of new chemical species was observed in flash spectroscopic or steady-state emission experiments. The results of these experiments suggest that the products of the quenching reaction form a strongly associated ion pair, Re2Cl8 D+. 

The salting-in effect may be used to increase the solubility of a drug substance through the formation of associated ion pairs, most commonly making use of anionic countering (hydrochloride being the most popular). Detailed reviews of pharmaceutical salts have been published, which contain extensive tables of anions and cations acceptable for pharmaceutical use [44,47]. These articles also describe useful processes for the selection of the most desirable salt... 

In the preceeding section mention was made of ion association (ion-pairing) which, for the purposes of this paper, will refer to coulombic entities with or without cosphere overlap. Experimental support for ion-pairing has come from sound attenuation (2). Raman spectroscopy (2) and potentiometry (2, 2). Credibility has resulted from the model of Fuoss (2) applied by Kester and Pytkowicz (2). 

Taken together these results suggest that the quaternary ammonium ions must not form closely associated ion pairs in acetone solutions. In this more polar solvent, it appears as if the reactants are present as solvent separated ion pairs and that the rate of reaction is, consequently, not effected by the structure of the quaternary ammonium ion. In methylene chloride solutions, where theory predicts tighter ion pairs (61), the ions must be intimately associated in either (or both) the ground state and the transition... 

Sakai et al. determined procaine by a colorimetric method that was proposed for the assay of procaine on the basis of solvent extraction [39]. Tetrabromophenolphthalein ethyl ester anion was added to an aqueous solution containing procaine, and the extract took on a red color (absorbance maximum of the extract at 580 nm). The optimal pH range for extraction of the drug from the aqueous solution was found to be 8-9. Procaine was found to form a 1 1 associated ion pair compound with the reagent in 1,2-dichloroethane. 

By analogy, one can define an association constant ion-pair formation. Thus, one can consider an equilibrium between free ions (the positive M" ions and the negative A ions) and the associated ion pairs (symbolized IP)... 

The activation parameters for the exchange reactions of 17 and 18 were determined by a combination of variable-temperature ll NMR lineshape analysis16 and spin saturation transfer experiments.17 Rate data for 17 were measured over a temperature range of 100 "C. Rates for compound 18 were measured over a 65 °C range. The enthalpy of activation was found to be considerably smaller in the case of 17 (12.2(2) kcal/mol) relative to 18 (17.6(3) kcal/mol). Ion pair dissociation is therefore facilitated by the presence of a lone pair of electrons on the boron substituent. The entropy of activation for 17 is -2.3(6) eu, while that of 18 is 8(1) eu. The more positive entropy of activation measured for 18 may be interpreted as the creation of two independent particles from a closely associated ion pair. 

The propagating anion and its counterion exist in relatively nonpolar solvents mainly in the form of associated ion pairs. Different kinds of ion pairs can be envisaged, depending on the extent of solvation of the ions. As a minimum, an equilibrium can be conceived between intimate (contact) ion pairs, solvent-separated ion pairs, and solvated unassociated ions. The nature of the reaction medium and counterion strongly influences the intimacy of ion association and the course of the polymerization. In some cases the niicrostructure of the polymer that is produced from a given monomer is also influenced by these variables. In hydrocarbon solvents, ion pairs are not solvated but they may exist as aggregates. Such inter-molecular association is not important in more polar media where the ion pairs can be solvated and perhaps even dissociated to some extent. 

The initiation and propagation processes are influenced by equilibria between various degrees of association of the active center and its counterion. As a minimum, it is necessary to conceive of the existence of contact (associated) ion pairs, solvent-separated ion pairs, and free solvated ions. A simplified reaction scheme [3] is presented in reaction (9-37). 

The nature of the cation is unimportant in aqueous or other highly polar solutions of borohydrides, but influences the rate of reaction in isopropanol or pyridine, where the reagent exists mainly as associated ion-pairs [44]. Lithium borohydride is more reactive than the sodium compound in these solvents Li+ can probably associate more closely than Na+ with the carbonyl oxygen, promoting polarisation of the C=0 group, and so aiding hydride transfer from the anion. Other cations [e.g. Ca +j and solvents e.g. dimethylformamide) provide variations in reactivity which can have valuable uses for selective reduction of carbonyl groups [42]. 

Values of n in equation 9 can vary from 1 for cryptate-coordinated lithium silanolates (25, 27) and R4NOSi= (35) to 4 for LiOSi=. The latter result was reported to be dependent on dilutions (36). In the absence of polar solvents, n = 2 is commonly reported for potassium silanolate initiators (37, 38). The fractional order is attributed to strongly associated ion pairs at the chain ends. These ion pairs must dissociate to provide a low concentration of unassociated ion pairs prior to propagation. In the case of potassium silanolate, this dissociation is pictured as the initiation of the polymerization (37), as shown by equations 10 and 11. 

Observed molar conductivities were analyzed by assuming the ion association (ion-pair formation) between the complex ions and the counter ions in the same manner as described previously. The closest distances of approach of ions (a) in the Robinson-Stokes conductivity equation and in the Debye-HUckel equation were taken as 6.8 and 7.3 A for chlorides and perchlorates of the tris(phen) complexes 6.6 and 7.1 A for those of the tris(bpy) complex, respectively, using the effective ionic radii of the complex ions, shown in T le 1, and those of Cl (1.81 A) and C104 (2.30A). The values of ref were estimated from the ionic partial molar volumes (f i°°) by use of Glueckauf equation. > ... 

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