Pressure Effects on the Hydrolysis of 3-Nitro-4-acyloxybenzoic Acids Catalyzed by Hexadecyltrimethylammonium Bromide Micelle

Abstract

Abstract The rates of the hydrolysis of 3-nitro-4-acetoxybenzoic acid (NABA), 3-nitro-4-propionyloxybenzoic acid (NPBA), 3-nitro-4-butyryloxybenzoic acid (NBBA), and 3-nitro-4-valeryloxybenzoic acid (NVBA), catalyzed by the hexadecyltrimethylammonium bromide (HTAB) micelle, were measured at pressures up to 2 kbar at 25 °C in Tris buffer solution (0.01 M, pH 8.7). The ester binding constant K increased and the rate constant of the hydrolysis catalyzed by micelle km decreased with longer alkyl chains of esters at 1 bar. The km values were separated into the contribution of kOH (hydroxide ion catalyzed hydrolysis) and kT (Tris catalyzed hydrolysis), which increased monotonically up to 2 kbar. On the other hand, the K values decreased and then inversed with minima at about 1.5 kbar except for that of NABA. From the pressure dependence of log K, the volume changes, ΔV, accompanying the ester binding to the HTAB micelle at 1 bar were estimated to be in the range of 11±2 cm3/mol for NABA to 23±2 cm3/mol for NVBA. The activation volumes for the hydrolysis catalyzed by micelle, were measured as −6±2 cm3/mol for ΔV*OH and −16±2 cm3/mol for ΔV*T for each ester. The activation volumes for the spontaneous hydrolysis were measured as −5±2 cm3/mol for ΔV*OH and −16±2 cm3/mol for ΔV*T for each ester. The reaction mechanism was discussed in terms of ΔV and ΔV*.