Kinetics for synthesizing benzyl salicylate via solid–liquid phase-transfer catalysis

Abstract

The kinetics for synthesizing benzyl salicylate (C 6 H 4 (OH)COOCH 2 C 6 H 5 , denoted as Ph(OH)COOR) from the carboxylation of benzyl bromide (C 6 H 5 CH 2 Br, RBr) via solid-liquid phase-transfer catalysis (SLPTC) was investigated. The key component to conduct the substitution reaction is the catalytic intermediate generated from solid sodium salicylate (C 6 H 4 (OH)COONa, denoted as Ph(OH)COONa) reacted with tetraalkylammonium salt. The concentration of catalytic intermediate remained nearly constant after the induction period and increased with the increase of the amount of catalyst employed. The reaction between catalytic intermediate and organic reactant is identified as the controlling step. Factors affecting the formation of the catalytic intermediate include the relative amounts of catalyst, solid reactant, organic reactant, and extra addition of inorganic salts. Different phase-transfer catalysts were employed to compare their catalytic efficiency: tetra-n-butylammonium bromide (Bu 4 NBr, TBAB or QBr in general) > tetra-n-butylammonium iodide (TBAI) > tetra-n-butylphosphonium bromide (TBPB) > aliquat 336 > tetra-n-butylammonium hydrogen sulfate (TBAHS) > polyethylene glycol (PEG) 1500. The product yield above 98% was achieved within 1 h at 70 • C by using tetra-n-butylammonium bromide as the catalyst. A kinetic model was developed for the solid-liquid reaction. The overall reaction can be described by pseudo-first-order kinetics with the apparent activation energy in chlorobenzene to be 62.52 kJ mol −1 .

Anion exchange cycle of catalyst in liquid–liquid phase-transfer catalysis reaction: Novel autocatalysis

“…Similarly, the induction period was also found in synthesizing benzyl salicylate from the carboxylation of benzyl bromide via solid-liquid PTC system [27]. Nevertheless, in hydroxide ion catalysed C-Alkylation of phenylacetonitrile, the appearance of an induction period was attributed to the time required for the saturation of phenylacetonitrile on the liquidliquid interface [28]. For the phase-transfer-catalytic preparation of benzyl phenyl ether, dibenzyl ether, and benzyl n-butyl ether, a first-order model after an induction period could describe the kinetics of the overall reaction very well [29].…”