The kinetics of the morpholine-catalysed solvolysis of some p-substituted (Z)-4-benzylidene-2-phenyloxazolin-5-one in water- dimethylsulfoxide mixtures (20-90% V/V) at different temperatures in the range (40-60°C) have been studied. There is an electronic effect in the predicted direction for para-substitution in the benzylidene moiety in different percentage compositions of binary aqueous solvent mixtures. An electron-withdrawing substituent in the benzylidene ring leads to rate enhancement while an electron-donating substituent retarded the rate. A good linear relationship was obtained from plots of log kobs against the σ-Hammett parameter with a positive slope (ρ) at the various experimental temperatures and solvent compositions. Plots of the reaction rate constant ρ against the dielectric constant and the reciprocal temperature gave straight lines. 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≠. A plot of ΔH≠versus ΔS≠ for the reaction in water–solvent mixtures gave good straight lines with isokinetic temperatures which are higher than the experimental temperature. The higher negative values of the entropy suggest selective solvation by the more polar water molecules.
