What are Ionic Liquids?
Ionic liquids refer to liquids composed entirely of ions, such as KCl and KOH at high temperatures, which are in liquid state. At this time, they are ionic liquids. Substances composed of ions that are liquid at room temperature or near room temperature are called room temperature ionic liquids, room temperature molten salts, organic ionic liquids, etc. There is no unified name at present, but it tends to be referred to as ionic liquids.
In ionic compounds, the force between anions and cations is Coulomb force, and its magnitude is related to the charge number and radius of anions and cations. The larger the ionic radius, the smaller the force between them, and the lower the melting point of this ionic compound. The anions and cations of some ionic compounds are large in volume and loose in structure, resulting in low force between them, so that the melting point is close to room temperature.
Types
Ionic liquids are composed of organic cations and inorganic or organic anions. Common cations include quaternary ammonium salt ions, quaternary phosphonium salt ions, imidazolium salt ions, etc., and anions include halogen ions, tetrafluoroborate ions, hexafluorophosphate ions, etc.
Among the ionic liquids studied so far, the cations are mainly imidazolium cations, and the anions are mainly halogen ions and other inorganic acid ions.
- According to the differentorganic cation matrixin the ionic liquid, the ionic liquid can be divided into imidazolium salts, pyridinium salts, quaternary ammonium salts and quaternary phosphonium salts.
- According to the different solubilityof ionic liquids in water, ionic liquids can also be divided into hydrophilicionic liquids and hydrophobic ionic liquids.
Application of ionic liquids in organic synthesis reactions
- Diels-Alder cycloaddition reaction
Advantages of carrying out Diels-Alder reaction in ionic liquids: the system has sufficiently low vapor pressure, can be recycled, is non-explosive, thermally stable and easy to operate. The first reaction studied was the reaction of cyclopentadiene with methyl acrylate and methyl ketone in [EtNH3][NO3](EAN). Compared with non-polar organic solvents, the reaction showed obvious high internal rotation product tendency and fast reaction rate characteristics.
- Alkylation reaction
The alkylation reaction of benzene with olefins or alkyl halides is a type of reaction catalyzed by Lewis acids, so acidic MCI/AICl3 ionic liquids can be used as new alternative catalysts. The use of HCI-modified MCI/AICL3 ionic liquids to catalyze the reaction of benzene with dichloromethane has high catalytic activity. The reaction takes only 5 minutes at room temperature to achieve a conversion rate of 99% and a yield of more than 95%, which is much higher than the 57% and 38.4% when anhydrous AlCl3 is used as a catalyst. In addition, the product can be easily separated.
- Esterification reaction
N-l-butylpyridinium chloride/AIClz ionic liquids are used as reaction media and catalysts to achieve a clean esterification process. The obvious advantages of this esterification process are: (1) The reaction product ester is insoluble in the ionic liquid, so it is easy to separate; (2) After the reaction is completed, the ionic liquid can be reused by heating it at 100°C to remove the water generated in the reaction.
- Beckmann rearrangement reaction
Beckmann rearrangement is a common method for laboratory synthesis and industrial production of internal amines such as ε-caprolactone. In the usual reaction, the reaction medium is a large amount of concentrated sulfuric acid, phosphorus pentachloride in ether or acetic acid and acetic anhydride in hydrogen chloride solution.
- Friedel-Crafts acetylation reaction
The hydrogen on the aromatic ring is replaced by an alkyl group under the action of B-acid or L-acid, which is called Friedel-Crafs reaction; if it is replaced by an acyl group, it is called Friedel-Crafts acylation reaction. The traditional Friede1-Crats catalyst is AIX3 type L-acid, and the reaction solvent is petroleum ether, chlorobenzene, etc. The general reaction time is 5h-6h, the yield is about 80%, and the product is a mixture of various isomers with poor selectivity. If it is carried out in [Emim][AICl] ionic liquid, the substance acts as both solvent and catalyst, and the reaction is completely converted (100% conversion) in just 3s, with high selectivity.
- Heck arylation reaction
Using Pd/C as a catalyst, aryl halides react with olefins in the ionic liquid [Bmim][PF6]. The advantage is that the catalyst is well dispersed in the ionic liquid to form a stable catalytic system. After the reaction is completed, the product is extracted with n-hexane. The catalyst can be recycled and the catalytic activity is not reduced.
- Acylation reaction
Many factories use HF or AlCl3 as a catalyst for the acylation of benzene derivatives, which will produce a large amount of waste. Replacing traditional catalysts with ionic liquids is a solution to this problem. Studies on the acylation of toluene, m-xylene, mesitylene, and methoxybenzene have shown that the ionic liquids [Bmim]FeCl3] and [Bmim][SnCl] are suitable catalysts for Friedel-Crafts acylation reactions, with high conversion rates and yields. Especially when the reaction is carried out in [Bmim][FeCl3], ionic liquids, except for the m-xylene acylation reaction, which has a selectivity of 80%, other reactions can obtain high selectivity and conversion rates.
Customized ionic liquids include:
Pyridine ionic liquids
Imidazole ionic liquids
Quaternary ammonium ionic liquids
Quaternary phosphonium ionic liquids
Pyrrolidine ionic liquids
Piperidine ionic liquids
Functionalized ionic liquids
Customized liquid synthesis
Functionalized ionic liquids:
olefin functionalized ionic liquids
Hydroxy functionalized ionic liquids
Ether functionalized ionic liquids
Ester functionalized ionic liquids
Carboxyl functionalized ionic liquids
Nitrile functionalized ionic liquids
Amino functionalized ionic liquids
Sulfonic acid functionalized ionic liquids
Benzyl functionalized ionic liquids
Amino functionalized ionic liquids
