Eyring Activation Energy Analysis of Acetic Anhydride Hydrolysis in Acetonitrile Cosolvent System

Acetic anhydride hydrolysis in water is considered a standard reaction for investigating activation energy parameters using cosolvents. Hydrolysis in water/acetonitrile cosolvent is monitored by measuring pH vs. time at temperatures from 15.0 to 40.0 °C and mole fraction of water from 1 to 0.750. This work utilizes a temperature controlled water bath and a Vernier glass-body pH probe with Vernier Logger Pro 3.10.1 software for automated data collection. Data analysis is used to determine observed kinetic rate constants under the assumption that hydrolysis is a pseudo-first-order reaction. Eyring plots were used to compare activation energy parameters under iso-mole fraction conditions. The hydrolysis reaction of acetic anhydride was enthalpically stabilized and entropically destabilized at mole fractions of acetonitrile greater than 0.17 and the reverse occurred at mole fractions less than 0.17. Activation enthalpy and entropy result in the reaction being less favorable to form products as mole fraction of acetonitrile increased.