phase transfer catalyst

Catalyst type

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phase transfer catalysis Phase transfer catalyst (PTC) can help reactant From one phase transfer To another level where reaction can take place, so as to speed up Out of phase system reaction rate A type of catalyst of. Generally, there are aqueous solutions and Organic solvent Two phase, ionic reactants often soluble to aqueous phase ， Insoluble to Organic phase The organic substrate is soluble in organic solvents. In the absence of phase transfer catalyst, the two phases are isolated from each other, several reactants cannot contact, and the reaction proceeds slowly. The existence of phase transfer catalyst can combine with the ions in the water phase (usually), and use its own Affinity Transfer the reactants in the water phase to the organic phase to promote the reaction.

Chinese name: phase transfer catalyst
Foreign name: Phase transfer catalyst

Abbreviation: PTC
Other types: Three-phase phase transfer catalysis

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brief introduction

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stay Organic synthesis Heterogeneous phase is often encountered in Organic reaction , such reactions usually have slow speed, yield Low and incomplete response. But if you use Water solubility inorganic salt , use organic solvent with low polarity Dissolved organic matter And a small amount (below 0.05mol) of quaternary ammonium salt or quaternary phosphorus salt is added to facilitate the reaction, which can promote the reaction rate And transfer between two phases anion Of Onium salt , called phase transfer catalyst.

Catalytic mechanism

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phase transfer catalysis The reaction is generally a two-phase reaction, Reaction process It mainly includes the transfer of reactants from one phase to another and the occurrence of transferred substances and untransferred substances chemical reaction 。 Has developed many different catalytic reaction mechanism:

1. Extraction mechanism

In 1971, Starks developed liquid liquid phase SN2 Nucleophilic substitution reaction The famous catalytic cycle principle was proposed, which laid the theoretical foundation for phase transfer catalytic reaction.

Take onium salt phase transfer catalyst as an example. Onium salt aqueous phase and Organic phase All have certain solubility , according to Solubility There are two different cycle models:

PTC is distributed in two phases phase transfer It is the onium salt that transports Y from the aqueous phase to the organic phase, and then the onium salt cation And send X - to the water phase

Phase transfer catalyst in model II Cation exchange It occurs on the interface, and the function of PTC is to ion pair Repeated extraction in the form of anion Y - entering the organic phase without the transfer of catalyst cation in the two phases

2. Interface mechanism

3 Other types

Three-phase phase transfer catalysis

Heteropolyacid phase transfer catalysis

ionic liquid phase transfer catalysis

advantage

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The phase transfer catalyst has the following outstanding advantages:

1. No water free operation required, phase transfer catalytic reaction Can be used in water and Organic solvent Two-phase reaction;

2. Accelerate reaction rate ；

3. Reduce the reaction temperature

4. The product yield is high, and the reaction is complete due to the full contact of reactants due to the role of phase transfer catalyst;

5. The synthesis operation is simple, the temperature and pressure are reduced, the strength of the equipment is low, and the operation is simple;

6. Avoid using the Dangerous reagent ；

7. Widely applicable to various Synthetic reaction , and it is possible to complete the synthesis reaction that cannot be realized by other methods;

eight side reaction Easy to control and improve selectivity.

Common types

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requirement

① With formation ion pair Conditions; Or can form with reactants Complex ion ；

② There are enough carbon atom To form ion pair It has the ability of organic solvent affinity;

③ Structure of R Steric hindrance It should be as small as possible, R-base Most of them are straight chains;

④ Under reaction conditions, it should be chemically stable and easy to recover.

type

1. Polyether

chain polyethylene glycol :H(OCH2CH2)nOH

Chain polyethylene glycol dialkyl ether: R (OCH2CH2) nOR

2. Annular crown ether Class: 18 crowns 6, 15 crowns 5 Cyclodextrin Etc.

3、 quaternary ammonium salt : The commonly used quaternary ammonium salt phase transfer catalyst is Benzyl triethyl ammonium chloride （ TEBA ）、 Tetrabutyl ammonium bromide （TBAB ）、 Tetrabutyl ammonium chloride 、 Tetrabutyl ammonium bisulfate 、 Trioctyl methyl ammonium chloride 、 Dodecyl trimethyl ammonium chloride 、 Tetradecyl trimethyl ammonium chloride Etc.

4、 Tertiary amine :R4N X , pyridine , Tributylamine Etc.

5、 Quaternary ammonium base (Its alkalinity is sodium hydroxide Similar) Easily soluble in water, strong Hygroscopicity 。

6. Quaternary phosphonate

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