Preparation3




Metathesis reactions
Acid-base neutralization reactions

 

The preparation of ionic liquids, even in large quantities, presents no significant difficulties. Provided they are of sufficient purity, most ionic liquids can be stored without decomposition for extended periods, although some are relatively hygroscopic.7

There are two basic methods for the preparation of ionic liquids:

  1. metathesis of a halide salt with, for instance, a silver, group 1 metal or ammonium salt of the desired anion;
  2. acid-base neutralization reactions;

The final method for the synthesis of ionic liquids is direct combination of a halide salt with a metal halide.





IL's preparation: metathesis reactions3



Many alkylammonium halides are commercially available or they can be prepared simply by the reaction of the appropriate halogenoalkane and amine. Preparation of the pyridinium and imidazolium halides can be achieved similarly.


For volatile halogenoalkanes, the low boiling points lead to preparations requiring either a sealed tube, such as in the synthesis of [emim]Cl (where [emim]+ is the 1-ethyl-3-methylimidazolium cation), or an elaborate reaction still.
Hence, the use of salts with longer chain substituents, such as [bmim]Cl (where [bmim]+ is the 1-butyl-3-methylimidazolium cation), that can be prepared in conventional glassware by heating under reflux has become popular.

In 1992 the first of the new ionic liquids, [emim][BF4], was prepared via metathesis of [emim]I with Ag[BF4] in methanol. This salt also has a melting point of 12 °C and may be prepared considerably more cheaply using [NH4][BF4] in acetone.


This ease of preparation, together with its relative moisture stability and its immiscibility with a number of organic solvents is leading to its increasing use in biphasic catalysis.

The preparation of [emim][PF6] shortly followed; this time it was prepared by reaction of [emim]Cl with HPF6. This salt has a melting point of 60 °C, which makes it slightly less attractive than the [BF4]- salt, if room temperature working is desired. Since then, thiocyanate, nonafluorobutanesulfonate, bis((trifluoromethyl)sulfonyl)imide,  tris((trifluoromethyl)sulfonyl)methide, trifluoroacetate, and heptafluorobutanoate salts have all been prepared by metathesis reactions.

These metathesis reactions are good candidates for those preparing new ionic liquids for the first time. However, they can leave the ionic liquids contaminated with a small amount of halide ions that may react with solute materials.

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OH...It's a base!

IL's preparation: acid-base neutralization reactions3



Monoalkylammonium nitrate salts are best prepared by the neutralization of aqueous solutions of the amine with nitric acid. The ionic liquids are isolated by removing excess water in vacuo. In a similar reaction, tetraalkylammonium sulfonates have been prepared by mixing equimolar amounts of the sufonic acid and the tetraalkylammonium hydroxide. Again, excess water was removed in vacuo. To ensure the purity of the ionic liquids, they were dissolved in either acetonitrile or tetrahydrofuran and treated with activated charcoal for at least 24 h, and finally the organic solvent was removed in vacuo.





The final method for the synthesis of ionic liquids is direct combination of a halide salt with a metal halide. This is how the halogenoaluminate(III) and the chlorocuprate(I) ionic liquids are prepared. The chlorocuprate(I) ionic liquids are particularly sensitive to oxygen and have not found widespread use in synthesis.

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