Toward Understanding the Mechanism of Phase Transfer Catalysis with Surface Second Harmonic Generation

Abstract

Phase transfer catalysis (PTC) is nowadays an important technique of organic synthesis. There are numerous indications that its key processes take place at the boundary between immiscible phases; however, this has not been proved by direct observations of interfaces separating the phases. Here, we demonstrate how surface second harmonic generation (SSHG) can be used for direct observation and identification of species adsorbed at the interface during a PTC reaction. The model system consisted of an organic (chlorobenzene) and aqueous (saturated solution of NaOH) phases with pH indicator (thymolphthalein, ThPh) used as a reactant. SSHG measurements allowed monitoring the surface concentration of ThPh anions at the interface and recording the kinetics of adsorption and deprotonation of ThPh. The results confirmed that deprotonation indeed occurs in the interfacial region. The proposed technique is relatively simple and general and can be used in a variety of PTC systems; therefore, its application will unve...