Unraveling Acid–Base Contributions in P-Nitrophenyl Acetate Hydrolysis: Kinetics, Mechanism, And Activation Parameters in Aqueous Media

Hydrolysis of p-nitrophenyl acetate (PNPA) serves as a classical model for studying acid–base catalysis in aqueous solution. Despite its widespread use in enzymology and mechanistic chemistry, comprehensive datasets unifying acid- and base-catalyzed pathways remain limited. In this study, PNPA hydrolysis was monitored spectrophotometrically across pH 2–11 and temperatures 20–45 °C. Pseudo-first-order rate constants (kobs) were determined from exponential fits of absorbance–time traces at 400 nm. A characteristic U-shaped pH–rate profile was obtained, confirming contributions from both acid- and base-catalyzed pathways. Arrhenius and Eyring analyses yielded an activation energy (E?) of 54.7 kJ·mol?¹, enthalpy of activation (?H) of 52.2 kJ·mol?¹, and entropy of activation (?S) of ?47.8 J·mol?¹·K?¹, consistent with a bimolecular, ordered transition state. The results bridge existing gaps by integrating acid- and base-mediated hydrolysis within a single mechanistic framework, providing a reference dataset for both physical organic chemistry and enzymatic catalysis.