Dissociation of ion-pairs

In the discussion of the relative acidity of carboxylic acids in Chapter 1, the thermodynamic acidity, expressed as the acid dissociation constant, was taken as the measure of acidity. It is straightforward to determine dissociation constants of such adds in aqueous solution by measurement of the titration curve with a pH-sensitive electrode (pH meter). Determination of the acidity of carbon acids is more difficult. Because most are very weak acids, very strong bases are required to cause deprotonation. Water and alcohols are far more acidic than most hydrocarbons and are unsuitable solvents for generation of hydrocarbon anions. Any strong base will deprotonate the solvent rather than the hydrocarbon. For synthetic purposes, aprotic solvents such as ether, tetrahydrofuran (THF), and dimethoxyethane (DME) are used, but for equilibrium measurements solvents that promote dissociation of ion pairs and ion clusters are preferred. Weakly acidic solvents such as DMSO and cyclohexylamine are used in the preparation of strongly basic carbanions. The high polarity and cation-solvating ability of DMSO facilitate dissociation... 

The results on the reaction of 1-octadecanol with octadecanoic acid in octadecyl octadecanoate22 are quite different from those relative to the same reaction carried out in benzophenone22 since the order with respect to acid is 1.5 in the first case and 2 in the second. Among the possible explanations of the lowering of the order in acid the most satisfactory is a non-negligible dissodation of the ion pair A and the formation of free RC(OH) . That such a process takes place in a non-polar medium (octadecyl octadecanoate) is rather surprising however, it can be supposed that all the reactive groups assemble in certain areas where they create a very polar medium and where water tends to be retained. In these areas, the dissociation of ion pairs would be easier and hence the overall order would decrease. 

The exceptionally low propagation constants of t-butyl and of phenyl methacrylate are notable. The polymerization of the former monomer was thoroughly examinedS5). At temperatures even as high as 25 °C this reaction, when performed in THF in the presence of salts depressing dissociation of ion-pairs, yields polymers of highly uniform size. The reaction is strictly first order in growing polymers and in monomer, and no... 

Polymerization of t-butyl methacrylate initiated by lithium compounds in toluene yields 100% isotactic polymers 64,65), and significantly, of a nearly uniform molecular-weight, while the isotactic polymethyl methacrylate formed under these conditions has a bimodal distribution. Significantly, the propagation of the lithium pairs of the t-Bu ester carbanion, is faster in toluene than in THF. In hydrocarbon solvents the monomers seem to interact strongly with the Li+ cations in the transition state of the addition, while the conventional direct monomer interaction with carbanions, that requires partial dissociation of ion-pair in the transition state of propagation, governs the addition in ethereal solvents. 

It is now clear that the equilibrium constant, which decreases with increasing temperature, is most probably that for the dissociation of ion-pairs. 

In the present context it will be useful to establish the conditions under which free cations or paired cations might be expected to determine the behaviour of a cationic polymerisation some aspects of this problem have been discussed previously [5]. Consider a system in which Pn+ are the growing polymer molecules and A is the anion derived from the catalyst or the syncatalytic system. Let [Pn+] + [Pn+ A"] = c, let [Pn+] = [A ] = i, [Pn+ A"] = q, and let K be the equilibrium constant for the dissociation of ion-pairs ... 

The effect of the dielectric constant D on enieidic polymerisations is analysed algebraically, with emphasis on the very complicated effect of D on the degree of dissociation of ion-pairs. 

The results of Enikolopyan and co-workers [27, 28] on the polymerisation of styrene by perchloric acid at high pressures shed some new light on the problem. Essentially their kinetic results agree with those of Pepper and Reilly and of ourselves. The important feature of their findings is that the extent of acceleration by pressure is merely that which can be attributed to increase of dielectric constant of the solvent. There was no effect which could be attributed to increasing abundance of free ions by increased dissociation of ion-pairs. This means that, if the propagating species are ions, then they are all free ions even at normal pressure (which is reasonable), or the propagating species is non-ionic. 

The mechanism is outlined in Scheme 1. The formation of triple ions increases the concentration of Cs+ ions and buffers, therefore, the dissociation of ion-pairs into free styryl ions which are the main contributors to the propagation. 

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). 

Tab. 3.1 Effect of solvents on the difficulties of ionization of acid HA and the dissociation of ion-pair (H+, A )so ...

The experimental result obtained was explained by the formation of ion pairs between the charges of a network and counter ions. The theoretical analysis of this problem has shown that the degree of ion pairs formation very strongly (exponentially) depends on e (cf. Sect. 2.2). Thus, if the precipitant has a small dielectric constant e (e.g. dioxane) the degree of dissociation of ion pairs is sufficiently small and this fact leads to the decrease of the osmotic pressure of counter ions which defines the swelling of the gel and the point of the transition in the collapsed state. As a result, in this case the degree of swelling of the gel near the transition point is less pronounced than for other solvents and only a relatively small amount of the precipitant is required to reach this point. In... 

