Ion pairs solvent separated

Contact Ion-pairs Solvent separated Free ions... 

An ion pair in which the constituent ions are separated by one or more solvent (or other neutral) molecules. If and Y represent the constituent ions, a loose ion pair is usually symbohzed by X+ Y. The constituent ions of a loose ion pair can readily exchange with other ions in solution this provides an experimental means for distinguishing loose ion pairs from tight ion pairs. In addition, there are at least two types of loose ion pairs solvent-shared and solvent-separated. See Ion Pair Tight Ion Pair Solvent-Shared Ion Pair Solvent-Separated Ion Pair... 

An ion pair in which the constituent ions are not separated by a solvent or other intervening molecule. Tight ion pairs are also referred to as contact ion pairs. If and represent constituent ions, then a tight ion pair would be symbolized by X+Y. An example of a tight ion pair would be the case in which an enzyme stabilizes a carbonium ion with juxtaposed negatively charged side-chain groups. See Loose Ion Pair Ion Pair Solvent-Shared Ion Pair Solvent-Separated Ion Pair. 

TIGHT ION PAIRS LOOSE ION PAIRS SOLVENT-SHARED ION PAIR SOLVENT-SEPARATED ION PAIR Ion pair return,... 

SOLVENT-SHARED ION PAIR SOLVENT-SEPARATED ION PAIR TIME... 

The addition of bromine to alkenes is a rapid, exothermic reaction usually taking place at room temperature. In contrast to chlorination, the rate law in bromination depends on the solvent used. On passing from hydroxylic to nonpolar aprotic solvents, the overall second-order changes to a rate law that is first-order in alkene and second-order in bromine.226 Alkene-bromine complexes with varying compositions were shown to form under reaction conditions3,218,227 228(Scheme 6.5). At low bromine concentration in protic solvents the reaction proceeds via a 1 1 complex (23). A 1 2 alkene-bromine complex (25) is involved at high bromine concentration in nonprotic solvents. The ionic intermediates (24, 26) were shown to exist as contact ion pairs, solvent-separated ions, or dissociated ions. 

The solution thus consists of different particles denoted as contact ion pairs, solvent-separated ion pairs and free ions. The fraction of the individual particles depends on the type of salt, type of solvent, polymerization system, temperature, and salt concentration. The catalytic effect of these particles may be very different as is evident in anionic polymerization of vinyl monomers. For instance, free polystyryl anion is 800times more reactive than its ion pair with the sodium counterion 60 . From this fact it follows that, although the portion of free ions is small in the reaction system, they may play an important role. On the other hand, anionic polymerization and copolymerization of heterocycles proceeds mostly via ion pairs. This is due to a strong localization of the negative charge on the chain-end heteroatom which strongly stabilizes the ion pair itself62. Ionic dissociation constants and ion contributions to the reaction kinetics are usually low. This means that for heterocycles the difference between the catalytic effect of ion pairs and free ions is much weaker than for the polymerization of unsaturated compounds. This is well documented by the copolymerization of anhydrides with epoxides where the substi-... 

The change in the nature of the tetrazole substrate on going from tetrazolide 7 to contact ion pairs, solvent-separated ion pairs with a metal cation, to complexes of 231 and 232 type, etc., can result in deviations from the canonical mechanisms like those described above (Schemes 22 and 23). Also, the possibility cannot be excluded that the ion pairs formed by anion 7 react with the electrophile concurrently by several alternative pathways. We believe that just this versatility of reaction routes explains the difference between the predicted rate and selectivity of the electrophilic attack under ideal conditions and the experimental result. In the light of these comments, new data on the application of ion pairs formed by anions of type 7 to the synthesis of N-substituted tetrazoles are discussed. As far as possible, attention is given to conclusions with respect to the regioselectivity of electrophile attack. 

Fig. 10.3. The structures of alkyllithium compounds in solution II contact ion pairs, solvent-separated ion pairs or lithium at-complexes. In THF solution sec- and tert-BuLi are present as contact ion pairs in the same way as phenyllithium (A) or (2,6-diisopropylphenyl) lithium (D) are.

Living polymers can only exist in aprotic solvents. They are killed by water, oxygen and a high number of electrophilic substances. Operating in absence of killing impurities one obtains stable living species they are ionic species whose exact form (free ion, contact ion-pair, solvent separated ion-pair1417 depends upon the concentration and the nature of the monomer, the counterion and the solvent polar... 

