Ion pair separation

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 ... 

Therefore, an ideal polymer electrolyte must be flexible (associated with a low Tg), completely amorphous, and must have a high number of cation-coordination sites to assist in the process of salt solvatation and ion pair separation (see Table 11). A review on this subject has been recently published by Inoue [594]. 

It is believed that in these systems the cations coordinate with the oxygen atoms of the methoxyethoxyethoxy side groups as indicated in Formula below, thus facilitating ion-pair separation. 

The metal pnictides discussed earlier are much more strongly aggregated than those of their monometalated analogues. The latter represent highly flexible systems in solution that undergo fast ion-pair separation processes (2). The multinuclear NMR spectra of the... 

The results of ion-pairing separation of this compound according to the method of Conboy et al. [36] with MS detection in the ESI—LC— MS(+) mode was not promising. Even at a concentration of 50 mg L 1 of this esterquat in the standard solution, the S/N ratio was poor [39]. The base peak was the ion at m/z 428, representative of the fragment ions (R(CO)OCH2CH2)-N (CH3)(CH2CH2OH)2 with R = C17H33. 

Much is known about the lifetimes of carbocation intermediates of solvolysis, and these data have proven critical in the design of experiments to estimate absolute rate constants for reorganization of ion pairs. Consider reorganization of an ion-pair reaction intermediate that exchanges the positions of the nucleophilic atoms of the leaving group (, Scheme 9) and that occurs in competition with diffusional separation to free ions (k-d) which is much faster than addition of solvent to the ion pair. Ion-pair separation is irreversible and will result in formation of solvolysis reaction products s ). Reorganization of the ion pair will result in formation of isomerization reaction product and the yield of this reaction product will provide a measure of the relative rate constant... 

The situation is different for solvolysis reactions in most other solvents, where the intermolecular interactions between ions at an ion pair are stronger than the compensating interactions with solvent that develop when the ion pair separates to free ions. This favors the observation of racemization during solvolysis. There are numerous reports from studies on solvolysis in solvents with relatively low dielectric constant such as acetic acid, of polarimetric rate constants (fe , s ) for racemization of chiral substrates that greatly exceed the titrimetric rate constant (fet, s ) for formation of acid from the solvolysis reaction. ... 

The ESR spectrum of the sodium salt of dibenzoyl ketyl in THF shows splitting due to Na, whereas in the ESR spectrum of the sodium salt of the ketyl of 2,2,6,6-tetramethylhexane-3,4-dione in THE, there is no splitting whatever due to Na (Luckhurst and Orgel 1963). Although dibenzoyl ketyl can give rise to the contact (or penetrate) ion pair, the formation of such a type of species from the 3,4-dione is difficult, owing to steric hindrance. This semidione exists in the form of a free ion (or an ion pair separated by solvent molecules) compare the two structures given in Scheme 3.48. 

Implying that the two Ion pair separation processes are not affected by the Ion pair present at the opposite side of the chain. The bolaform salt with n 2 Is the only exception. 

It Is clear that the reactions with divalent cations are considerably more complex than those Involving monovalent cations, especially when ligands are added to promote Ion pair separation (32). Complex rate phenomena can be expected in anionic vinyl polymerization In the presence of divalent counterions. Some of these Interesting systems will be described elsewhere by other Investigators In this symposium. 

Tire preferred type of reversed-phase sorbents is Cjg bonded silica (Table 29.4). Using this reversed-phase sorbent, ion-pair separation of lincomycin (154), spiramycin (138), and tylosin (145) residues has also been reported through use of octanesulfonate, heptanesulfonate, and tetrabutylammonium pairing ions, respectively. Phenyl-bonded silica or polymeric stationary phases have also been described for the separation of tilmicosin (133) and lincomycin (146) residues, respectively. 

In the liquid chromatographic methods, reversed-phase columns are commonly used for the separation of the -agonists molecules due to their hydrophobic interaction with the Cis sorbent. Efficient reversed-phase ion-pair separation of -agonists has also been reported using sodium dodecyl sulfate as the pairing ion (477). 

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