Kinetics and mechanism of amine-catalysed solvolysis of azlactone in water-dioxan mixtures

Abstract

The effect of replacing solvent water by dioxan up to 50% v/v on the rate of catalytic-amine solvolysis of azlactone has been measured spectrophotometrically in the temperature range 40-60ºC. Addition of an organic solvent to water largely decreases the rate of ring- opening process. Such an effect was found to diminish in the region of lower relative permittivity. The reaction rate constants are analyzed and discussed in terms of solvent effect and nucleophilicity of amines. ΔG≠ gradually decreases as a function of increasing [water], due to a complex and quasi-mirror image compensation of ∆H≠ and ΔS≠. The variation of ΔH≠ versus ΔS≠ is linear. The catalytic coefficient, kAm, for amine has been estimated from the general linear equation, kobs= k o + kAm [Amine]. The binary solvent mixtures used give an unusual range of properties. The plot of log kAm versus log [H₂O] show two linear segments with intersection at 33.3 mol dm⁻³[H₂O]. The correlation between log kAm and reciprocal of the relative permittivity is non-linear, while that between log kAm versus pKa is linear. All these observations are consistent with the suggested mechanism.