Phase-transfer catalyzed benzoylation of 4-chloro-3-methylphenol sodium salt in liquid-liquid system

Abstract

The reaction of sodium 4-chloro-3-methylphenoxide (ArONa) and benzoyl chloride (ROCl) to synthesize 4-chloro-3-methylphenyl benzoate (ArOOR) was investigated by liquid-liquid phase-transfer catalysis in the present study. The catalytic intermediate was synthesized and its behaviors during the reaction were explored. The product yield was above 97.2% within 0.5 h of reaction at 10 degrees C and 100 rpm using tetrabutylammonium bromide as the catalyst. The benzoylation reaction was governed by the interfacial reaction and the intrinsic reaction in the bulk phase. During the reaction, a fraction (0.55 - 0.65) of the catalyst was observed in the form of catalytic intermediate, tetrabutylammonium 4-chloro-3-methylphenoxide (ArOQ), and its concentration would be near constant after 6 min of induction period. A reaction mechanism and pseudo-first-order kinetics were proposed to describe this phase-transfer catalyzed benzoylation successfully. Extra additions of NaCl and NaBr in the aqueous phase would increase the concentration of ArOQ in the organic phase to enhance the overall reaction, while a poison effect was observed for extra addition of NaI. A more hydrophilic catalyst, tetrabutylammonium hydrogen sulfate, was also favorable to conduct this type of interfacial intrinsic reaction with the apparent activation energy 7.13 kcal/mol.