Modeling of microwave irradiated liquid–liquid–liquid (MILLL) phase transfer catalyzed green synthesis of benzyl thiocyanate

Abstract

Intensification of multiphase reactions with 100% selectivity to the desired product is a cherished goal in fine chemical industry. In this regard, converting bi-liquid phase transfer catalysis (L–L PTC) into a tri-liquid PTC can dramatically intensify the reaction rates and give 100% selectivity. The novelty of liquid–liquid–liquid phase transfer catalysis (L–L–L PTC) is that the catalyst forms a third middle liquid phase, that can be repeatedly reused. Additionally the aqueous phase can also be reused. Microwave technology is a novel approach towards clean and green chemistry and is relatively a very convenient, safe and rapid method. In the current work, synergistic combination of low energy microwave irradiated liquid–liquid–liquid (MILLL) PTC reaction has been brought out in intensification of reaction rates and selectivity towards benzylthiocyanate in nucleophilic substitution reaction of benzyl chloride. Tetra- n-butylammonium bromide (TBAB), ethyl triphenyl phosponium bromide (ETPB) and Cyphose 163 were evaluated as phase transfer catalysts. ETPB was the BEST catalyst. Effects of various parameters were studied. A mechanism of the reaction is proposed and the kinetics is established and validated against experimental data. Waste minimization was achieved in two ways in this process, namely, the aqueous and the catalyst rich middle phases were reused several times which reduces the waste. There is 100% selectivity to benzyl thiocyanate due to the creation of the third phase. The creation of third phase allows minimization of waste with complete selectivity, thereby improving profitability and environmental benefits.