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 S_N1 and S_N2 mechanisms of benzyl bromide were determined and the S_N1 pathway was found to contribute to a negligible extent. The S_N2 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.