Ions and Ion Pairs

M. Szwarc, Ions and Ion Pairs in Organic Reactions, John Wiley Sons, New "Vbrk, 1972. 

Ions and ion pairs interact strongly with the solvent, and hence an ionic polymerization is greatly influenced by the environment. Solvation tends to separate the ions and thus the system approaches a state which would be expected in a hypothetical solution deprived of gegen ions. At the same time formation of a solvation shell around the growing center probably slows down the addition. This effect is particularly notable in the termination step and will be discussed further in the next section of this paper. 

Becker and Israel (1979) have studied the influence of the solvent in more detail. They determined the constant KD of the equilibrium between free ions and ion pairs (Schemes 10-12 and 10-13) conductometrically in five solvents (H20, MeCN, MeOH, EtOH, and Me2CO). An inverse linear relationship was found between the ratio of products [ArOS]/[ArF] (where ArOS is the product of heterolytic solvolysis) and Kd/e (e = dielectric constant). This result indicates that solvolysis products are formed mainly from free diazonium ions, whereas fluoro-de-diazoniation takes place in the ion pair. Of the solvents used, acetone gives the lowest value of KD, and thus the yield of the fluorinated product is highest in this solvent. 

The following treatment has been suggested by Shiu et al. (1994) and is reproduced briefly below. The simplest, first-order approach is to take into account the effect of dissociation by deducing the ratio of ionic to non-ionic species I, the fraction ionic x and the fraction non-ionic xN for the chemical at both the pH and temperature of experimental data determination (/D, xID, xND) and at the pH and temperature of the desired environmental simulation (/E, xIE, xNE). It is assumed that dissociation takes place only in aqueous solution, not in air, organic carbon, octanol or lipid phases. Some ions and ion pairs are known to exist in the latter two phases, but there are insufficient data to justify a general procedure for estimating the quantities. No correction is made for the effect of cations other than H+. This approach must be regarded as merely a first correction for the dissociation effect. An accurate evaluation should preferably be based on experimental... 

Unlike the carbon acids of simpler structure, the triarylmethanes form colored salts. No quantitative results are available, but the appearance of the color has been used qualitatively as an indication of the formation of the ions and ion pairs. Table VIII gives the colors of some organometallic compounds. 

An important consequence of quantal charge transfer between ions and ion pairs (dipoles) is the appearance of non-pairwise-additive cooperative or anticooperative contributions that have no counterpart in the classical theory. These nonlinear effects strongly stabilize closed-CT systems in which each site is balanced with respect to charge transfers in and out of the site, and disfavor open-CT systems in which one or more sites serves as an uncompensated donor or acceptor. This CT cooperativity accounts for the surprising stability of cyclic (LiF) clusters, which are strongly favored compared with linear structures. 

A mechanistic scheme, (Scheme 4) similar to that previously reported (72) for nucleophilic substitution, in which pre-association, free-ion and ion-pairs pathways compete, can therefore also be proposed for bromination. Based on this scheme, the stereochemical behaviour depends on both solvent and substituents. In trifluoroethanol, the reaction occurs independently of the substituents on the double bond, via free ions ... 

The DPs obtained in cationic polymerizations are affected not only by the direct effect of the polarity of the solvent on the rate constants, but also by its effect on the degree of dissociation of the ion-pairs and, hence, on the relative abundance of free ions and ion-pairs, and thus the relative importance of unimolecular and bimolecular chain-breaking reactions between ions of opposite charge (see Section 6). Furthermore, in addition to polarity effects the chain-transfer activity of alkyl halide and aromatic solvents has a quite distinct effect on the DP. The smaller the propagation rate constant, the more important will these effects be. 

If the equilibrium (XVI) is the only one, the relation between K and the concentrations of free ions and ion-pairs is given by some elementary relations which, however, it is useful to set out in full. Let the concentrations be denoted by... 

