Saturday, 22 March 2014

Liquid Ammonia as a Solvent

                                

Liquid Ammonia as a Solvent

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Ammonia has a reasonable liquid range (-77 to –33 °C), and as such it can be readily liquefied with dry ice (solid CO2, Tsub = -78.5 °C), and handled in a thermos flask. Ammonia’s high boiling point relative to its heavier congeners is indicative of the formation of strong hydrogen bonding, which also results in a high heat of vaporization (23.35 kJ/mol). As a consequence ammonia can be conveniently used as a liquid at room temperature despite its low boiling point.
Liquid ammonia is a good solvent for organic molecules (e.g., esters, amines, benzene, and alcohols). It is a better solvent for organic compounds than water, but a worse solvent for inorganic compounds. The solubility of inorganic salts is highly dependant on the identity of the counter ion 


Soluble in liquid NH3
Generally insoluble in liquid NH3
SCN-, I-, NH4+, NO3-, NO2-, ClO4-
F-, Cl-, Br-, CO32-, SO42-, O2-, OH-, S2-
Table 1: General solubility of inorganic salts in liquid ammonia as a function of the counter ion.
The difference in solubility of inorganic salts in ammonia as compared to water, as well as the lower temperature of liquid ammonia, can be used to good advantage in the isolation of unstable compounds. For example, the attempted synthesis of ammonium nitrate by the reaction of sodium nitrate and ammonium chloride in water results in the formation of nitrogen and water due to the decomposition of the nitrate, Equation 1. By contrast, if the reaction is carried out in liquid ammonia, the sodium chloride side product is insoluble and the ammonium nitrate may be isolated as a white solid after filtration and evaporation below its decomposition temperature of 0 °C, Equation 2


Ammonation


Ammonation is defined as a reaction in which ammonia is added to other molecules or ions by covalent bond formation utilizing the unshared pair of electrons on the nitrogen atom, or through ion-dipole electrostatic interactions. In simple terms the resulting ammine complex is formed when the ammonia is acting as a Lewis base to a Lewis acid, Equation 3 and Equation 4, or as a ligand to a cation, e.g., [Pt(NH3)4]2+, [Ni(NH3)6]2+, [Cr(NH3)6]3+, and [Co(NH3)6]3+.
(3)
(4)
Ammonolysis
Ammonolysis with ammonia is an analogous reaction to hydrolysis with water, i.e., a dissociation reaction of the ammonia molecule producing H+ and an NH2- species. Ammonolysis reactions occur with inorganic halides, Equation 5 and Equation 6, and organometallic compounds, Equation 7. In both case the NH2- moiety forms a substituent or ligand.
(5)
(6)
(7)
The reaction of esters, Equation 8, and aryl halides, Equation 9, are also examples of ammonolysis reactions.
(8)
(9)
Homoleptic amides
A homoleptic compound is a compound with all the ligands being identical, e.g., M(NH2)n. A general route to homoleptic amide compounds is accomplished by the reaction of a salt of the desired metal that is soluble in liquid ammonia (Table 1) with a soluble Group 1 amide. The 

solubility of the Group 1 amides is given in Table 2. Since all amides are insoluble (except those of the Group 1 metals) are insoluble in liquid ammonia, the resulting amide may be readily isolated, e.g., Equation 10 and Equation 11.
(10)
(11)
Amide
Solubility in liquid ammonia
LiNH2
Sparingly soluble
NaNH2
Sparingly soluble
KNH2
Soluble
RbNH2
Soluble
CsNH2
Soluble
Table 2: Solubility of Group amides in liquid ammonia.
Redox reactions
Ammonia is poor as an oxidant since it is relatively easily oxidized, e.g., Equation 12 and Equation 13. Thus, if it is necessary to perform an oxidation reaction ammonia is not a suitable solvent; however, it is a good solvent for reduction reactions.
(12)
(13)
Liquid ammonia will dissolve Group 1 (alkali) metals and other electropositive metals such as calcium, strontium, barium, magnesium, aluminum, europium, and ytterbium. At low concentrations (ca. 0.06 mol/L), deep blue solutions are formed: these contain metal cations and solvated electrons, Equation 14. The solvated electrons are stable in liquid ammonia and form a complex: [e-(NH3)6].
(14)
The solvated electrons provide a suitable and powerful reducing agent for a range of reactions that are not ordinarily accomplished, e.g., Equation 15 and Equation 16

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