Solvated ion pair

Figure 1 shows the mechanistic picture developed by C. M. Starks (1,2) for Hquid—Hquid PTC in a graphical form. The catalyst cation extracts the more hpholilic anion Y from the aqueous to the nonpolar organic phase where it is present in the form of a poorly solvated ion pair Y ]. This then reacts rapidly with RX, and the newly formed ion pair X ] returns to the aqueous phase for another exchange process X — Y . In practice most catalyst cations used are rather lipophilic and do not extract strongly into the aqueous phase so that the anions are exchanged at the phase boundary. 

It is also of significance that in the dilute gas phase, where the intrinsic orientating properties of pyrrole can be examined without the complication of variable phenomena such as solvation, ion-pairing and catalyst attendant on electrophilic substitution reactions in solution, preferential /3-attack on pyrrole occurs. In gas phase t-butylation, the relative order of reactivity at /3-carbon, a-carbon and nitrogen is 10.3 3.0 1.0 (81CC1177). 

In any solvent system, the essential factors required for dissolution of cellulose include adequate stabihty of the electrolyte/solvent complex cooperative action of the solvated ion-pair on hydrogen bonding of cellu-... 

This distinction is meaningful if the resultant distribution function is of the type shown in Figure 4.7 (Szwarc, 1965). This figure shows that there is a high probability that the cation and anion are either in contact, separated by a solvent molecule or far apart (Szwarc, 1965). Intermediate positions are improbable. The structure of solvated ion-pairs has been studied by Grunwald (1979) using dipole measurements. 

Winstein Robinson (1958) used this concept to account for the kinetics of the salt effects on solvolysis reactions. They considered that carbonium ions (cations) and carbanions could exist as contact ion-pairs, solvated ion-pairs and as free ions and that all these forms participated in the reactions and were in equilibrium with each other. These equilibria can be represented, thus ... 

It is well known that lyophilic sols are coagulated by the removal of a stabilizing hydration region. In this case, conversion of a sol to a gel occurs when bound cations destroy the hydration regions about the polyanion, and solvated ion-pairs are converted into contact ion-pairs. Desolvation depends on the degree of ionization, a, of the polyacid, and the nature of the cation. Ba ions form contact ion-pairs and precipitate PAA when a is low (0-25), whereas the strongly hydrated Mg + ion disrupts the hydration region only when a > 0-60. 

Grunwald, E. (1979). Structure of solvated ion pairs from electric dipole moments. Journal of Pure and Applied Chemistry, 51, 53-61. 

At high pressures, a non-covalent ionic complex can be regarded as a microsolvated ion. It represents the simplest model for ions generated in a dynamic environment, such as in a solvent cage in solution. The main difference is that the behavior of a microsolvated ion is not perturbed by those environmental factors (solvation, ion pairing, etc.) which normally affect the fate of intimate ion-dipole pairs in solution. Hence, a detailed study of the dynamics and the reactivity of microsolvated ions may provide valuable information on the intrinsic factors governing the reaction and how these factors may be influenced by the solvent cage in solution.4 493... 

Surprisingly, there are only a few catalysts known capable of hydrogenating ketones in fully or largely aqueous systems. For example, most of the water-soluble rhodium, mthenium and indium phosphine complexes preferentially hydrogenate the C=C bonds in unsaturated ketones, as does the solvated ion pair formed from aqueous rhodium trichloride and Aliquat-336 [206]. 

It has been found that the Li quadrupole parameters x( Li) and /]( Li) are sensitive probes of solid state structures of organolithium compounds, for example with respect to aggregate size, solvation, ion pair structure and the X-Li-X structural angle. These results will be discussed in the following sections. 

The same type of addition—as shown by X-ray analysis—occurs in the cationic polymerization of alkenyl ethers R—CH=CH—OR and of 8-chlorovinyl ethers (395). However, NMR analysis showed the presence of some configurational disorder (396). The stereochemistry of acrylate polymerization, determined by the use of deuterated monomers, was found to be strongly dependent on the reaction environment and, in particular, on the solvation of the growing-chain-catalyst system at both the a and jS carbon atoms (390, 397-399). Non-solvated contact ion pairs such as those existing in the presence of lithium catalysts in toluene at low temperature, are responsible for the formation of threo isotactic sequences from cis monomers and, therefore, involve a trans addition in contrast, solvent separated ion pairs (fluorenyllithium in THF) give rise to a predominantly syndiotactic polymer. Finally, in mixed ether-hydrocarbon solvents where there are probably peripherally solvated ion pairs, a predominantly isotactic polymer with nonconstant stereochemistry in the jS position is obtained. It seems evident fiom this complexity of situations that the micro-tacticity of anionic poly(methyl methacrylate) cannot be interpreted by a simple Bernoulli distribution, as has already been discussed in Sect. III-A. 

