Preparative and practical aspects

The ability of micellized surfactants to catalyze, or inhibit, reactions and to control stereochemistry and product composition, suggests that these agents could have a useful role in organic synthesis. A micelle can speed a desired reaction and inhibit an undesired one, and, for example, cationic micelles can control the ratio of unimolec-ular, Sfjl, substitution to bimolecular, E2, ehmination [54,55]. Micellization is of great importance in emulsion polymerization, but little use has been made of aqueous micelles in synthesis. 

There are two obvious limitations to the use of surfactants in preparative chemistry. The first is that the high molecular weight of surfactants makes it impracticable to use them in large excess over reactants, and second, surfactants complicate product isolation. The isolation problem can often be solved by precipitating the surfactant, e.g., alkyl sulfates can often be precipitated from water as their potassium salts, or alkylammonium ions as their perchlorates, but both limitations are neatly solved using the two-phase system of phase-transfer catalysis. 

One way to use surfactants in synthesis is to immobilize them. For example surfactant moieties can be bound covalently to a solid polymeric support [176]. Alternatively a surfactant or a hydrophobic cation can be supported on a solid support such as an ion-exchange resin [106] or silica gel [177]. 

In all these systems the aim is to design a reusable catalyst which can easily be separated from the products. 

A different system, but akin to micellar catalysis, is the technique of triphase catalysis, in which reaction is carried out in a mixture of water and an apolar organic 

---