Medium Effects on the Initial and Transition States on the Amine-Catalysed Ring-Opening of Azlactone in Water–Acetonitrile Mixtures

The kinetics of ring opening of azlactone in water – acetonitrile mixtures in the presence of morpholine have been studied under pseudo-first order condition in the temperature range 40-60°C. Non-linear plots of log k_obs with the reciprocal of relative permittivity of the solvent were obtained while linear plots of log k_obs against the Y value were observed. The thermodynamic parameters ΔH^≠, ΔS^≠ and ΔG≠ have been determined; ΔG^≠ increases gradually as the mole fraction of the cosolvent increase, due to a complex quasi-mirror image compensation of ΔH^≠ and ΔS^≠. The negative values of the entropy of activation and the non-linear relation of log k_obs with the reciprocal of relative permittivity suggested selective solvation by the more highly polar water molecules. The isokinetic temperature obtained indicated that the reaction was enthalpy-controlled. The presence of an electron-releasing or electron-withdrawing para-substituent in the benzylidene moiety affected the reaction rate, i.e. it being accelerated by p-Br but retarded by p-OMe substituents. Also the rate was decreased by increasing the content of organic cosolvent. The reactivity was analysed in the light of various simple and multiple regression equations using Kamlet–Taft solvatochromic parameters which were applied successfully in mixed aqueous – acetonitrile mixtures.