Ammonia as a solvent

Scheme 12 Synthesis of metal polysulfido complexes using supercritical ammonia as a solvent...

These optimistic views received a first blow by the discovery of another category of nucleophilic aromatic photosubstitutions occurring in liquid ammonia as a solvent and nucleophile. In this medium the nitroanisoles still show the pattern that had become familiar, i.e., photosubstitution of OCH3 by NH2 with preference for reaction at the position meta with respect to the nitro-group. However, nitrobenzene, dinitrobenzenes, and nitrohalogenobenzenes... 

Acetylenic compounds are conveniently prepared with the aid of liquid ammonia as a solvent. The preparation of a simple acetylenic hydrocarbon (a-butylacetylene or 1-hexyne) and also of phenylacetylene is described. Experimental details are also given for two acetylenic carbinols, viz., 1-ethynyl-cycfohexanol and 4-pentyn-l-ol. It will be noted that the scale is somewhat large smaller quantities can readily be prepared by obvious modifications of the directions. 

Considerable use has been trade of these analogies, especially with reference to nitrogen compounds and their relation to liquid ammonia as a solvent.6... 

The acid and base analogs of ammonia as a solvent is specified by this equilibrium as NH41 and NH inns. All substances which undergo ammonolysis and hence bring about an increase in the ammonium ion concentration yield acitl solutions Thus P S, dissolves in liquid ammonia to give an acid solution as follows. 

There are three groups of substrates for which SNH reactions are especially characteristic (i) neutral azines and azoles (ii) azinium and azolium salts and (iii) nitro-arenes. Their electron-deficiency and, thus, their ability to react with nucleophiles strongly differ from each other. Azinium salts are the more electrophilic and are able to add even neutral nucleophiles very easily. Triazines,. v-tetrazines and polynitro-arenes also possess high electrophilicity. At the same time substrates such as pyridine (82JHC1285, 72JA682) and cinnoline (03CHE87), at a low temperature (e.g. in liquid ammonia as a solvent), do not react even with sodium amide. It should also be... 

Liquid Ammonia as a Solvent. Liquid ammonia (b.p. --38.4 C) has a high dielectric constant, and is a good solvent for salts, forming ionic solutions. It also has the unusual power of dissolving the alkali metals and alkaline-earth metals without chemical reaction, to form blue solutions which have an extraordinarily high electrical conductivity and a metallic luster. These metallic solutions slowly decompose, with evolution of hydrogen, forming amides, such as sodium amide, NaNH ... 

The H2/Pd hydrogenolysis system for the removal of the Z group from peptides is limited to sulfur-free peptides. In order to overcome this limitation, Meienhofer successfully conducted hydrogenolysis in liquid ammonia as a solvent to remove the Z group from Cys- or Met-containing peptides.As shown in Scheme 23, protected somatostatin was prepared in a stepwise manner starting from H-Cys(tBu)-OtBu. 

Water-free inorganic solvents, such as ammonia, sulfur dioxide, and hydrazine, have been tested in terms of their suitability for electrolytic metal deposition. Liquid ammonia is used for a series of electrolytic metal deposition processes. Besides the low boiling point (- 33 °C) of this solvent its toxicity is disadvantageous. It has been reported that group lA and IIA metals, such as hthium, sodium, magnesium, and beryllium can be deposited from solutions based on ammonia as a solvent [45]. However, only thin or incoherent layers are thus produced [43, 44]. Because it is possible to form anions of molybdenum, lead, selenium, and tellmium in anunonia, these elements can be anodically deposited. Thus, deposition of the semiconductor lead selenide has also been achieved with ammonia as a solvent. 

When using water instead of ammonia as a solvent in such clusters, no evidence for proton transfer was found in time-dependent dynamic studies for n = 20. 

Compare liquid ammonia as a solvent with water. 

As might be expected, alkali metals react with cyclopentadiene much more readily in a solvent in which the alkali metal cyclopentadienide has some solubility. Fischer and Jira (7-9) first used liquid ammonia as a solvent for this reaction and obtained high yields of the ionic cyclopentadienides of Li, Na, and K. It is interesting that the reaction of sodium with cyclopentadiene in liquid ammonia does not produce hydrogen, but yields cyclopentene according to the stoichiometry of the following equation 10,11)... 

Recently, the use of liquid ammonia as a solvent for substitutions has been advocated [96, 97], and the displacement of fluoride from 2-nitro- and 4-nitro-fluorobenzene by alkoxide and phenoxide ions has been achieved without competing solvolysis. 

The use of liquid ammonia as a solvent for Ar reactions has been explored. With 4-fluoronitrobenzene and chloropyrimidines, solvolysis occurs although substitution by added nucleophiles such as phenoxide or triazolate ions can compete successfully. Kinetic studies of the reactions of 2-chloro- and 2-ethoxy-3,5-dinitropyridine with substituted anilines in DMSO in the presence of DABCO indicate a base-catalysed pathway, which is likely to involve rate-limiting proton transfer from the zwitterionic intermediate to base." ° In the reaction of 2,6-bis(trifluoromethanesulfonyl)-4-nitroanisole, (26), with substituted anilines in DMSO/methanol mixtures, nucleophilic attack is rate-limiting. Rate constants have higher values in DMSO than in methanol, and in DMSO-rich mixtures, there is evidence, from changes in slope of Brpnsted and Hammett plots, that formation of the zwitterionic intermediate involves an SET process." ... 

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