Study on catalytic property of NaOH-cationic surfactant solutions for efficient, green and selective synthesis of flavanone

Abstract

The catalytic activity of aqueous NaOH–quaternary ammonium surfactant (NaOH–QAS) solutions was studied for efficient and selective synthesis of flavanone by cross aldol reaction of benzaldehyde and o-hydroxy acetophenone (as model reaction) under ambient condition (at 30°C). The surfactant's nature (alkyl chain length and head group size) and concentration influence the reaction in NaOH–QAS micellar solutions. The NaOH–QAS systems, with surfactant concentration below critical micelle concentration (CMC), showed high activity giving high conversion and flavanone selectivity. This result was interpreted as substrate induced micellization and favorable interactions of surfactant molecules with reactants and intermediates. The reduction in conversion and selectivity of flavanone was observed with NaOH-QAS solutions having surfactant concentration at CMC or above CMC, which was due to strong solubilization of o-hydroxy acetophenone and intermediates in the micelles. The highest conversion (92% in 24h) and selectivity of flavanone (98%) were obtained with NaOH–cetylpyridinium chloride (NaOH–C16PQAS) system at surfactant concentration below CMC. The –OH group present in o-hydroxy acetophenone was observed to be responsible for its less reactivity in the aldol reaction. The reusability of NaOH-cationic surfactant solution was examined for three reaction cycles.