198
T. J. S. Schubert
atmosphere. This dangerous operation requires a lot of experience. Even traces of
oxygen lead to the formation of phosphine oxides as side products.
8.2.2.2 Anion Metathesis
The anion metathesis, which should not be confused with ring-closing metathesis, is
a type of reaction where the anion of an intermediate (as defined in Sect. 8.2.2.1) is
replaced by a different anion. It is important to differentiate whether a hydrophobic
or a hydrophilic ionic liquid is formed as a product.
The better the anion metathesis works, the higher the lattice energy of the corresponding waste salt, such as sodium chloride or lithium bromide. In case of a
hydrophobic product, these highly water-soluble side products can be easily separated from the product-containing phase by extraction or separation. Depending on
each product, a small amount of cross-contamination by the reaction-specific waste
salts occurs that can be reduced to values below 10 ppm by washing the product with
deionized water.
The situation is different for hydrophilic ionic liquids. In this case, both intermediate and product ionic liquids as well as the waste salt are soluble in water, so that
it is not possible to separate the product from the waste salt by phase separation.
Consequently, it is necessary to choose an organic solvent that dissolves the target
ionic liquid sufficiently but has only a poor tendency to dissolve the waste salt. For
this purpose, polar aprotic solvents with low boiling points give good results, but
the best solvent must be identified for each case. Nevertheless, the halide content of
hydrophilic ionic liquids produced by using anion metathesis is in nearly all cases
higher than that for hydrophobic ionic liquids because of the comparably high solubility of waste salts in the product phase. Therefore, to achieve low halide content
for hydrophilic ionic liquids, further purification is necessary (see Sect. 8.3).
An alternative method for performing an anion metathesis is the use of ionexchange resins. This method has not been applied to the commercial production
of ionic liquids, but has, if automated, a significant potential because it is of interest for synthesizing hydrophilic ionic liquids. Furthermore, it is also a very good
approach for preparing hydroxide-based ionic liquids, which can be used as intermediates and transferred into a broad variety of ionic liquids by a simple acid–base
reaction.
8.2.2.3 Acid–Base Reactions
Acid–base reactions are another commonly used type of synthesis for commercially
relevant ionic liquids.
T. J. S. Schubert
atmosphere. This dangerous operation requires a lot of experience. Even traces of
oxygen lead to the formation of phosphine oxides as side products.
8.2.2.2 Anion Metathesis
The anion metathesis, which should not be confused with ring-closing metathesis, is
a type of reaction where the anion of an intermediate (as defined in Sect. 8.2.2.1) is
replaced by a different anion. It is important to differentiate whether a hydrophobic
or a hydrophilic ionic liquid is formed as a product.
The better the anion metathesis works, the higher the lattice energy of the corresponding waste salt, such as sodium chloride or lithium bromide. In case of a
hydrophobic product, these highly water-soluble side products can be easily separated from the product-containing phase by extraction or separation. Depending on
each product, a small amount of cross-contamination by the reaction-specific waste
salts occurs that can be reduced to values below 10 ppm by washing the product with
deionized water.
The situation is different for hydrophilic ionic liquids. In this case, both intermediate and product ionic liquids as well as the waste salt are soluble in water, so that
it is not possible to separate the product from the waste salt by phase separation.
Consequently, it is necessary to choose an organic solvent that dissolves the target
ionic liquid sufficiently but has only a poor tendency to dissolve the waste salt. For
this purpose, polar aprotic solvents with low boiling points give good results, but
the best solvent must be identified for each case. Nevertheless, the halide content of
hydrophilic ionic liquids produced by using anion metathesis is in nearly all cases
higher than that for hydrophobic ionic liquids because of the comparably high solubility of waste salts in the product phase. Therefore, to achieve low halide content
for hydrophilic ionic liquids, further purification is necessary (see Sect. 8.3).
An alternative method for performing an anion metathesis is the use of ionexchange resins. This method has not been applied to the commercial production
of ionic liquids, but has, if automated, a significant potential because it is of interest for synthesizing hydrophilic ionic liquids. Furthermore, it is also a very good
approach for preparing hydroxide-based ionic liquids, which can be used as intermediates and transferred into a broad variety of ionic liquids by a simple acid–base
reaction.
8.2.2.3 Acid–Base Reactions
Acid–base reactions are another commonly used type of synthesis for commercially
relevant ionic liquids.