Kinetics and mechanism of cymoxanil degradation in buffer solutions

The kinetics and mechanism(s) of the hydrolytic degradation of a compound are needed to evaluate a compound’s abiotic degradation in the environment. In this paper, the hydrolysis of cymoxanil [2-cyano-N-[(ethylamino)carbonyl]-2-(methoxyimino) acetamide] was investigated in dark sterile aqueous solutions under a variety of pH conditions (pH 2.8-9.2) and temperatures (15-50 °C). Hydrolysis of cymoxanil was described by first-order kinetics, which was dependent on pH and temperature. Cymoxanil degraded rapidly at pH 9 (half-life ) 31 min) and relatively slowly at pH 2.8 (half-life ) 722 days). The effect of temperature on the rate of cymoxanil degradation was characterized using the Arrhenius equation with an estimated energy of activation of 117.1 kJ mol-1. An increase in temperature of 10 °C resulted in a decrease in half-life by a factor of 5. Three competing degradation pathways are proposed for the hydrolysis of cymoxanil, with two of the pathways accounting for 90% of cymoxanil degradation. These two pathways involved either initial cyclization to 1-ethyldihydro-6-imino-2,3,5(3H)-pyrimidinetrione-5-(O-methyloxime) (1, Figure 1) or direct cleavage of the C-1 amide bond to form cyano(methoxyimino) acetic acid (7). The third pathway of degradation involved initial cyclization to 3-ethyl-4-(methoxyimino)-2,5-dioxo-4-imidazolidinecarbonitrile (8), which rapidly degrades into 1-ethyl-5-(methoxyimino)-2,4-imidazoline-2,4-dione (9). All three pathways eventually lead to the formation of the polar metabolite oxalic acid.