In the styrene (MJ-indene (Mz) system, rx increased with the field. This result shows that the dissociation of ion pairs at the growing chain ends, the terminal group of which is styrene, was enhanced by the field. As was mentioned above, the field has no effect on the homopolymerization of styrene by boron trifluoride etherate in nitrobenzene (see Fig. 5). This result of the homopolymerization seems to be inconsistent with that obtained for the copolymerization, but can be accounted for as follows. The field-accelerating effect decreases as kp/kp decreases, when an enhancement of the degree of dissociation with the electric field is given. The fact that no field effect was observed on the homopolymerization of styrene with boron trifluoride etherate in nitrobenzene may be attributed to a fairly small value of kp/kp, in addition to the factor af 1. On the other hand, the field enhanced the polymerization of indene by boron trifluoride etherate in nitrobenzene (16). The difference in the field effects of the two monomer systems suggests that the following relation must hold, ... 

Electrophoresis does not show the presence of uncharged species, such as undissociated metal alcoholate or carbohydrate-metal hydroxide adducts. These species are probably present in alcoholic solutions, but their concentration has not yet been ascertained. Their presence is suggested by the relatively low mobility of carbohydrates in alcoholic solutions of alkali metal hydroxide. In aqueous media, where greater dissociation of ion pairs should occur, the mobility is extremely high. The possible existence of free carbohydrate-hydroxide ion species cannot be disregarded, because of the hydrogen-bonding properties of the hydroxide ion. 

In acetic acid it is possible to measure separately the equilibrium constant of proton transfer to form an ion pair and the constant for dissociation of ion pairs to form free ions. [I. M. Kolthoff and S. Bruckenstein, J. Amer. Chem. Soc., 78, I (1956) S. Bruckenstein and I. M. Kolthoff, J. Amer. Chem. Soc., 78, 10 (1956)]. G. W. Geska and E. Grunwald, J. Amer. Chem. Soc., 89, 1371, 1377 (1967) applied this technique to a number of substituted anilines and concluded that the equilibrium constant of the ionization step, rather than the overall acid dissociation constant, is the quantity that should be considered in discussions of effects of structural changes on acidity. 

The water in the RMs is considered to be a composite of two different types the "bound water" region, and the remaining "free water" region. On the basis of the IR data up to a Wo = 4, the water solvates the AOT ion-pair, further increasing in the water concentration up to a Wo = 10, probably giving rise to a hydration shell around the new-separated ions of AOT. Further increasing water concentration gives rise to the so called "free water". It has been shown by various physico-chemical techniques that the water of the reverse micelle behaves differently from normal water, especially at low concentrations (Wo < 10). Solubilization of water by such micelles promotes dissociation of ion pairs in the micelle to form micellar free ions. 

There is also only limited information available on the dissociation of ion pairs to free carbenium ions, especially from macromolecular systems (cf., Sections II.D and IV.B). The dissociation constants in cationic polymerizations of styrene are approximately KD = 10-6il moFL in CH2CI2, depending on the temperature and structure of the counteranions... 

Three approaches have been used to decrease the molecular weight distributions. One approach suppresses dissociation of ion pairs to free ions by adding salts with common counteranions however, this may cause a special salt effect [274]. Addition of a common ion salt shifts the equilibrium between ions and ion pairs toward the latter by mass law. (In spite of speculation to the contrary [275], the common ions can not influence the equilibrium between covalent species and ion pairs.) The kinetics of association is also affected because ion pair formation is a bimolecular reaction whose rate increases with increasing anion concentration, This decreases the lifetime of free ions. In such systems, kdeact in Eq. (68) should be replaced with the product of /cdeacl and deactivator [D] = [A ], in which the deactivator is a counteranion (DP ./DP = 1 + [l]0 p/ ([D] deacl)). 

Figure 9 shows evolution of molecular weights with conversion for a hypothetical system, in which ions and ion pairs have the same reactivities (kp+ = kp = 10s mol L sec-1), covalent species are inactive, the ionization equilibrium constant is Ki = 10 5 mol-, L, and the dissociation constant is Kd = 10 7 mol/L. Kf is defined by the ratio of the rate constant of ionization to that of recombination of counterions within the ion pair (Kj = kj/kr). Kd is defined by the ratio of the rate constants of the dissociation of ion pair and that of the association of free ions (Kd... 

However, as an ionic dipole, it may rotate in an external electrical field. On the average, a precise population of these duplexes exists, although the formation and dissociation of ion-pairs are incessant. The degree of association can be easily obtained if the medium dielectric constant and the ion sizes, and the electroly tel concentration... 

Herel andl are rate constants for ion pair and free ion propagation, respectively, and K is the equilibrium constant for dissociation of ion pairs into solvated free ions. 

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