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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