Alkali Metal - Solvent Systems

The solubility of alkali metals in most of the solvents used in anionic polymerization is exceedingly low. Liquid ammonia and hexamethylphosphoric-triamide are exceptional in this respect relatively high concentrations of the metals can be attained in these media. Several distinct species co-exist in alkali metal solutions. A minute but constant concentration of unionized atoms, Met, is maintained when a solvent is kept in contact with solid alkali metal. These in turn are in equilibrium with the products of their ionization  

Higher aggregates need not be considered since most of the investigated solutions are rather dilute. 

Due to the common ion effect, the concentration of solvated electrons is depressed by the presence of ionized salts of the appropriate alkali. This in turn depresses the concentration of the negative alkali ions. However, as long as contact with the solid metal is maintained, the concentrations of the unionized atoms as well as of the ion-pairs remain unaffected. The latter are given by  

The equilibrium concentration of the unionized atoms depends mainly on the heat of sublimation of the pertinent metal their solvation energy is probably small. Hence [Cs]e [Rb]e [K]e [Na]e and the equilibrium concentration of Li atoms is probably the smallest, since no Li hyperfine ESR lines were observed in solutions of metallic lithium. 

The available data suggest the following order of solvent s capacity to ionize alkali metals, namely 

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Comparisons were not made for Et3N and benzene since MgCl2 reduction was incomplete in these solvents. In addition to these solvents, other alkali metal-solvent systems also work, such as Na-diglyme, but little research with these systems has been done. 

If a mercury cathode is expected to be necessary, the aprotic solvent-alkali-metal salt system appears to be inconvenient since many compounds are cathodically cleaved, reduced, or/and deprotected at potentials beyond that of alkali-metal amalgam formation. nevertheless, in certain cases the use of lithium salts as an electrolyte possessing strong electrophilic properties appears necessary in order to avoid the possibility of a Hofmann degradation of the tetraalkylammonium ion by electrogenerated bases. Under such experimental conditions, the cathodic synthesis of some aza and aza-oxa ligands [31] has been successfully achieved from the corresponding and readily obtained poly-... 

The alkali metals have the interesting property of dissolving in some non-aqueous solvents, notably liquid ammonia, to give clear coloured solutions which are excellent reducing agents and are often used as such in organic chemistry. Sodium (for example) forms an intensely blue solution in liquid ammonia and here the outer (3s) electron of each sodium atom is believed to become associated with the solvent ammonia in some way, i.e. the system is Na (solvent) + e" (sohem). 

The compounds can therefore be used as nonaqueous ionizing solvent systems (p. 424). For example the conductivity of ICl is greatly enhanced by addition of alkali metal halides or aluminium halides which may be considered as halide-ion donors and acceptors respectively ... 

A stream of monomer (or mixture of monomers) is made to flow rapidly over the surface of an alkali metal. If the reaction with the metal is sufficiently slow, a low concentration of monomer" ions will result. In view of the high concentration of the monomer, the monomeric" ions would add further monomer to form the dimeric and polymeric radical ions. Of course, the final product is not a radical, but it would result from a polymerization which took place to some extent on the radical ends. The mixture of monomers may be recirculated many times to increase the conversion and a solvent may be added to the system when necessary. 

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. 

The crowns as model carriers. Many studies involving crown ethers and related ligands have been performed which mimic the ion-transport behaviour of the natural antibiotic carriers (Lamb, Izatt Christensen, 1981). This is not surprising, since clearly the alkali metal chemistry of the cyclic antibiotic molecules parallels in many respects that of the crown ethers towards these metals. As discussed in Chapter 4, complexation of an ion such as sodium or potassium with a crown polyether results in an increase in its lipophilicity (and a concomitant increase in its solubility in non-polar organic solvents). However, even though a ring such as 18-crown-6 binds potassium selectively, this crown is expected to be a less effective ionophore for potassium than the natural systems since the two sides of the crown complex are not as well-protected from the hydro-phobic environment existing in the membrane. 

The alkali metals in liquid ammonia give deep coloured solutions which have been shown to contain solvated electrons. The unsaturated system takes up an electron to give an anion radical. There is evidence for this species from electron spin resonance studies. It accepts a proton from the solvent to give a radical which is reduced to a carbanion by another sodium atom. Finally the addition of a proton gives the reduced product. This proton is supplied by a protic solvent like enthanol and not from NH3. 

Onium salts, crown ethers, alkali metal salts or similar chelated salts, quaternary ammonium and phosphonium are some of the salts which have been widely used as phase transfer catalysts (PTC). The choice of phase transfer catalysts depends on a number of process factors, such as reaction system, solvent, temperature, removal and recovery of catalyst, base strength etc. 

Recently it was shown that introduction of fluorescent groups into ionophoric calix[4]arenes provides new systems for recognition of alkali metal ions.<59 62) Because of the mobility of the phenyl units, conformational changes can be induced by solvents and metal ions. Examples are given in Figure 2.15. 

In asymmetric hydrogenation of olefins, the overwhelming majority of the papers and patents deal with hydrogenation of enamides or other appropriately substituted prochiral olefins. The reason is very simple hydrogenation of olefins with no coordination ability other than provided by the C=C double bond, usually gives racemic products. This is a common observation both in non-aqueous and aqueous systems. The most frequently used substrates are shown in Scheme 3.6. These are the same compounds which are used for similar studies in organic solvents salts and esters of Z-a-acetamido-cinnamic, a-acetamidoacrylic and itaconic (methylenesuccinic) acids, and related prochiral substrates. The free acids and the methyl esters usually show appreciable solubility in water only at higher temperatures, while in most cases the alkali metal salts are well soluble. 

Low-lying vacant orbitals of alkali metal cations can, consequently, accept an unpaired electron density even if it is delocalized over an extended n system of carbon chains. The anion-radical of 1,4-diphenylbutadiene can exist in i-trans and in -cis forms. The relative amounts of these geometrical isomers appear to depend highly on the counterion/solvent system. Li and K+ were studied as counterions THF, 2-MeTHF, and DME were employed as solvents (Schenk et al. 1991). Interaction between the anion-radical and the cation contributes to a stabilization of... 

The choice of base used in the Ter Meer reaction is important for two reasons. First, studies have found that strong bases, such as alkali metal hydroxides, inhibit the reaction and promote side-reactions, whereas the weaker alkali metal carbonates generally give higher yields.Secondly, if the m-nitronitronate salt needs to be purified by filtration it should be sparingly soluble in the reaction solvent and both the reaction solvent and the counterion of the gm-nitronitronate salt affect this solubility. Use of the potassium salt is advantageous for aqueous systems where the em-nitronitronate salts are usually only sparingly soluble, whereas the sodium salt can be used for nonaqueous reactions. 

Lariat ethers of structure 8 were found to be selective toward Li ion and the lariat crown ether-Li+ complexes are more stable than the corresponding complexes with Na or K+, in methanol. Nevertheless, experiments conducted in aqueous solution showed that Na+ had a better complexation ability than the other two alkali metal cations. Hence, selective complexation of lariat crown ethers with cations changes with the solvent system this may be due in part to the difference in solvation between solvent and cation (Figure 9 f. ... 

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