Novelties of kinetics and mechanism of liquid–liquid phase transfer catalysed reduction of p-nitroanisole to p-anisidine

Abstract

Anisidines, which are valuable intermediates in dyestuff industry, are synthesised from nitroanisoles. The reduction of nitroanisoles and other substituted nitroaromatics to the corresponding amines can be affected by aqueous inorganic sulphides and polysulphides and the rates of these biphasic reductions are amenable to intensification under phase transfer catalysis (PTC). The selectivity in the reduction of substituted nitroaromatics is greatly affected by the type and number of phases involved in the PTC process. The kinetics and mechanisms of complex L–L PTC processes are not properly understood. This paper delineates novelties of the kinetics and mechanism of L–L PTC reduction of the model compound p-nitroanisole to p-anisidine wherein the sulphide ions get transformed into bisulphide in the aqueous phase and are extracted as ion pairs in the organic phase. A series of reactions of the bisulphide occur to reduce the nitroaromatic to the corresponding amino compound and the Q⁺HSO₃⁻ ion pair is transferred across the interface to react with S²⁻ leading to the formation of the Q⁺SH⁻ ion pair and S₂O₃²⁻. The effects of different parameters are studied to establish this mechanism. The selectivity to the product is 100%.