Ammonia, liquid solvate formation

The two free hydroxy groups are First protected with acetic anhydride. In a second step the acetyl group is reductively cleaved by a Birch reduction with lithium in liquid ammonia.19 Lithium dissolves in the ammonia with the formation of solvated electrons. Stepwise electron transfer to the aromatic species (a SET process) leads first to a radical anion, which stabilizes itself as benzylic radical 38 with loss of the oxygen substituent. A second SET process generates a benzylic anion, which is neutralized with ammonium chloride acting as a proton source (see Chapter 12). 

Alkali and alkaline earth metals dissolve in liquid ammonia with the formation of solvated electrons. These solvated electrons constitute a very powerful reducing agent and permit reduction of numerous conjugated multiple-bond systems. The technique, named for Birch provides selective access to 1,4-cydohcxiidicnes from substituted aromatics.8 In the case of structures like 21 that are substituted with electron-donating groups, electron transfer produces a radical anion (here 22) such that subsequent protonation occurs se lectively in the ortho position (cf intermediate 23) A second electron-transfer step followed by another protonation leads to com pound 24... 

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... 

Events of electron photoemission from a metal into an aqueous solution had first been documented in 1966 by Geoffrey C. Barker and Arthur W. Gardner on the basis of indirect experimental evidence. The formation of solvated electrons in nonaque-ous solutions (e.g., following the dissolution of metallic sodium in liquid ammonia) had long been known, but it was only in the beginning of the 1950s that their existence in aqueous solutions was first thought possible. It is probably for this reason that even nowadays in aqueous solutions we more often find the term solvated than hydrated electrons. 

Electron-transfer initiation from other radical-anions, such as those formed by reaction of sodium with nonenolizable ketones, azomthines, nitriles, azo and azoxy compounds, has also been studied. In addition to radical-anions, initiation by electron transfer has been observed when one uses certain alkali metals in liquid ammonia. Polymerizations initiated by alkali metals in liquid ammonia proceed by two different mechanisms. In some systems, such as the polymerizations of styrene and methacrylonitrile by potassium, the initiation is due to amide ion formed in the system [Overberger et al., I960]. Such polymerizations are analogous to those initiated by alkali amides. Polymerization in other systems cannot be due to amide ion. Thus, polymerization of methacrylonitrile by lithium in liquid ammonia proceeds at a much faster rate than that initiated by lithium amide in liquid ammonia [Overberger et al., 1959]. The mechanism of polymerization is considered to involve the formation of a solvated electron ... 

One of the most interesting processes in electrically initiated polymerization was an initiation with the solvated electron proposed by Laurin and Parravano (22), who studied electro-anionic polymerization of 4-vinylpyridine in liquid ammonia solution of alkali metal salts in the temperature range — 33 to — 78° C. Rapid and efficient polymerization occurred and conversions of monomer to polymers formed exclusively at the cathode in the form of an orange-red, porous, solid deposit, suggesting the formation of a pile of living polymers. 

Almost all of the reactions that the practicing inotganic chemist observes in the laboratory take place in solution. Although water is the best-known solvent, it is not the only one of importance to the chemist. The organic chemist often uses nonpolar solvents sud) as carbon tetrachloride and benzene to dissolve nonpolar compounds. These are also of interest to Ihe inoiganic chemist and, in addition, polar solvents such as liquid ammonia, sulfuric acid, glacial acetic acid, sulfur dioxide, and various nonmctal halides have been studied extensively. The study of solution chemistry is intimately connected with acid-base theory, and the separation of this material into a separate chapter is merely a matter of convenience. For example, nonaqueous solvents are often interpreted in terms of the solvent system concept, the formation of solvates involve acid-base interactions, and even redox reactions may be included within the (Jsanovich definition of acid-base reactions. 

In general, from among the protic solvents, only liquid ammonia (the first used)1 is particularly useful, and is still used more than any other solvent despite the low temperature at which reactions have to be carried out (b.p. -33 °C) and the fact that solubilities of some aromatic substrates and salts (M+Nu-) are poor. Ammonia has the added advantage of being easily purified by distillation, being an ideal system for production of solvated electrons, and has very low reactivity with basic nucleophiles and radical anions, and aryl radicals. Also, poor solubilities can sometimes be ameliorated by use of cosolvents such as THF. In addition it can be used as a solvent for the in situ reductive generation of nucleophiles such as ArSe- and ArTe- ions, e.g. the formation of PhTe- from diphenyl ditelluride (equation 16).54 55... 

Comparison of the properties of metal alkoxides with their structures permits a conclusion that the polymeric nature does not always lead to chemical inertness. The major role appears to be played by the nature of the M-OR bonding. Solubility in alcohols and liquid ammonia of the methoxides of alkaline and alkaline earth metals and that in hydrocarbons ofthe isopropoxides of K, Rb, Cs (isostructural with the corresponding methoxides), and also M(OC2H4OMe)n, M = Pb, Bi indicates the easy oligomerization due to solvation or chelation. At the same time the methoxides and ethoxides of Al, Cr, Fe, and so on, forming the strongest covalent bonds in the [MOs/6] octahedra (and not prone to solvation in alcohols), appear almost inert. They can be dissolved only due to complexation or partial destruction with formation ofoxobridges. 

In the S l mechanism of aromatic substitution the initiating step is the formation of a radical anion. In order to distinguish the process from the route described above (SR+N1) in which a radical cation plays a crucial role, the symbol S l has been used17. Creation of the radical anion can occur by several procedures. The reaction can be electrochemically initiated, a solvated electron in a solution of alkali metal in liquid ammonia may be involved or a radical anion may be used as the source of electrons. The most common source of electrons is, however, the nucleophile itself involved in the substitution reaction. In many cases the electron transfer from nucleophile to substrate is light-catalysed and the process is then sometimes referred to as S l Ar. Although the nucleofugic group in S l... 

