An inorganic nonaqueous solvent is a
solvent other than water, that is not an
organic compound
Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...
. These solvents are used in chemical research and industry for reactions that cannot occur in aqueous solutions or require a special environment. Inorganic nonaqueous solvents can be classified into two groups, protic solvents and aprotic solvents. Early studies on inorganic nonaqueous solvents evaluated ammonia, hydrogen fluoride, sulfuric acid, as well as more specialized solvents, hydrazine, and selenium oxychloride.
Protic inorganic nonaqueous solvents
Prominent members include
ammonia,
hydrogen fluoride,
sulfuric acid,
hydrogen cyanide
Hydrogen cyanide (formerly known as prussic acid) is a chemical compound with the chemical formula, formula HCN and structural formula . It is a highly toxic and flammable liquid that boiling, boils slightly above room temperature, at . HCN is ...
.
Ammonia (and several amines as well) are useful for the generating solutions of highly reducing species because the N-H bond resists reduction. The chemistry of
electrides and
alkalides relies on amine solvents.
The combination of HF and SbF
5 is the basis of a
superacid
In chemistry, a superacid (according to the original definition) is an acid with an acidity greater than that of 100% pure sulfuric acid (), which has a Hammett acidity function (''H''0) of −12. According to the modern definition, a superacid i ...
solution. Using this mixture, the conjugate acid of
hydrogen sulfide can be isolated:
:H
2S + HF + SbF
5 →
3S">3SbF
6
Autoionization
The limiting acid in a given solvent is the solvonium ion, such as H
3O
+ (
hydronium) ion in water. An acid which has more of a tendency to donate a
hydrogen ion
A hydrogen ion is created when a hydrogen atom loses or gains an electron. A positively charged hydrogen ion (or proton) can readily combine with other particles and therefore is only seen isolated when it is in a gaseous state or a nearly particl ...
than the limiting acid will be a strong acid in the solvent considered, and will exist mostly or entirely in its dissociated form. Likewise, the limiting base in a given solvent is the solvate ion, such as OH
− (
hydroxide) ion, in water. A base which has more affinity for protons than the limiting base cannot exist in solution, as it will react with the solvent.
For example, the limiting acid in liquid ammonia is the
ammonium ion, NH
4+ which has a p''K''
a value in water of 9.25. The limiting base is the
amide ion, NH
2−. NH
2− is
a stronger base than the hydroxide ion and so cannot exist in aqueous solution. The p''K''
a value of ammonia is estimated to be approximately 34 (''cf.'' water, 14).
Aprotic inorganic nonaqueous solvents
Prominent members include
sulfur dioxide
Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a colorless gas with a pungent smell that is responsible for the odor of burnt matches. It is r ...
,
sulfuryl chloride fluoride,
dinitrogen tetroxide,
antimony trichloride, and
bromine trifluoride. These solvents have proven useful for study highly electrophilic or highly oxidizing compounds or ions. Several (SO
2, SO
2ClF, N
2O
4) are gases near room temperature, so they are handled using
vacuum-line techniques.
The generation of
7">S7sup>+ and
7">rS7sup>+ are illustrative. These highly electrophilic salts are prepared in SO
2 solution.
The preparation of
3">Br3sup>+ salts also calls for a mixed solvent composed of SO
2 and SO
2FCl. Sulfuryl chloride fluoride is often used for the synthesis of
noble gas compounds.
Autoionization
Many inorganic solvents participate in
autoionization reactions. In the solvent system definition of acids and bases, autoionization of solvents affords the equivalent to acids and bases. Relevant autoionizations:
: 2BrF
3 BrF
2+ + BrF
4−
: N
2O
4 ⇌ NO
+ (
nitrosonium) + NO
3− (
nitrate)
: 2SbCl
3 ⇌ SbCl
2+ + SbCl
4−
: 2POCl
3 ⇌ POCl
2+ + POCl
4−
According to the
solvent-system definition, acids are the compounds that increase the concentration of the solvonium (positive) ions, and bases are the compounds that result in the increase of the solvate (negative) ions, where solvonium and solvate are the ions found in the pure solvent in equilibrium with its neutral molecules:
The solvent SO
2 is relatively uncomplicated, it does not autoionize.
See also
*
Nonaqueous titration
*
Protic solvent
References
External links
*
{{Chemical solutions
Solvents