Separated ion pair

Winstein suggested that two intermediates preceding the dissociated caibocation were required to reconcile data on kinetics, salt effects, and stereochemistry of solvolysis reactions. The process of ionization initially generates a caibocation and counterion in proximity to each other. This species is called an intimate ion pair (or contact ion pair). This species can proceed to a solvent-separated ion pair, in which one or more solvent molecules have inserted between the caibocation and the leaving group but in which the ions have not diffused apart. The free caibocation is formed by diffusion away from the anion, which is called dissociation. 

Attack by a nucleophile or the solvent can occur at either of the ion pairs. Nucleophilic attack on the intimate ion pair would be expected to occur with inversion of configuration, since the leaving group would still shield the fiont side of the caibocation. At the solvent-separated ion pair stage, the nucleophile might approach fiom either fece, particularly in the case where solvent is the nucleophile. Reactions through dissociated carbocations should occur with complete lacemization. According to this interpretation, the identity and stereochemistry of the reaction products will be determined by the extent to which reaction occurs on the un-ionized reactant, the intimate ion pair, the solvent-separated ion pair, or the dissociated caibocation. 

If it is assumed that ionization would result in complete randomization of the 0 label in the caihoxylate ion, is a measure of the rate of ionization with ion-pair return, and is a measure of the extent of racemization associated with ionization. The fact that the rate of isotope exchange exceeds that of racemization indicates that ion-pair collapse occurs with predominant retention of configuration. When a nucleophile is added to the system (0.14 Af NaN3), k y, is found to be imchanged, but no racemization of reactant is observed. Instead, the intermediate that would return with racemization is captured by azide ion and converted to substitution product with inversion of configuration. This must mean that the intimate ion pair returns to reactant more rapidly than it is captured by azide ion, whereas the solvent-separated ion pair is captured by azide ion faster than it returns to racemic reactant. 

The ion-pair return phenomenon can also be demonstrated by comparing the rate of loss of enantiomeric purity of reactant with the rate of product formation. For a number of systems, including 1-aiylethyl tosylates, ftie rate of decrease of optical rotation is greater than the rate of product formation. This indicates the existence of an intermediate that can re-form racemic reactant. The solvent-separated ion pair is the most likely intermediate in the Winstein scheme to pl this role. 

Stabilization of a carbocation intermediate by benzylic conjugation, as in the 1-phenylethyl system shown in entry 8, leads to substitution with diminished stereosped-ficity. A thorough analysis of stereochemical, kinetic, and isotope effect data on solvolysis reactions of 1-phenylethyl chloride has been carried out. The system has been analyzed in terms of the fate of the intimate ion-pair and solvent-separated ion-pair intermediates. From this analysis, it has been estimated that for every 100 molecules of 1-phenylethyl chloride that undergo ionization to an intimate ion pair (in trifluoroethanol), 80 return to starting material of retained configuration, 7 return to inverted starting material, and 13 go on to the solvent-separated ion pair. 

In solvents containing low concentrations of water in acetic acid, dioxane, or sulfolane, most of the alcohol is formed by capture of water with retention of configuradon. This result has been explained as involving a solvent-separated ion pair which would arise as a result of concerted protonation and nitrogen elimination. ... 

The species R X is called an internal, contact, or intimate ion pair, and R (s),X (s), sometimes symbolized R X , is an external or solvent-separated ion pair. 

According to Eigen and Tamm [87,88], ion-pair formation proceeds stepwise, starting from separated solvated ions which form a solvent-separated ion pair [C+SSA ]°, followed by a solvent-shared ion pair [C+SA ]° and finally a contact ion pair, [C+A ]° [Eqs. (4)-(6)]. All these species are solvated. The types of ion pair formed depend on the relative strength of the interaction of the involved species. 

R is the distance parameter, defining the upper limit of ion association. For spherical ions forming contact ion pairs it is simply the sum of the crystallographic radii of the ions a — a+ + a for solvent-shared and solvent-separated ion pairs it equals a + s or a + 2s respectively, where s is... 

In reactions in which separated ion pairs are involved, e.g., R4N+, K or Na +, and as a borderline case, Li +, the cation does not contribute to the adjustment of the reaction partners in a dense, well-ordered transition state poor selcctivities arc usually the result of these carbanionic carbonyl additions. Further, the high basicity of such carbanionic species may cause decomposition or racemization of sensitive reactions partners. 

