Kinetics of the reaction of the hydrophobic ester 4-nitrophenyl decanoate with long-chain N-alkylimidazoles above and below their critical micelle concentration

Abstract

The kinetics of the reaction of 4-nitrophenyl decanoate with a series of straight-chain N-alkylimidazoles have been studied at 25� in water and aqueous ethanol. In water, below the critical micelle concentration of the N-alkylimidazole, hydrophobic interaction between the hydrocarbon chains of the reactants caused substantial increases in the reaction rate (up to about 200-fold) compared with the rate of reaction of 4-nitrophenyl acetate with N-methylimidazole. The rate constants, though, differed from those previously reported which were measured with a higher initial concentration of ester. No increase in rate with increasing hydrocarbon chain length could be detected in the presence of a large concentration of ethanol (mole fraction of 0.31) but a rate increase did occur in the presence of a more moderate concentration of ethanol (mole fraction of 0.10), confirming that hydrophobic interactions persist in this mixed solvent. The long-chain ester reacts rapidly with N-alkylimidazole micelles. Association constants (K) for binding the ester to the micelles and rate constants for the reaction of the bound ester (km) were estimated by following the conventional treatment of the kinetics of micelle-catalysed reactions. The value of K was found to increase sharply with increasing hydrocarbon chain length of the micelle but km showed the opposite trend.