Study on hydrolysis of novel herbicide O,O-dimethyl-1-(2,4-dichlorophenoxy-acetoxy) ethyl phosphonate in buffered solutions

HW-02,O,O dimethyl-1-(2,4-dichlorophenoxy-acetoxy) ethyl phosphonate,is a novel herbicide discovered and developed by Huazhong Normal University,China.The hydrolysis kinetics and mechanisms of HW-02 were studied in buffered aqueous solutions under pH 5-9 at ambient temperature and under pH 7 with the stored temperature of 25℃-55℃.The results showed that HW-02 hydrolyzed very rapidly in the buffered solutions.Its hydrolysis rate was characterized by the first-order kinetics and accelerated with the increasing of pH and temperature.The calculated half lives of HW-02 hydrolysis in the solutions of pH 5,7 and 9 were 191.3 h,6.2 h and 0.2 h at 25℃,respectively.It is indicated that HW-02 was not stable in the aqueous environments.HW-02 hydrolyzed much faster in the basic solution than in acidic and neural solution at the same temperature.The hydrolysis rate of HW-02 increased as the temperature increases and its half lives in the buffered solution of pH 7 were 6.1 h,4.2 h,1.8 h and 0.7 h at 25℃,35℃,45℃ and 55℃,respectively.The temperature coefficient of hydrolysis for HW-02 was 1.46-2.64.There were no obvious correlation between the activation enthalpy of HW-02 hydrolysis and temperatures.Its average activation energy and activation enthalpy were 60.51 kJ/mol and 57.70 kJ/mol,respectively.The activation entropy absolute values of HW-02 hydrolysis increased with the increasing temperatures in the environment.The average value of-81.00 kJ/(mol·K) suggested that hydrolysis of HW-02 in the aqueous solutions was driven by the activation entropy.Through isolation and identification of HW-02 hydrolytic products using GC-MS detection,it was found that HW-02 was hydrolyzed through ester cleavage.The main degradation product was 2,4-dicholorphenoxyaceate.