Phase-transfer catalysis interfaces

Phase Transfer Catalysis in Industry, PTC Interface, Inc., Marietta, Ga. 

The effect of ultrasound on liquid-liquid interfaces between immiscible fluids is emulsification. This is one of the major industrial uses of ultrasound (74-76) and a variety of apparatus have been devised which will generate micrometer-sized emulsions (9). The mechanism of ultrasonic emulsification lies in the shearing stresses and deformations created by the sound field of larger droplets. When these stresses become greater than the interfacial surface tension, the droplet will burst (77,78). The chemical effects of emulsification lie principally in the greatly increased surface area of contact between the two immiscible liquids. Results not unlike phase transfer catalysis may be expected. 

Uniquely interesting, complex and useful activities and phenomena occur at interfaces one need only to look at the interfaces between the land, the atmosphere, and the sea to find this truth. The same truth occurs in chemical interfaces, although sometimes it is the lack of activity that draws our attention. In many chemical situations where two species cannot collide and therefore cannot react because they are separated by an interface, the lack of activity has been overcome by use of the technique of PHASE TRANSFER CATALYSIS (PTC), which not only allows reaction to occur, but often to occur in very selective ways. 

Because the cellulose ether alkoxide is present entirely in the aqueous phase, the rate-limiting step may be the partitioning (phase transport) of the hydrophobic electrophile across the interface from the organic to aqueous phase. If the reaction rate is controlled by diffusion of the electrophile across the interface, then one would expect a correlation between water solubility of the hydrophobe and its alkylation efficiency. The fact that the actual alkylation reaction is probably occurring in the aqueous phase (or at the interface) yet the electrophile itself is principally soluble in the organic phase has important mechanistic ramifications. This type of synthetic problem, in which one reactant is water soluble and the other organic soluble, should be amenable to the techniques of phase transfer catalysis (PTC) to yield significant improvements in the alkylation efficiency. 

Most reactions in two-phase systems occur in a liquid phase following the transfer of a reactant across an interface these are commonly known as extractive reactions. If the transfer is facilitated by a catalyst, it is known as phase-transfer catalysis [2]. Unusually, reactions may actually occur at an interface (interfacial reactions) examples include solvolysis and nucleophilic substitution reactions of aliphatic acid chlorides [3 ] and the extraction of cupric ion from aqueous solution using oxime ligands insoluble in water [4], see Section 5.2.1.3(ii). 

Starks, C.M., Liotta, C.L. and Halpern, M. (1994) Phase-Transfer Catalysis. Chapman and Hall, London. Volkov, A.G. (Ed.) (2001) Liquid Interfaces in Chemical, Biological and Pharmaceutical Applications. Dekker, New York. 

Figure 3.11 illustrates a scenario where OH ions are transported from the aqueous into the chloroform phase by tetraalkylammonium cations. There, the tetraalkylammonium hydroxide is the base and is available for deprotonation in the entire chloroform phase—a process that was previously limited to just the interface. The C13C so formed could undergo fragmentation to dichlorocarbene, which could then add to the alkene to be cyclopropanated. This scenario provides a plausible explanation of the reaction mechanism for dichlorocyclo-propanations, which in practice are usually performed under phase-transfer catalysis (cf. Figure 3.13 for an example). 

The use of a two-phase system with added phase transfer catalyst and the use of a microemulsion are two alternative approaches to overcome reagent incompatibility problems in organic synthesis. Both routes have proved useful but on entirely different accounts. In phase transfer catalysis the nucleophilic reagent is carried into the organic phase where it becomes highly reactive. In the microemulsion approach there is no transfer of reagent from one environment to another the success of the method relies on the very large oil-water interface at which the reaction occurs. 

It has become Increasingly evident that the surfactants are accomplishing more than the solubilization of the organic compounds. Certainly phase transfer catalysis would be expected to occur In the emulsion system and this has been proposed In several organic synthesis studies (21-26). The term micelle catalysis, has not been used to any extent In electrochemistry. Instead terms such as Ion pairing and Ion bridging have been used to explain the acceleration of electrode reactions by the presence of a variety of Ions In the Interface between the solution and the electrode (40-A2). Obviously these processes are the same king of processes one postulates In micelle catalysis. 

This book is about homogeneous reactions, that is, all kinds of reactions that occur within a single fluid phase. The term excludes reactions at interfaces, among them reactions of solids with fluids, heterogeneous catalysis, and phase-transfer catalysis. It does not exclude reactions in which a dissolved reactant is resupplied from another phase, as is the case, for example, in homogeneous hydrogenation or air oxidation reactions in the liquid phase in contact with a gas phase. 

Understanding chemical reactivity at liquid interfaces is important because in many systems the interesting and relevant chemistry occurs at the interface between two immiscible liquids, at the liquid/solid interface and at the free liquid (liquid/vapor) interface. Examples are reactions of atmospheric pollutants at the surface of water droplets[6], phase transfer catalysis[7] at the organic liquid/water interface, electrochemical electron and ion transfer reactions at liquidAiquid interfaces[8] and liquid/metal and liquid/semiconductor Interfaces. Interfacial chemical reactions give rise to changes in the concentration of surface species, but so do adsorption and desorption. Thus, understanding the dynamics and thermodynamics of adsorption and desorption is an important subject as well. 

From an electrochemical point of view it is easily inferred that the solution in a cell near an electrode is separable into two parts a stagnant layer adjacent to the electrode in which no convective motions occur, and the remainder of the solution, which is homogeneous (bulk solution). Yet this is not a particularity of electrochemical methods since the same phenomena occur at any solid/liquid interface, as when metal particles (reductions by Zn or Na, for example) or any heterogeneous reagent is used in organic homogeneous chemistry, as well as in phase-transfer catalysis or related methods. 

Phase transfer catalysts transport reactants across a liquid-liquid interface ", and are used when the desired reaction involves reactants with far different solubility characteristics, and which, therefore, cannot be brought simultaneously to a high concentration in any single liquid phase. The use of phase transfer catalysis may eliminate the need for anhydrous solvents. 

Phase-Transfer Catalysis. Since the efficiency of the reaction requires that bisphenoi A be used in the form of water-soluble phenolate anions, while phosgene must be dissolved in a chemically inert and hence water-insoluble solvent, the desired reaction would have to depend on the diffusion rate of the two reactants to the interface between the immiscible solvents. The area of the interface can be increased by vigorous stirring of the two-phase reaction mixture, but a more efficient way to accelerate the process is to induce one of the reactants to migrate into a phase that is not particularly receptive to it. In this example, the sodium phenolate ions are in equilibrium with a phase-transfer catalyst such as tetra-n-butylammonium chloride, and while one of the products of the equilibrium (sodium chloride) remains in the aqueous phase, the other products of the equilibrium (the BPA anion-tetra-rc-butylammonium cation ion pairs) are of sufficiently covalent character to migrate into the nonaqueous phase where they encounter phosgene and the reaction takes place. 

---