Ammonia, liquid alkali metal solutions

D. Fluorocarbon Polymers. Four different fluorocarbons account for the bulk of the laboratory applications polytetrafluoroethylene, Teflon PTFE po-ly(chlorotrifluoroethylene), KEL-F tetrafluoroethylene-hexafluoropropylene copolymer, Teflon FEP and tetrafluoroethylene-perfluorovinyl ether copolymer, PFA. These polymers are inert with most chemicals and solvents at room temperature and exceptionally inert with oxidizing agents. They also have an exceptional resistance to temperature extremes. However, they are decomposed by liquid alkali metals, solutions of these metals in liquid ammonia, and carban-ion reagents such as butyllithium. Teflon retains some of its compliance at liquid hydrogen temperature. The maximum temperature which is recommended for continuous service is 260°C for Teflon PTFE and PFA, and about 200°C for Kel-F and Teflon FEP. 

The dependence of the shape, intensity, and energy of the absorption band on temperature was investigated using a 2.83 X 10 AM solution of sodium in liquid ND8. The spectrum of this solution at several temperatures is shown in Figure 6b. A plot of the energy of the absorption maximum vs. temperature is shown in Figure 7, and a least squares treatment of these data indicates that the temperature dependence of the band maximum is — 14.3/cm./deg. Values of —9.7/cm./deg. for sodium (6), —9.1/cm./deg. for potassium (2), and — 12.7/cm./deg. for the alkali metal solutions (9) in liquid ammonia have been reported. No detectable decomposition of the solution occurred during the four hours required to take the measurements between —69° and —31° C. The intensity of the band, as measured by both the absorbance (1.48 0.04)... 

As early as 1969, Pedersen was intrigued by the intense blue colour observed upon dissolution of small quantities of sodium or potassium metal in coordinating organic solvents in the presence of crown ethers. Indeed, the history of alkali metal (as opposed to metal cation) solution chemistry may be traced back to an 1808 entry in the notebook of Sir Humphry Davy, concerning the blue or bronze colour of potassium-liquid ammonia solutions. This blue colour is attributed to the presence of a solvated form of free electrons. It is also observed upon dissolution of sodium metal in liquid ammonia, and is a useful reagent for dissolving metal reductions , such as the selective reduction of arenes to 1,4-dienes (Birch reduction). Alkali metal solutions in the presence of crown ethers and cryptands in etheric solvents are now used extensively in this context. The full characterisation of these intriguing materials had to wait until 1983, however, when the first X-ray crystal structure of an electride salt (a cation with an electron as the counter anion) was obtained by James L. Dye and... 

After Abe s work the problem again lay dormant for a number of years until it was taken up by Wilmarth and his co-workers. Claeys, Baes, and Wilmarth (29) in 1948 reported that a liquid ammonia solution of potassium metal rapidly catalyzed o-p H2 conversion, a half-time in solution of 37 sec. being obtained at —53°. In order to establish that this result was due to dissolved metal and not to amide ion impurity, Claeys, Dayton, and Wilmarth (30) studied the o-p H2 conversion in the presence of potassium amide in liquid ammonia. Rates were obtained comparable with those occurring with the metal solution. The mechanism of the conversion was different for the two cases, however, since the amide solution also catalyzed exchange between gaseous deuterium and liquid ammonia, while the metal solution did not. It was assumed that the metal acted by a paramagnetic mechanism and the amide ion by a chemical mechanism. In the same note Claeys, Dayton, and Wilmarth (30) reported confirmation of Wirtz and Bonhoeffer s results on the aqueous alkali system and questioned the validity of Abe s objections. 

Ammonia dissolves alkali metals, barium, calcium and strontium and forms an unstable blue solution. This solution contains the metal ion and free electrons that slowly decompose, release hydrogen and form the metal amide. Compared to water, liquid ammonia is less likely to release protons (H+ ions), is more likely to take up protons (to form NH4+ ions) and is a stronger reducing agent219. 

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 ... 

Since the first preparation of alkali-metal intercalates by the liquid ammonia technique, many preparation methods have been proposed. The most commonly used fall into four main groups (i) use of alkali-metal solutions in liquid ammonia, (ii) use of organometallic reagents, (iii) solid-state techniques and (iv) electrochemical processes. 

Carbonyls may be reduced by alkali metal solutions in liquid ammonia to give carbonyl metallates. These are salt-like compounds containing the metals with negative oxidation numbers in the complex anions ... 

It should also be noted that some nonradical ionic and condensation reactions of monomers with cellulose are used to modify the properties of cellulosic products. In one type of anionic-initiated reaction of monomers, cellulose is reacted with concentrated aqueous solutions of alkali metal hydroxides to yield cellulose copolymer. Free alkali metal in liquid ammonia or alkali metal alkoxides in nonaqueous systems may also be used as initiators of cellulose alkoxide derivatives. In cationic-initiated formation of copolymers, cellulose is reacted with an acid, such as boron trifluoride, to yield a cellulosic carbonium ion which initiates reactions with vinyl monomers. Condensation reactions of cyclic monomers with cellulose also form copolymers. Cellulose is usually slightly oxidized and also has reactive hydroxyl groups on carbons C-2, C-3 and C-6 of the anhydroglucose unit. The reactions of cyclic monomers are initiated at these carbonyl groups. A heating step may increase cellulosic oxidation and thereby increase the yield of these condensation products of cellulose and cyclic monomers." ... 

