Ionic liquids are salts composed of cations such as imidazolium ions, pyridinium ions and anions such as BF₄^-, PF₆^- and are liquid at relatively low temperatures. Their characteristic properties include; (1) essentially no vapor pressure, (2) non-flammability, (3) high thermal stability, (4) relatively low viscosity, (5) liquid state over extended temperature ranges, (6) high ionic conductivity. Applications of ionic liquids with these features are being vigorously explored in various fields. When an ionic liquid is used as a reaction solvent, the solute is solvated by ions only. Thus, the reaction proceeds in an environment totally different from that when water or ordinary organic solvents are used and therefore, high selectivity is possible. To date application in the Friedel-Crafts reaction, Diels-Alder reaction, metal-catalyzed asymmetric synthesis and so forth, have been reported.¹⁾ Furthermore, some ionic liquids have low solubility in water and low polar organic solvents. By using this property, ionic liquids can be recovered and reused after reaction product is extracted with organic solvents. Recently, methods are being studied which reduce the load on the environment from the viewpoint of green chemistry. Ionic liquids are receiving much attention as a superb solvent from the point of safety, separation/purification procedures, and recyclability.²⁾
Ionic liquids are used not only as a reaction solvent but also electrochemically, for example, as electrolytes of rechargeable batteries.³⁾ Watanabe and co-workers have developed a novel solid electrolyte in which an ionic liquid is confined in a network polymer called “ion gel”.⁴⁾ This ion gel is expected to contribute to solidification of various electrochemical systems and safety improvement as it exhibits exceedingly high ion-conductivity compared with conventional solid polymer electrolytes. Recently, Hamaguchi and co-workers reported that 1-butyl-3-methylimidazolium tetrachloroferrate is magnetic ionic liquid.⁵⁾ The new magnetic ionic liquid overcomes various problems of traditional magnetic fluids, and is expected to be applied to many fields including the use as a sealing agent for the motor axis.

Below shows the major ionic liquids. The melting points are shown as the reference value. These are not specification value. The salts that form liquid state at room temperature are given the description “liquid”.

Imidazolium Salts

Pyrrolidinium Salts

Pyridinium Salts

Ammonium, Phosphonium, Sulfonium Salts

Imidazolium Salts

B2473	B2474	B2475
B2193	B2194	B2320
B2708	B2672	B2195
B3596	B2337	B3542
D3341	D3240	D3903
E0543	E0490	E0680
E0650	E0493	E0754
E0556	E0755	E0706
E0496	E0494	E0836
H1227	H1097	H1098
H1099	M1904	M2062
M2063	M1440

Pyrrolidinium Salts

B2851	B3427	B3358
M2098

Pyridinium Salts

B2700	B3104	B3425
B3426	B2701	B1743
B1329	B2196	B3232
E0682	E0681	E0171
E0544

Ammonium, Phosphonium, Sulfonium Salts

A2274	C1966	M1660
T0054	T0055	T1124
H1047	T2564	T2679
T2680	T2314	T2761

Pyridinium Compounds