Among possible alternative isomers, the preferential formation of diene 11, with the indicated location of the double bonds, is determined by the structure of the initially formed, most stable intermediate, radical-anion 14. Thus, the reduction of a single bond of toluene, as is represented in equation 5, requires the presence of an electron source (sodium), a solvent capable of electron solvation (liquid ammonia), and a proton donor (alcohol). 

In 1921, by dissolving an alkali metal in liquid ammonia, C Kraus and W Lucasse observed a volume expansion of the solution greater than that obtained for the dissolution of ordinary salts.They attributed this volume expansion to the formation of the solvated electron, which is regarded as a particle, since the electron itself has a negligible volume. For example, the dissolution of three moles of sodium in... 

The cathode potential necessary for the production of solvated electrons is rather negative the standard potential of the hydrated electron has been calculated to be —2.68 V versus NHE. Also, in other solvents compatible with its formation, very negative potentials must be used for example, in liquid ammonia the generation of ens is achieved at —2.47 V versus Ag/AgN03 (0.1 M) [306], but the dissolution standard potential measured in HMPA was found to be —3.44 V versus Ag/AgC104 (0.1 M) [307]. Similarly, in methy-lamine f — 50°C), a potential of —2.90 V versus Ag/AgN03 was reported [308]. 

The formation of alkali-metal and alkaline-earth-metal sulphides and polysulphides from the elements in liquid ammonia has been extensively studied in the past, but the reactions between the metals and hydrogen sulphide in liquid ammonia have drawn detailed attention only recently. It has been suggested that the equilibrium of H2S in this solvent to give the solvated hydrosulphide ion accounts for the formation of KSH even with an excess of metal. With the alkaline-earth metals, effective preparative methods have been developed for the sulphides from H2S in liquid ammonia but anhydrous hydrosulphides have not been obtained. Now, hydrosulphides have been prepared of the form M(SH)2,xNH3 (M = Ca, Sr, or Ba x — 4, 6, or 0, respectively) from the metals with H2S in ammonia, but the compounds are stable only at low temperatures. Those of Ca and Sr are stable at —45 °C but decompose to the monosulphides at room temperature. Ba(HS)2 decomposes to BaS at 100 °C with evolution of a mole of H2S. For M (SH) (M = Rb or Cs), thermal decomposition gives polysulphides. The hydrosulphides of Rb, Cs, Sr, and Ba hydrolyse rapidly in moist air.74... 

Potassium ions are also present in this solution, but because of the high dielectric constant and high solvating capability of liquid ammonia the propagating species formed here is truly a free ion, not more than very loosely associated with the cation. The less polar the solvent used, the greater would be the degree of association between the propagating anion and the cation present. Thus, the polarity of the solvent chosen can affect the rate of formation and sometimes also the stereochemistry of the anionic polymerization product. 

In 1984, Edwards and coworkers reported the formation of Na43+ in zeolites Y and A and the formation of K43 1 in K+-exchanged zeolite Y. Later on, they found that if the zeolite was Na-Y, the formed metal ion clusters are Na43+ no matter what (Na or K) the reaction vapor was whereas if the zeolite used was K-Y, the obtained metal clusters will be K43+. That is, the formed metal cluster species is not related with the vapor but simply depends on the type of cation in the zeolite used. In fact, through variation of the reaction condition, various M,(i ion clusters can be prepared. If M = Na, n = 2 6, whereas if M = K, n 3,4. After the alkali metals enter the channels or cages of zeolites to form metal ion clusters, the electrons on the original metal atoms will be released to be shared by more than one metal atom. It has been confirmed that these free electrons actually occupy the holes formed by the metal atoms (ions). Therefore, these electrons are also called solid solvated electrons (in analogy with the solvated electrons formed by alkali metals in solvents such as liquid ammonia),[7] and the formed compounds are called solid electrides. 

In 1921, by dissolving an alkali metal in liquid ammonia, C. Kraus and W. Lucasse observed a volume expansion ofthe solution greater than that obtained for the dissolution of ordinary salts [3], They attributed this volume expansion to the formation of the solvated electron with a cavity, regarded as a particle since the electron itself has a negligible volume. For example, the dissolution of 3 moles of sodium in one litre of liquid ammonia induces an increase in volume of 43 cm compared to the pure liquid. Assuming that all the metal is dissociated, it may be deduced that in ammonia the electron occupies a spherical volume with a radius of 0.18 nm. In fact, the cavity radius of the solvated electron in ammonia is greater than that value and is about 0.3 nm. 

The best known and most studied systems in which the formation of solvated electrons is observed are the alkali metal — liquid ammonia systems. Shortly after his discovery of alkali metals, Davy initiated studies into their reactions with dry gaseous anunonia. In November 1808 he noticed that potassium assumed a beautiful metallic appearance and gradually became of a fine blue colour when heated in an ammonia atmosphere. It is today difficult to interpret the processes that occured in his experiment. Most probably, he noticed a form of the electron localized in a condensed phase. Unfortunately, this and other analogous records of Davy remained unknown for long only in 1980 were they discovered and later published... 

Unlike the solvents enumerated in Table 4, in liquid ammonia only one absorption band caused precisely by solvated electrons is observed, irrespective of the nature of cation. The shift of the peak and the decrease in the fraction of paramagnetic particles as the alkali metal concentration is increased are attributed by a number of authors to the formation of associates, the associate being a combination of two solvated electrons. The contribution of alkali metal anions to the formation of the single absorption band in liquid ammonia seems to be scarcely probable. Usually the location of the M absorption band is significantly different from that for solvated electrons. Rough estimates made by different authors in the last few years reveal that the formation of alkali metal anions in liquid ammonia is... 

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