Alkenyllithium derivatives, carrying carbanion-stabilizing substituents, which facilitate the formation of solvent-separated ion pairs, can also exhibit preparatively useful configurational stability in respect to the double bond of the precursor. 

Sodium peroxide [2 Na+, 02 ] is obtained. This indicates that Na + also induces the disproportionation of superoxide [59], presumably due to the large difference of interaction energy between the contact ion pair [Na+,02] and the solvent-separated ion pair [Na+/THF/02]. 

Consequently, due to preferred cis-cis orientation a dimeric structure is observed for the indium complex and an unprecedented cis-trans arrangement in the thallium structure leads to a polymeric aggregate. Further N-NMR spectroscopic studies show that the aluminum and gallium complexes are stable contact ion pairs even in solution whereas the indium and thallium compounds are solvent-separated ion pairs in THE solution. 

In this scheme, RS and SR represent enantiomers, and so on, and 5 represents some fraction. The following are the possibilities (1) Direct attack by SH on RX gives SR (complete inversion) in a straight Sn2 process. (2) If the intimate ion pair R X is formed, the solvent can attack at this stage. This can lead to total inversion if Reaction A does not take place or to a combination of inversion and racemization if there is competition between A and B. (3) If the solvent-separated ion pair is formed, SH can attack here. The stereochemistry is not maintained as tightly and more racemization (perhaps total) is expected. (4) Finally, if free R" " is formed, it is planar, and attack by SH gives complete racemization. 

Molecular orbital calculations made on t-BuCl show that the C—Cl distance in the intimate ion pair is 2.9 A and the onset of the solvent-separated ion pair takes place at 5.5 A (cf. the C—Cl bond length of 1.8 A). 

Niecke et al. have prepare polyimido analogues of the metaphosphate ion, PO3, bylithiation ofthe corresponding amido compounds [16]. Thus the monomeric solvent-separated ion pair [(THF)4Li][P(NMes )3] (10) is obtained by treatment of (Mes N)2P(NHMes ) with "BuLi [16]. A monomeric contact ion pair (11) containing the unsymmetrical anion [P(N Bu)2(NMes )]" has also been reported [16]. By contrast the dilithium derivative of the trisimidometaphosphate [P(N Bu)3]" forms a dimer (12) [17], with a cubic structure reminiscent of that of (7). 

The first example of a tetrakisimido analogue of the orthophosphate ion, PO, the solvent-separated ion pair [(THF)4Li][(THF)4Li2P(Nnaph)4] (16), was reported by Russell et al. [21]. This complex was isolated in low yield from the reaction of P2I4 with a-naphthylamine in THF/NEt3, followed by the addition of "BuLi. The mechanism of this remarkable redox process is not understood. 

To ensure that proton transfer takes place from the protonated catalyst 64-H and not from the acidic reagent itself, apolar solvents favoring contact rather than solvent separated ion pairs as well as a slow addition of the acidic substrate RX-H are required. In addition, it was sometimes found beneficial to lower the basicity of the catalyst, thus rendering the protonated species [catalyst-H" ] more acidic for the stereo-determining protonation of the enolate. This was accomplished by formally replacing NR2 by Me (see 64e, Fig. 36). 

It has been suggested by Ikegami (1968) that the carboxylate groups of a polyacrylate chain are each surrounded by a primary local sphere of oriented water molecules, and that the polyacrylate chain itself is surrounded by a secondary sheath of water molecules. This secondary sheath is maintained as a result of the cooperative action of the charged functional groups on the backbone of the molecule. The monovalent ions Li", Na and are able to penetrate only this secondary hydration sheath, and thereby form a solvent-separated ion-pair, rather than a contact ion-pair. Divalent ions, such as Mg " or Ba +, cause a much greater disruption to the secondary hydration sheath. 

Eigen Tamm (1962a,b) and Atkinson Kor (1965, 1967) envisage a more complex situation and consider that there are two kinds of solvent-separated ion-pairs those with one intervening molecule of solvent and others where the ion-pair is fully solvated (Wilson Crisp, 1977). 

Figure 4.7 Distribution function for contact and solvent-separated ion-pairs.

Counterions can affect the strocture of hydration regions, and conversely hydration regions can affect ion binding. We have already touched on this subject in discussing contact and solvent-separated ion pairs in Section 4.2.8. 

It is, perhaps, more in line with other thinking to represent form (I) as a solvent-separated ion-pair COO" HjO Me HjO "OOC, and form (II) as a contact ion-pair COO" Me "OOC. Thus, precipitation occurs when a solvent-separated ion-pair is desolvated. 

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