Solvents and Solvating Agents

Provided that KTf rSBa2+Si 1 1, the rate of polymerization, attributed to the free, unassodated s ion, is independent of the salt concentration, i.e. 

This treatment accounts for the experimental findings. Nevertheless, it is questionable because the association of the free S ion with the cyclic form of the salt forms a new species, viz. a kind of triple ion. Since the concentration of the open, free S ions is still given by K lrS,Ba2+,Sil, the rate of propagation should be proportional to the concentration of the barium salt, whether a further association with the cyclic form does or does not take place, provided that the bulk of the salt is still present in the conventional cyclic form. 

Anionic polymerization of lithium polystyrene in mixtures of THF and dioxane was investigated by Van Beylen et al.327 and in the THF-benzene solutions by Worsfold and Bywater328). Physical properties of those solvents, such as density, viscosity and dielectric constants, were determined over the whole range of compositions and found to monoton-ically vary with the volume percent of THF. 

Conductometric measurements were feasible down to 50 volume percent of THF, and the dissociation constants of lithium ion-pairs were determined from those data. The log Kdiss were shown to be linear with reciprocal of the appropriate dielectric constant - a relation commonly reported284) although not obvious. The solvating power of a medium is not determined uniquely by the bulk dielectric constant, D the more sophisticated Kirkwood function (D - l)/(2 D + l)329) could provide a better measure of that property. However, since the Kirkwood function is nearly linear with 1/D in the investigated range of dielectric constants, the discrimination between them is not practical. 

The propagation constant, k , of ion-pairs is strongfy affected by the composition of the solvent. In THF-benzene solutions its value decreases from 170 M-1s-1 to 0.6 M 1s as the mole fraction of THF drops from 0.74 to 0.01. Plots of log k vs. (D - 1)/(2D + 1) 

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The quantitative effects of a-substituents in decreasing the rates of these reactions are not additive and also depend considerably on solvent and alkylating agent. They are low in liquid sulfur dioxide as a solvent where solvation effects are small and the high dielectric constant increases the bond breaking in the transition state. For 3- and 4-substituted pyridines a Bronsted correlation exists between the rates of quatemization and the pKa values (78AHC(22)86). 

The crown ethers and cryptates are able to complex the alkaU metals very strongly (38). AppHcations of these agents depend on the appreciable solubihty of the chelates in a wide range of solvents and the increase in activity of the co-anion in nonaqueous systems. For example, potassium hydroxide or permanganate can be solubiHzed in benzene [71 -43-2] hy dicyclohexano-[18]-crown-6 [16069-36-6]. In nonpolar solvents the anions are neither extensively solvated nor strongly paired with the complexed cation, and they behave as naked or bare anions with enhanced activity. Small amounts of the macrocycHc compounds can serve as phase-transfer agents, and they may be more effective than tetrabutylammonium ion for the purpose. The cost of these macrocycHc agents limits industrial use. 

The lack of solvent separated pairs raises the question whether some alternative mode of solvation should be considered. The ester group of the penultimate unit of the polymer, or the one preceeding it, could act as a solvating agent. The idea of intramolecular solvation was proposed by several workers in the field 37) and it is supported by the results of nmr studies of polymethyl methacrylate formed under various experimental conditions 38). Hypothetical structures such as those depicted below were proposed 39 h... 

In dimethoxyethane DME, a more powerful solvating agent than THF, solvation by solvent molecules competes with the intramolecular solvation, increasing the reactivity of ion-pairs. Indeed, the propagation constants of Na+ and Cs+ salts of polymethyl methacrylate are higher in that solvent than in THF, although again both salts are nearly equally reactive 39) as shown in Fig. 5. 

Korotkov offered an ingenious explanation for this phenomenon. The monomers were treated as solvents, with butadiene believed to be a less reactive monomer than styrene, but treated as the preferential solvating agent for Li+. Thus butadiene was expected to be present virtually exclusively in the vicinity of the growing polymer ends, and hence it polymerizes preferentially, albeit slowly. On its exhaustion styrene reaches the reactive centers and, being assumed to be the more reactive monomer, it polymerizes rapidly speeding up the reaction. 

