Solid–liquid phase transfer kinetics of p-nitrobenzoate esterification catalysed by crown ethers; a comparison of the catalytic efficiency of various crown ethers
Abstract
The kinetics of the esterification reaction between potassium p-nitrobenzoate and benzyl bromide catalysed by various crown ethers have been studied in a solid–chloroform phase-transfer system at 25 °C. The reaction was found to be first order in benzyl bromide, and the observed pseudo-first-order rate constants obtained at constant crown ether concentration were found to depend linearly on the concentration of the crown ether–salt complexes in the organic phase. Rate constants for both SN1 and SN2 mechanisms of benzyl bromide were determined and the SN1 pathway was found to contribute to a negligible extent. The SN2 rate constants in the presence of four crown ethers decreased in the order: oxydimethylenebisbenzo-15-crown-5 dicyclohexano-18-crown-6 > 18-crown-6 ∼ 4′-methylbenzo-18-crown-6. Differences in the catalytic efficiencies of the crown ethers were ascribed to their different effects on the interionic ion pair distance R-CO2–⋯ M+ on complexation.