Use of alkali metal solutions in liquid ammonia that leads to fast reactions. A good procedure consists of using thick-walled sealed Pyrex tubes through chosen temperature gradients to separate ammonia from IC product [26]. 

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). 

Solutions of alkali metals in liquid ammonia are used in organic chemistry as reducing agents. The deep blue solutions effectively contain solvated electrons (p. 126), for example... 

The last isomerization is remarkable in that the triple bond can shift through a long carbon chain to the terminus, where it is fixed as the (kinetically) stable acetylide. The reagent is a solution of potassium diami no-propyl amide in 1,3-di-aminopropane. In some cases alkali metal amides in liquid ammonia car also bring about "contra-thermodynamic" isomerizations the reactions are successful only if the triple bond is in the 2-position. 

The chemical resistance of PTFE is exceptional. There are no solvents and it is attacked at room temperature only by molten alkali metals and in some cases by fluorine. Treatment with a solution of sodium metal in liquid ammonia will sufficiently alter the surface of a PTFE sample to enable it to be cemented to other materials using epoxide resin adhesives. 

The above results are concordant with the recent finding" that saturated alkyl fiuorides are not reduced by alkali metals in liquid ammonia at — 33°, although unsaturated fiuorides are reduced rapidly. All types of fiuoro compounds are reported to be reduced by metal-ammonia solutions at 0-25°. 

The interpretation of these remarkable properties has excited considerable interest whilst there is still some uncertainty as to detail, it is now generally agreed that in dilute solution the alkali metals ionize to give a cation M+ and a quasi-free electron which is distributed over a cavity in the solvent of radius 300-340 pm formed by displacement of 2-3 NH3 molecules. This species has a broad absorption band extending into the infrared with a maximum at 1500nm and it is the short wavelength tail of this band which gives rise to the deep-blue colour of the solutions. The cavity model also interprets the fact that dissolution occurs with considerable expansion of volume so that the solutions have densities that are appreciably lower than that of liquid ammonia itself. The variation of properties with concentration can best be explained in terms of three equilibria between five solute species M, M2, M+, M and e ... 

Solutions of alkali metals in liquid ammonia have been developed as versatile reducing agents which effect reactions with organic compounds that are otherwise difficult or impossible/ Aromatic systems are reduced smoothly to cyclic mono- or di-olefins and alkynes are reduced stereospecifically to frani-alkenes (in contrast to Pd/H2 which gives cA-alkenes). 

These compounds are in many ways analogous to the solutions of alkali metals in liquid ammonia (p. 77). 

Deprotonation of H2O2 yields OOH , and hydroperoxides of the alkali metals are known in solution. Liquid ammonia can also effect deprotonation and NH4OOH is a white solid, mp 25° infrared spectroscopy shows the presence of NH4+ and OOH ions in the solid phase but the melt appears to contain only the H-bonded species NH3 and H202. " Double deprotonation yields the peroxide ion 02 , and this is a standard route to transition metal peroxides. 

Solution of alkali metals in liquid ammonia, containing the so-called solvating electrons, may be used as an alternative homogeneous system to initiate polymerization by an electron transfer process. This system suffers, however, from complications resulting from proton transfer from ammonia leading to the formation of NH2- ions, which in turn initiate further polymerization.4... 

The alkali metals also release their valence electrons when they dissolve in liquid ammonia, but the outcome is different. Instead of reducing the ammonia, the electrons occupy cavities formed by groups of NH3 molecules and give ink-blue metal-ammonia solutions (Fig. 14.14). These solutions of solvated electrons (and cations of the metal) are often used to reduce organic compounds. As the metal concentration is increased, the blue gives way to a metallic bronze, and the solutions begin to conduct electricity like liquid metals. 

Initiation presumably involves metal alkyls as the primary source of carbanions. These are immediately available from the Grignard reagents, organosodium compounds, or sodium amide used as catalysts when the alkali metal itself or its solution in liquid ammonia is used, addition to the monomer may precede actual initiation. ... 

In solutions neither H+ nor e can exist in a free state they will be donated only if they are accepted within the solution, e.g., by another acceptor, which may be the solvent and thus cause solvation here the mere solvation of electrons is an exceptional case, but may occur, e.g., in liquid ammonia, where according to Kraus82 the strongly reducing alkali metals dissolve while dissociating into cations M+ and solvated electrons e, which, however, are soon converted into NH2" and H2 gas. Further, from the analogy with acid-base reactions and the definition of... 

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