Kinetics for synthesizing benzyl salicylate by third-liquid phase-transfer catalysis

The kinetics of esterification of sodium salicylate with benzyl bromide to produce benzyl salicylate was investigated via third-liquid phase-transfer catalysis. The formation of the third-liquid phase from the interaction of aqueous reactant, inorganic salts, organic solvent, and catalyst was investigated to find the characteristics of catalytic intermediate in the tri-liquid system. The minimum quantity of tetra-n-butylphosphonium bromide (TBPB) required to form the third-liquid phase was affected by the addition of NaBr, but the variation was insignificant with greater amount of TBPB. The volume of third-liquid increased with increasing amount of TBPB, and using 20–70 cm3 of water for 30 cm3 of n-heptane solvent favors the formation of the third-liquid phase. The interfacial tension between the aqueous and the third-liquid phases was 1–3 mN/m, much less than 9–12 mN/m for the third-liquid/heptane interface, indicating that the mass transfer resistance dominates in the heptane side. The product can be present in both organic and third-liquid phases, and above 85% of the product yield in the organic phase can be obtained using 0.006 mol of TBPB to form the third-liquid phase. The distribution of the catalytic intermediate between the aqueous and the third-liquid phases was measured, and above 86% of catalyst in the third-liquid phase is in the form of catalytic intermediate. The reaction mechanism and kinetic model were proposed and the pseudo-first-order kinetics was successfully applied, the apparent activation energy in heptane being 73.94 kJ/mol when TBPB was used as the catalyst.

Graphical abstractThird-liquid phase-transfer catalyzed esterification of sodium salicylate to synthesize benzyl salicylate:Figure optionsDownload full-size imageDownload as PowerPoint slideAqueous reactant: sodium salicylate (ArCOONa)Organic reactant: benzyl bromide (RBr)Phase-transfer catalyst: tetra-n-butylphosphonium bromide, etc. (QBr)Product: benzyl salicylate (ArCOOR)Reaction mechanism:Figure optionsDownload full-size imageDownload as PowerPoint slide