Solvent-separated ions

In all the schemes written above the final products (active species) are assumed to be carbenium ions, and although they have been shown as ion pairs, it is implicit that their other forms, free ions, solvent-separated ion pairs, etc., are also present. Details about the precise nature of these species will only be discussed whenever some evidence is available (mostly from kinetic data) to suggest the predominance of a specific form. Due to the high reactivity of carbenium moieties, we think that in general all available states are in fact reactive, but of course the propagation rate constant may vary considerably from one to another (see Sect. Il-J and other quantitative a >ects in Chaps. 

Free ions, solvent-separated ion pairs, contact ion pairs, and ionic associates can often be experimentally distinguished from each other with the aid of uv, ir, Raman, or nuclear magnetic resonance spectroscopy. According to nmr measurements, poly(styryl anions), I, and poly(dienyl anions), II, are strongly charge delocalized consequently, they exhibit a strong absorption band, sometimes several bands, in the near-ultraviolet region ... 

So the kinetic scheme and parameters not only depend on M, but they may differ considerably according to the initiator used. Furthermore, the active centers are not uniform as in free-radical polymerization, but because of their ionic nature there usually exists a complex chemical equilibrium between different species, even if the initiator has a unique stmcture. This equilibrium [Eq. (3)] between free ions, solvent-separated ion pairs, contact ion pairs, covalent polarized bonds, and between associated and non-associated species, as well as the concentration of these species strongly depend on the polarity or solvating power of the solvent system, the solvent itself, and the presence of other salts. 

Free-ions Solvent-separated Solvent-separated Contact ion-pair (1) ion-pair (2) ion-pair... 

In GC-MS effluent from the column is introduced directly into the mass spectrometer s ionization chamber in a manner that eliminates the majority of the carrier gas. In the ionization chamber all molecules (remaining carrier gas, solvent, and solutes) are ionized, and the ions are separated by their mass-to-charge ratio. Because each solute undergoes a characteristic fragmentation into smaller ions, its mass spectrum of ion intensity as a function of mass-to-charge ratio provides qualitative information that can be used to identify the solute. 

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. 

The concept of ion pairs in nucleophilic substitution is now generally accepted. Presumably, the barriers separating the intimate, solvent-separated, and dissociated ion pairs are quite small. The potential energy diagram in Fig. 5.4 depicts the three ion-pair species as being roughly equivalent in energy and separated by small barriers. 

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 kinetic method of determining relative acidity suffers from one serious complication, however. This complication has to do with the fate of the ion pair that is formed immediately on removal of the proton. If the ion pair separates and difiuses into the solution rapidly, so that each deprotonation results in exchange, the exchange rate is an accurate measure of the rate of deprotonation. Under many conditions of solvent and base, however, an ion pair may return to reactants at a rate exceeding protonation of the carbanion by the solvent. This phenomenon is called internal return ... 

In the first step the hydrated ion and ligand form a solvent-separated complex this step is believed to be relatively fast. The second, slow, step involves the readjustment of the hydration sphere about the complex. The measured rate constants can be approximately related to the constants in Scheme IX by applying the fast preequilibrium assumption the result is k = Koko and k = k Q. However, the situation can be more complicated than this. - °... 

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. 

The results of ion exchange separations may be influenced by varying the pH, the solvent or eluant, the temperature, the nature of the ion exchange resin, the particle size, the rate of flow of eluant, and the length of the column. 

An interesting consequence of selective sorption is that conditions for partition chromatography arise which may enhance the normal ion exchange separation factors. This aspect has been utilised by Korkisch34 for separation of inorganic ions by the so-called combined ion exchange-solvent extraction method (CISE). 

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... 

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. 

The reaction (Eq. (5)) in THF yields labile THF adducts which are converted into the more stable HMPA adducts by addition of HMPA. The various equilibria existing between Na2Fe(CO)4 and several donor solvents are described in a detailed paper by Collman in HMPA, the solvent-separated supernucleophilic ion pair [Na+x HMPA x Fe(CO)4 ] is the kinetically dominant species, with no kinetic contribution from free [Fe(CO)J2 . In THF, Na2Fe(CO)4 is much less dissociated, with tight-ion paired [NaFe(CO)4] as the kinetically important species [96],... 

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]. 

Mattay, J., and Vondenhof, M. Contact and Solvent-Separated Radical Ion Pairs in Organic Photochemistry. 159, 219-255 (1991). 

FIGURE 1.2 Sodium chloride consists of sodium ions and chloride ions. When sodium chloride comes in contact with water (left), the ions are separated by the water molecules, and they spread throughout the solvent (right). The solution consists of water, sodium ions, and chloride ions. There are no NaCI molecules present. 

Contact Ion-pairs Solvent separated Free ions... 

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. 

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