Ammonia, liquid solvent properties

Ammonia is a pungent, toxic gas that condenses to a colorless liquid at — 33°C. The liquid resembles water in its physical properties, including its ability to act as a solvent for a wide range of substances. Because the dipole moment of the NH3 molecule (1.47 D) is lower than that of the H20 molecule (1.85 D), salts with strong ionic character, such as KCI, cannot dissolve in ammonia. Salts with polarizable anions tend to be more soluble in ammonia than are salts with greater ionic character. For example, iodides are more soluble than chlorides in ammonia. Liquid ammonia undergoes much less autoprotolysis than water ... 

Ethyleneimine (El) and its two most important derivaiives. 2-methyla/iri-dine (propyleneintinei. ami I, (2-hydroxyeihyl)aziridinc (HEA) are colorless liquids. They are miscible in all proportions with water and ihe majority of organic solvents. Ethyleneimine is not miscible with concenlrated aqueous NaOH solutions ( > 17(F by weight). Ethyleneimine has an odor similar to ammonia. The physical properties of ethyleneimine and the derivaiives mentioned are given in Table I. 

H. P. Cady and R. Taft sought if electrolytic oxidations can occur in systems containing no oxygen, and found that with liquid ammonia as solvent, thallous iodide, cuprous iodide, hydrazobenzene, and methyl- and ethylamine hydrochlorides can be oxidized. The properties of soln. of salts, etc., in liquid ammonia were studied by F. W. Bergstrom, T. J. Webb, E. C. Franklin and C. A. Kraus, etc. 

Uses of ammonia. The various uses of ammonia include the use of the compound both as such and in the form of other compounds made from ammonia. In the liquid state, much ammonia is used as the refrigerant liquid in commercial refrigeration plants and in the manufacture of ice. Some liquid ammonia is used both in the laboratory and commercially as a solvent, and its solvent properties are in many respects similar to those of water. Great quantities of ammonia are used in the manufacture of nitric acid, sodium hydrogen carbonate, normal sodium carbonate, aqueous ammonia (or ammonium hydroxide), ammonium salts for use as fertilizers, and many other useful chemicals. 

The solubility of amides in liquid ammonia differs appreciably and increases from lithium to rubidium. Because of the high reactivity of amides and the excellent solvent properties of liquid ammonia, amide-ammonia solutions are highly reactive and react with most substances.f ... 

We believe this to be the first monograph devoted to the physicochemical properties of solutions in organic solvent systems. Although there have been a number of books on the subject of non-aqueous solvents they have been devoted, almost entirely, to inorganic solvents such as liquid ammonia, liquid sulphur dioxide, etc. A variety of new solvents such as dimethylformamide, dimethylsulphoxide and propylene carbonate have become commercially available over the last twenty years. Solutions in these solvents are of technological interest in connection with novel battery systems and chemical synthesis, while studies of ion solvation and transport properties have fostered academic interest. 

Methacrylonitrile polymerizes readily in inert solvents. The polymer, depending on the initiator and on reaction conditions, is either amorphous or crystalline. Polymerizations take place over a broad range of temperatures from ambient to —5°C, when initiated by Grignard reagents, triphenyl ethylsodium, or sodium in liquid ammonia [264]. The properties of these polymers are essentially the same as those of the polymers formed by free-radical mechanism. 

Liquid ammonia has certain solvent properties like those of water however, liquid ammonia will dissolve the alkali metals (sodium, potassium, etc.) and the heavier alkaline earth metals (calcium, strontium, barium) to give intensely blue, conducting solutions. The sodium solution is widely used as a reducing agent in organic syntheses. The unbalanced equation is presented in textbooks as follows ... 

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). 

W. L. Jolly and C. J. Hallada, Liquid ammonia. Chap. 1 in T. C. WaDDINGTON (ed.), Non-aqueous Solvent Systems, pp. 1-45, Academic Press, London, 1965. J. C. Thompson, The physical properties of metal solutions in non-aqueous solvents. Chap. 6 in J. Lagowski (ed.). The Chemistry of Non-aqueous Solvents, Vol. 2, pp. 265-317, Academic Press, New York, 1967. J. Jander (ed.). Chemistry in Anhydrous Liquid Ammonia, Wiley, Interscience, New York, 1966, 561 pp. 

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 ... 

Liquid ammonia is the best-known and most widely studied non-aqueous ionizing solvent. Its most conspicuous property is its ability to... 

The problem with the Arrhenius definitions is that they are specific to one particular solvent, water. When chemists studied nonaqueous solvents, such as liquid ammonia, they found that a number of substances showed the same pattern of acid-base behavior, but plainly the Arrhenius definitions could not be used. A major advance in our understanding of what it means to be an acid or a base came in 1923, when two chemists working independently, Thomas Lowry in England and Johannes Bronsted in Denmark, came up with the same idea. Their insight was to realize that the key process responsible for the properties of acids and bases was the transfer of a proton (a hydrogen ion) from one substance to another. The Bronsted-Lowry definition of acids and bases is as follows ... 

If then, one considers these two compounds as ionizing solvents, it should be possible to prepare derivatives containing either the cation or anion of the solvent which, in the parent solvent, would have properties analogous to those of either an acid or a base. Here one is guided by the analogy with solvents such as liquid ammonia, in which anuno-nium salts behave as acids and metal amides as bases. This expectation was, in fact, realised in full. 

It was also observed, in 1973, that the fast reduction of Cu ions by solvated electrons in liquid ammonia did not yield the metal and that, instead, molecular hydrogen was evolved [11]. These results were explained by assigning to the quasi-atomic state of the nascent metal, specific thermodynamical properties distinct from those of the bulk metal, which is stable under the same conditions. This concept implied that, as soon as formed, atoms and small clusters of a metal, even a noble metal, may exhibit much stronger reducing properties than the bulk metal, and may be spontaneously corroded by the solvent with simultaneous hydrogen evolution. It also implied that for a given metal the thermodynamics depended on the particle nuclearity (number of atoms reduced per particle), and it therefore provided a rationalized interpretation of other previous data [7,9,10]. Furthermore, experiments on the photoionization of silver atoms in solution demonstrated that their ionization potential was much lower than that of the bulk metal [12]. Moreover, it was shown that the redox potential of isolated silver atoms in water must... 

Physical properties of the solvent are used to describe polarity scales. These include both bulk properties, such as dielectric constant (relative permittivity), refractive index, latent heat of fusion, and vaporization, and molecular properties, such as dipole moment. A second set of polarity assessments has used measures of the chemical interactions between solvents and convenient reference solutes (see table 3.2). Polarity is a subjective phenomenon. (To a synthetic organic chemist, dichloromethane may be a polar solvent, whereas to an inorganic chemist, who is used to water, liquid ammonia, and concentrated sulfuric acid, dichloromethane has low polarity.)... 

The synthesis of sodium amide, NaNH2 (or sodamide ), by passing ammonia over heated sodium metal, was first reported almost two centuries ago. A number of studies have since been made of its properties, but no crystal structure has been reported. Sodamide is used as a strong base in organic chemistry (often in liquid ammonia solution). In contrast, sodium bis(trimethylsilyl)amide NaN(SiMe3)2 (or sodium hex-amethyldisilazide , NaHMDS), whose crystal structure is discussed later, is widely used for deprotonation reactions or base catalysed reactions due to its solubility in a wide range of non-polar solvents. An overview of some of the types of chemical reactions in which NaHMDS is used is presented in Scheme 2.3. 

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