Energetic considerations based on the separation of solvated ions at the encounter distance a show that solvated ion-pair formation from 1M is sufficiently exothermic in polar solvents to effectively prevent the production of excited singlet states 1M by the reverse process. Table XVIII lists values for free energies AGIM of ion-pair formation in acetonitrile estimated24 from the oxidation and reduction potentials, D/D+ and EA-tA, of donor and acceptor using the relationship... 

The gas-phase formation of metalloporphyrin ions (M(P)+) has been known for a long time, since the early mass-spectra studies of metalloporphyrins (97,98). Such gas-phase studies allow determination of the intrinsic properties of the species, unaffected by solvation, ion pairing, and other effects common to solution chemistry. 

Thus the growing anionic chain can assume at least two identities the free anion and the anion-cation ion pair (several types of solvated ion-pairs can also be considered). Furthermore, the kinetics of these propagation reactions, which generally show a fractional dependency on chain-end concentration ranging from one-half to unity, can best be explained by assuming that the monomer can react with both the free anion and the ion-pair (4, 5, 60, but at different rates. Thus, for example, in the polymerization of styrene by organosodium, the rate of polymerization (Rp) can be expressed as... 

If we Investigate the Influence of the solvent, It Is necessary to take Into account the nature of the species In the medium. In non-polar solvent we have aggregated speclesi when we add tetramethylenediamine (THEDA) we destroy the aggregates, and we have an equilibrium between Ion pairs and complexed species.In polar solvent, we have another equilibrium between the Ion pairs, the solvated Ion pairs and loose Ion pairs.11 If we compare the oxidation results (Table IV), we observe that for similar oxygen concentration, the coupling yield and the hydroperoxide functionality Increase like the lonlclty of the C-Ll linkage. 

This means that the active propagating sites such as free alkoxy ions and ion pairs are solvated with the hydroxyl groups. This must lead to an increase in the reactivity of the solvated ion pair as compared with that of the contact ion pair and decrease in the free ion reactivity, i.e. ultimately to the levelling off of the reactivity differences of these particles. 

Small amounts of polar solvents such as tetrahydrofuran, ether, dioxane and triethylamine have been shown to break down the association of organo-lithium compounds in non-polar solvents, and to greatly increase the rate of chain initiation. In polar solvents, therefore, one expects rapid initiation and a polymerization rate essentially determined by the rate of chain propagation of solvated ion-pairs. 

All the absolute values of k reported in the literature are collected in Table 1. In all cases the figures should refer to the reaction of the ion-pair with monomer. The results are too fragmentary and in some cases of uncertain accuracy for a detailed discussion of the effect of environment on reactivity. A few points are clear. The reactivity of the relatively unsolvated ion-pair in hydrocarbon solvents is relatively large and may even be comparable with that of the solvated ion-pair in tetrahydrofuran despite the large difference in dielectric constant. The reactivity of the ether-solvated ion-pair in solvents of lower dielectric constant is lower than either. The first effect of etherate formation is to decrease the reactivity of the ion-pair which can be increased again by an increase of the dielectric constant of the solvent. 

The most common electrochemical effects exerted in bulk solution are related to association (solvation, ion-pairing, complex formation, etc.) with the electroactive substance or electrochemically generated intermediates [4,19]. The importance of solvation can be gauged by comparing calculated and measured values of the parameter AE1/2 (defined as the difference, in volts between the half-wave potentials of the first and second polarographic waves) exhibited by polycyclic aromatic hydrocarbons (PAH) in dipolar aprotic solvents [46,47], It can be shown that AE1/2 is related to the equilibrium constant for disproportionation of the aromatic radical anion into neutral species and dianion, that is,... 

Figure 5.12 Projection of the limiting SW1 reaction coordinate. Transition state of the ratedetermining step is . The central minimum, o(, is the solvated ion pair.

The interpretation is based on the balance of competition between nucleophile-led reactions (via contact ion pairs or concerted and tending towards ElcB) and electrophile-led reactions (the El extreme, via solvated ion pairs). The solvated ion pairs are favoured with better nucleofuges, and/or greater solvating power, and collapse via loss of the more hydride-like /1-hydrogen collapse of the contact ion pairs involves preferential loss of the most acidic /1-hydrogen, and demands interaction with the nucleophile. 

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