In contrast to the allyl system, where the reduction of an isolated double bond is investigated, the reduction of extensively delocalized aromatic systems has been in the focus of interest for some time. Reduction of the systems with alkali metals in aprotic solvents under addition of effective cation-solvation agents affords initially radical anions that have found extensive use as reducing agents in synthetic chemistry. Further reduction is possible under formation of dianions, etc. Like many of the compounds mentioned in this article, the anions are extremely reactive, and their intensive studies were made possible by the advancement of low temperature X-ray crystallographic methods (including crystal mounting techniques) and advanced synthetic capabilities. 

Nuclear Magnetic Resonance Spectroscopy Measured in a Chiral Solvent or with a Chiral Solvating Agent. One method of NMR analysis for enantiomer composition is to record the spectra in a chiral environment, such as a chiral solvent or a chiral solvating agent. This method is based on the diastereomeric interaction between the substrate and the chiral environment applied in the analysis. 

The situation would be completely different for oxycarbenium ions in a highly polar solvent such as sulphur dioxide which could compete effectively as solvating agent with the DCA and their polymers. In such systems one could envisage that both the solvent-solvated oxycarbenium ions and also the solvent-solvated teJt.-oxonium ions could coexist in a true equilibrium, and that each would react according to its own characteristics. This is an area which remains very largely unexplored, although Penczek has made a start in this direction and these considerations arose from discussions with him of his exploratory experiments with sulphur dioxide as solvent. 

In Table 6 the differences of free enthalpies of solvation for several anion ligands in a donor solvent D and in AN are given. HMPA shows very weak solvation whereas water is a very strong solvating agent for anions. The free enthalpies of solvation of halide and pseudohalide ions are by 4 to 15 keal/mol more negative than in aprotic donor solvents. 

Barretta and coworkers63 reported a direct determination of the enantiomeric purity of chiral trisubstituted allenes by using permethylated cyclodextrin as a chiral solvating agent. They found that the heptakis ft-cyclodextrin TRIMEB discussed above can be successfully used as a chiral solvating agent (CSA) for the NMR determination of the enantiomeric purity of trisubstituted allenes llOa-f. An accurate analysis of the experimental conditions (molar ratio aliene/TRIMEB, temperature and solvent) required to optimize the enantioseparation has been carried out. The XH NMR spectra of TRIMEB, allenes llOa-f, and the mixtures TRIMEB/allene have been recorded at 300 MHz in CD3OD as solvent. 

Nitronium salts are efficient and powerful nitrating agents. Nitronium tetrafluoroborate is the most commonly used nitronium salt for nitration and is commercially available as a solid or as a solution in sulfolane in which the ions are highly solvated and exist as an ion pair. Nitronium tetrafluoroborate shows poor solubility in most organic solvents and so the more soluble nitronium hexafluorophosphate is sometimes preferred for nitrations. Olah and co-workers - have studied the nitration of a vast array of aromatics with nitronium salts. Solutions of nitronium salts in aprotic organic solvents are useful for the nitration of acid sensitive or readily oxidized substrates. Nitronium tetrafluoroborate has been used for the... 

Reactions of a, [1, /J-trifl uorostyrenc and chloro-a, /i-difluorostyrene with 9-methylfluoren-9-yl anion (potassium salt) are accelerated by ion-pair solvating agents (18-crown-6, [2.2.2]-cryptand) and retarded by KBPI14 as a common-ion reagent.5 This was interpreted as indicating that solvent-separated ion pairs are more reactive than contact ion pairs in these reactions. 

An example of coordinative extraction with solvating agents is shown in Eq. (4). The difference with physical extraction is that the capacity and extraction power of the liquid neutral ion exchanger is much higher than with any bulk organic solvent (toluene, xylene, butanol, etc.) used in physical extraction. Alkyl-substituted... 

The stereoelectronic effect of the RO-group is less pronounced, when bulkier electrophiles are employed (Table 3, entries 23,27,28), but is increased when the well solvating agent hexamethyl phosphorous amide (HMPA) is used as an additive 61,68). On the other hand, if one performs the deprotonation/alkylation sequence in the unpolar solvent pentane, a complete reversal of the stereochemical outcome provides the coproduct in excess (Eq. 35)61 Now a coordination of the lithium cation to the siloxy function might favour structures like 109 (or its oligomers) and cause predominant formation of cO-cyclopropanes. 

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