Heteroaromatic azo-activated substitutions. Part 4. Kinetics and mechanism of the hydrolysis of 3-(4-methoxyphenylazo)-5-methylisoxazole in aqueous sulphuric acid media

Abstract

The kinetics of the hydrolysis of the title compound, (3), have been investigated in moderately concentrated aqueous sulphuric acid media at 30 °C. Activation parameters have also been determined. Rate correlation by the Cox–Yates excess acidity method shows that hydrolysis occurs from the monoprotonated substrate by the A-S_E2 mechanism of the S_NAr type. Nucleophilic attack by water at the aryl carbon and a subsequent proton-transfer equilibrium are fast processes which precede the electrophilically catalysed separation of the leaving group, in which the functional catalysts are all general acids in solution. An abnormal value of 1.4 is obtained for the slope parameter m^‡(Kresge's α_A); this is discussed in terms of differential solvation of the initial and transition states. The Bunnett–Olsen slope parameter (φ_‡–φ_e) of –2.0 indicates that the transition state of the reaction is substantially less solvated than its initial state. The values of ΔH^‡ and ΔS^‡ vary to compensate each other and the decreasing ΔS‡ values accord with the ordered transition state proposed.