Several factors that are less important in the case of neutral cycloadditions have to be considered because they can potentially be exploited for greater selectivity. As a result of the charged nature of the radical cations, solvents and the solvation state of the ions as contact ion pairs, solvent separated ions pairs, or free ion pairs involved in the reaction now play a much greater role than for the cycloadditions of a neutral substrate, which for the most part show only a weak solvent dependence. The... 

Winstein et al. [45] first presented evidence for the concept that different types of electrophilic species, each with distinct reactivities, may participate in reactions involving cationic intermediates. As shown in Eq. (36), Winstein et al. proposed that four species are in equilibrium, including covalent electrophiles, contact ion pairs, solvent-separated ion pairs, and free ions. In addition, ion pairs may aggregate in more concentrated solutions- According to this concept, electrophilic species do not react with a continuous spectrum of charge separation, but rather in well-quantified minima in the potential energy diagram. 

The propagating anion and its counterion exist in relatively nonpolar solvents mainly in the form of associated ion pairs. Different kinds of ion pairs can be envisaged, depending on the extent of solvation of the ions. As a minimum, an equilibrium can be conceived between intimate (contact) ion pairs, solvent-separated ion pairs, and solvated unassociated ions. The nature of the reaction medium and counterion strongly influences the intimacy of ion association and the course of the polymerization. In some cases the niicrostructure of the polymer that is produced from a given monomer is also influenced by these variables. In hydrocarbon solvents, ion pairs are not solvated but they may exist as aggregates. Such inter-molecular association is not important in more polar media where the ion pairs can be solvated and perhaps even dissociated to some extent. 

The initiation and propagation processes are influenced by equilibria between various degrees of association of the active center and its counterion. As a minimum, it is necessary to conceive of the existence of contact (associated) ion pairs, solvent-separated ion pairs, and free solvated ions. A simplified reaction scheme [3] is presented in reaction (9-37). 

All these processes, namely, the formation of the encounter complex, collision complex, contact ion pairs, solvent-separated ion pairs, and free-radical ion pairs, are reversible. For generation of free radicals in good yields, forward electron-transfer processes have to compete efficiently with the energy wasting back-electron-transfer processes [142]. 

Note that the ion-radical pairs may have different degrees of solvation depending on the electron-transfer pathway in Scheme 1. Thus, they may exist as contact ion pairs, solvent-separated ion pairs, or fully solvated ion radicals,... 

Besides the dissolving power of a solvent toward substrate and supporting electrolyte, the ability to solvate intermediate cations and anions is also important. The chemical properties of charged species are dependent on whether there are formed tight ion pairs, solvent-separated ion pairs, or symmetrically solvated ions. Redox potentials of ions are... 

In this scheme R X, R //X , and R" -I- X represent contact ion pairs, solvent-separated ion pairs, and the free ions, which can all lead to products. Nevertheless, relatively little is known about the details of the corresponding free energy surfaces in solution. Abraham suggested a profile similar to Fig. 4 for the solvolysis of f-butyl chloride (TBC). However, about all that is well established in water is AG = 19.5 kcal/mol, and the transition state is structurally close to the contact ion pair with about 80% charge separation. - ... 

It is the similarity of these data with those of the equilibrium 171 a Na+ (THF) 17 la (Na+ (THF) which led to the suggestion that there exists a contact ion pair -solvent-separated ion pair relationship also between 777a Na+ and 171a Na+ (and the other species mentioned above). 

Li, MMc4 [S Li] [MMe4] [S Li] [MMe4] unsolvated solid tight ion-pair solvent-separated ion-pair... 

The large dipole moment of ion pairs causes them to interact strongly with polar molecules with the result that small amounts of polar compounds profoundly affect the course of the polymerization. In polar solvents the following dynamic equilibrium will be set up involving ion pairs, solvent separated ion pairs and free ions ... 

In real ionic polymerization systems, however, more than one kind of ion occurs, especially in the case of anionic polymerizations. A distinction is made between free ions, solvated ion pairs (solvent separated or loose ion pairs), contact ion pairs (intimate ion pairs or tight ion pairs), polarized molecules, and ionic associates of three or more ions. A rapid dynamic equilibrium often occurs between these ionic forms ... 

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