Organic Synthesis in Nonionic Microemulsions
The aim of this work has been to explore the use of microemulsions as reaction media for organic synthesis. Another important aim has been to improve the understanding of the reaction kinetics and the mechanism of the reaction in this type of surfactant system.
Three different types of reactions have been investigated in this work, which has been performed mainly with nonionic microemulsions. As expected, the reactions studied were much faster in a microemulsion compared to the same reactions performed in a corresponding two-phase system. Most reaction rates have been monitored by 1H-NMR, but in one study UV spectroscopy was employed as method.
It has been shown that the reactions proceed well in microemulsions based on nonionic surfactants; however, the reaction rate is very dependent on surfactant type. To be more specific, a large difference in reaction rate was found when performing a SN2 reaction in a microemulsion based on an alcohol ethoxylate compared with the same reaction in a very similar microemulsion based on a sugar surfactant. The high reactivity in microemulsions based on alcohol ethoxylates is believed to be due to a favourable microenvironment in the reaction zone. However, it was shown that in both microemulsion types the reaction rate was almost independent of the choice of counterion to the attacking nucleophile. This indicates that complexation of the cation with the surfactant headgroup seems not to be of importance. The reactions in the microemulsions based on alcohol ethoxylates ran surprisingly fast compared to reactions in protic solvents such as methanol. The reaction kinetics was also compared with those found in other self-assembly structures. The reactivity order was micelle > one-phase microemulsion = Winsor system > liquid crystalline phase. Other parameters that affected the reaction kinetics were interfacial area, local concentration of reactants within the subvolumes of the microemulsion, and temperature.
The combination of phase transfer catalysis and the microemulsion approach, i.e., performing a reaction in a microemulsion in presence of a phase transfer agent was shown to be a successful concept. The reaction in the microemulsion was accelerated when a phase transfer agent was added and was faster than the reaction in the two-phase system with phase transfer agent.