i am working on synthesis of ionic liquids. how could we generally classify ionic liquids and what are the general criteria under which we can say a compound to be ionic liquid
The general requirement for a compound to be categorized as an ionic liquid is being a salt that melts below 100 °C, but you will find many authors describing compounds as being ionic liquids even though their melting points are higher than 100 °C just because they are similar in molecular structure. One of the most common types of ionic liquids are imidazolium salts, therefore, some researchers tend to call all imidazolium salts as ionic liquids, but this is not correct, since one of the main requirements for being a ionic liquid is having a melting point lower than 100 °C. This categorization by melting point is of course arbitrary, since it is based on the boiling point of water, therefore, just because a compound is not strictly speaking an ionic liquid (meaning it has a melting point higher than 100 °C) it doesn't mean it will necessarily behave in a completely different way when compared to compounds considered to be ionic liquids, since there can be many chemical similarities between them. In summary, you should only call a compound a ionic liquid if it is a salt that melts below 100 °C.
Ionic liquids are salts, which are liquid in the ambient temperature. Melting below 100 (sometimes, even below 0) and boiling above 100. One of the interesting problem of IL is the search for a ionic liquid, capable to maintain its state during moon days and moon nights.
Low-temperature ionic liquids can be compared to ionic solutions, liquids that contain both ions and neutral molecules, and in particular to the so-called deep eutectic solvents, mixtures of ionic and non-ionic solid substances which have much lower melting points than the pure compounds. Certain mixtures of nitrate salts can have melting points below 100 °C
Ionic liquids are often moderate to poor conductors of electricity, non-ionizing (e.g., non-polar), highly viscous and frequently exhibit low vapor pressure. Their other properties are diverse: many have low combustibility, are thermally stable, with wide liquid regions, and favorable solvating properties for a range of polar and non-polar compounds. Many classes of chemical reactions, such as Diels-Alder reactions and Friedel-Crafts reactions, can be performed using ionic liquids as solvents
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