Another important feature is that all the terms are composite, and therefore it is not to be expected that the Arrhenius plots for the intercepts and slopes of the Mayo plots will be rectilinear. In fact, historically, the nonlinearity of such Arrhenius plots obtained for the polymerization of isobutene by titanium tetrachloride and water gave the first clue to the existence of simultaneous propagation by free ions and ion-pairs in that system [23]. 

It was shown by means of conductivity measurements that (Xxr)0 = c0, the initial catalyst concentration, and that initiation is almost instantaneous, but the question to what extent free ions and ion-pairs are involved is not yet entirely clear, though it seems probable that free ions are dominant because of this uncertainty we will denote these propagation constants by k. For rapid and completely efficient initiation, and termination by reaction with polymer... 

For cationic polymerisation of olefins in solvents of DC appreciably less than ca. 10 and for those of heterocyclic monomers in all solvents of DC up to perhaps 15-20, this is not so. For such systems the polymerisations are probably at least dieidic (free ions and ion-pairs) and a lowering of the temperature will increase the DC of the ion pairs. Thus in such systems the change of temperature affects not only k p and k"p, but also the relative abundance of the different types of chain-carriers therefore the proper interpretation of the apparent activation energies is difficult and by no means obvious. 

A plot of the spectroscopically computed maximum ionic concentrations [SD ions]m (sum of free ions and ion-pairs) at the end of the reactions (m denotes maximum values throughout this paper, the subscript 0 denotes initial values) against the final values of the specific conductivity Km. for the equivalent runs (Tables 1 and 2) gave a straight line through the origin, showing that virtually only free ions were present in our systems if there had been important quantities of ion-pairs, this plot would have been markedly curved. 

Table 27 The secondary a-deuterium and primary nitrogen KIEs for the free ion and ion-pair SN2 reactions between benzyldimethylphenylammonium nitrate and p-substituted thiophenoxide ions in methanol at 2Q°C.a...

The scaling tendency of the lime or limestone processes for flue gas desulfurization is highly dependent upon the supersaturation ratios of calcium sulfate and calcium sulfite, particularly calcium sulfate. The supersaturation ratios cannot be measured directly. They are determined by measuring experimentally the molalities of dissolved sulfur dioxide, sulfate, carbon dioxide, chloride, sodium and potassium, calcium, magnesium, and pH. Then by calculation, the appropriate activities are determined, and the supersaturation ratio is determined. Using the method outlined in Section IV, the concentrations of all ions and ion-pairs can be readily determined. The search variables are the molalities of bisulfite, bicarbonate, calcium, magnesium, and sulfate ions. The objective function is defined from the mass balance expressions for dissolved sulfur dioxide, sulfate, carbon dioxide, calcium, and magnesium. This equation is... 

The absorption of sulfur dioxide into flue gas scrubbing slurries is enhanced considerably because of the reactions of dissolved sulfur dioxide with various dissolved alkaline species. The alkaline species of interest are the following ions and ion-pairs OH-, SO", HCO-, CO--, MgS0°, MgC0°,... 

The most significant chemical equilibria present in flue gas scrubbing slurries are outlined. Expressions for temperature dependent equilibrium constants are presented that are suitable for the temperature ranges encountered in scrubbing applications. Expressions for activity coefficients of ions and ion-pairs are presented that are suitable for the ranges of ionii strengths encountered for this type of applications. 

The nucleophile in the S.v2 reactions between benzyldimethylphenylammonium nitrate and sodium para-substituted thiophenoxides in methanol at 20 °C (equation 42) can exist as a free thiophenoxide ion or as a solvent-separated ion-pair complex (equation 43)62,63. The secondary alpha deuterium and primary leaving group nitrogen kinetic isotope effects for these Sjv2 reactions were determined to learn how a substituent on the nucleophile affects the structure of the S.v2 transition state for the free ion and ion-pair reactions64. 

Kebarle, P. Ions and ion-solvent molecule interaction in the gas phase. In Szwarc, M. (ed.), Ions and ion pairs in organic reactions, Vol. 1, pp. 27—83. New York Wiley-Interscience 1972. 

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