Isoxazoles. VII: Hydrolysis of 4-Methyl- 5-isoxazolylnaphthoquinone Derivatives in Aqueous Solutions

The kinetics for the degradation of 2-(4-methyl-5- isoxazolylamine)-A/-(4-methyl-5-isoxazolyl)-1,4-naphthoquinone-4- imine (1) in solution were investigated at 70 °C and at a constant ionic strength of 0.5 over a pH range of 1.75 to 12.85. The degradation rates were determined by absorption and second-derivative UV spectrometry. Two degradation products were identified in acidic and neutral pHs; they are 4-/V-(4-methyl-5-isoxazolyl)-1,2-naphthoquinone (2) and 2-methyl- cyanoacetamide (5), respectively. In alkaline pH, two degradation products, 2-hydroxy-A/-(4-methyl-5-isoxazolyl)-1,4-naphthoquinone-4- imine (3) and 5-amino-4-methylisoxazole (4), were isolated. The pathway for degradation of 1 in acidic and neutral pH followed consecutive first-order kinetics since 2 undergoes hydrolysis giving 2-hydroxy-1,4- naphthoquinone (6) ano 2-methylcyanoacetamide (5). No appreciable buffer effect on the degradation of 1 and 2 was observed for any of the buffer species in this study. The pH-rate profiles exhibited specific acid and specific basic catalysis for 1 and specific acio catalysis for 2. The maximum stability for 1 and 2 occurred in the neutral pH region.