Ion-pair dissociation constant

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),... 

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... 

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 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. 

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

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. 

The solvated electrons in the flash transient may be present either as free or paired ions. The extent of pairing may be estimated in a given experiment from the extinction coefficient of the solvated electron (taken to be 1.0 X 104 at 980 m/z) (10), from the potassium amide dissociation constant and from the ion pair dissociation constant for the process [M + e ] - M+ + e. Taking this to be about 3 X 10-3 M (1,12,14), we find for a typical flashed solution in which ADm 0.02 and Dm = 0.9, that the ratio (unpaired/paired) solvated electrons is about 30. This result is difficult to confirm, at least by spectroscopic methods, owing to the obedience to Beer s Law in the pairing region (13). 

Table I. Ion-Pair Dissociation Constants (Kd)b for Hexachloroanti-monate Salts of Stable Organic Cations in CH2CI2 (34)...

This in no way detracts from the proved ability of the cyclohepta-trienyl cation to oxidize reactive amines, and consequently there is always doubt as to the true nature of the reaction mechanism when tropylium ion initiates polymerization of N-vinylcarbazole. We have made a detailed kinetic study of this polymerization (34), using an adiabatic calorimetric technique. Some typical data are shown in Table VII. Initiation is instantaneous and complete, there is no termination, and kp is evaluated readily as 4.6 X 10+5M-1 sec. 1 at 0°C. with Ea = 6 kcal./mole. By comparing data for the ion pair dissociation constant of C7HT"SbCl 6 (Table I) with the catalyst concentrations employed (Table VII) it is apparent that free tropylium ions are the dominant initiating speices. It... 

Contact Ion Pair Dissociation Constants of Charge-Transfer Salts in Polar and Nonpolar Solvents ... 

Table 7 Molar conductivities, ion pair dissociation constants, and thermodynamic parameters for the dissociation of carbenium ion salts in dichloromethane...

GiUcerson, W.R. Application of free volume theory to ion-pair dissociation constants. 

Fig. 16. Temperature dependence of ion-pair dissociation constants of fluorenyl...

This is true for all the values reported in Table 6 with one exception. Only for solvents such as diethylether and dioxane, where the free anion contribution to the rate is negligible, do they represent true values corresponding to a single species, in this case the ion-pair. If the reactivity ratios are determined from measurements of the rates of the four individual reaction steps, then it is possible to analyze the results, if each is carried out at a series of concentrations, in the manner described in Section 4.2, for kxy and k y. If the reactivity ratios alone are required it is not necessary to measure the ion-pair dissociation constants because these cancel in the determination of and rj, e.g. 

The value of, the ion pair dissociation constant, varies according to the nature of R" and X , the solvating and dissociating ability of the solvent employed and the temperature. Dennison and Ramsey [48] have treated quantitatively the effect of solvent and have shown the following relationship to hold ... 

There seems little doubt that in radiation induced polymerizations the reactive entity is a free cation (vinyl ethers are not susceptible to free radical or anionic polymerization). The dielectric constant of bulk isobutyl vinyl ether is low (<4) and very little solvation of cations is likely. Under these circumstances, therefore, the charge density of the active centre is likely to be a maximum and hence, also, the bimolecular rate coefficient for reaction with monomer. These data can, therefore, be regarded as a measure of the reactivity of a non-solvated or naked free ion and bear out the high reactivity predicted some years ago [110, 111]. The experimental results from initiation by stable carbonium ion salts are approximately one order of magnitude lower than those from 7-ray studies, but nevertheless still represent extremely high reactivity. In the latter work the dielectric constant of the solvent is much higher (CHjClj, e 10, 0°C) and considerable solvation of the active centre must be anticipated. As a result the charge density of the free cation will be reduced, and hence the lower value of fep represents the reactivity of a solvated free ion rather than a naked one. Confirmation of the apparent free ion nature of these polymerizations is afforded by the data on the ion pair dissociation constant,, of the salts used for initiation, and, more importantly, the invariance, within experimental error, of ftp with the counter-ion used (SbCl or BF4). Overall effects of solvent polarity will be considered shortly in more detail. 

Coupled with these considerations is the effect of solvent on the ion pair dissociation constant, which governs the proportion of each species present in a system at any instant. Obviously the more polar the medium the more highly dissociated an ionic species and vice versa. Since free ions... 

Some ion pair dissociation constants obtained from conductivity data are given in Table II (13). As might be expected, Na2[Fe(CO)4] has a very low dissociation constant in THF solution around room temperature. Under similar conditions, Li[Mn(CO)5] has a larger dissociation constant than Na[Mn(CO)5]. Apparently THF competes more favorably than Mn(CO)s for coordination with the small Li+ ion than with the larger Na+. 

The 1 1 ion-pair dissociation constants are determined by use of conductivity measurements or are estimated theoretically, for example, by use of the Fuoss model of ion pairs (eq 12) [61]. 

In NMP or THF the values of the two successive ion-pair dissociation constants of [Fe(CO)J " shown in (12) were estimated from a combination of conductometric titrations in the presence and absence of the cryptand,... 

Conductance and chemical relaxation were measured for TBAP ion-pairs in benzene-chlorobenzene mixtures, with composition as given in Table I, over a large concentration and pressure range. At low concentration and/or higher dielectric constant the simple ion-pair dissociation is the main ionisation process. From the conductimetrie data the ion-pair dissociation constant was obtained at different pressures in all solvent-mixtures. 

The tendency for ion pair dissociation strongly depends on the nature of anion and cation. Thus, fQ should sttongly depend on the ion pair dissociation constants of metal-free initiator and the enolate ion pair. The tetrabutylammonium cation is quite large, leading to a larger interionic distance. 

---