Kinetics and hydrolysis mechanism of chlorsulfuron and metsulfuron‐methyl

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Abstract
In acidic media, hydrolysis of chlorsulfuron and metsulfuron‐methyl occurs via two consecutive reactions which were followed by ultraviolet spectrophotometry. For these two reactions, the pseudo‐first‐order rate constants increase proportionally to the concentration of hydronium ion in the more acidic media and to the square of this concentration at higher pH values.A kinetic study by HPLC shows that the first reaction leads to the formation of 4‐methoxy‐6‐methyl‐1,3,5‐triazin‐2‐amine and (o‐chlorophenylsulfonyl) carbamic acid for chlorsulfuron or (o‐methoxycarbonylphenylsulfonyl) carbamic acid for metsulfuronmethyl. The second reaction is the conversion of these sulfonylcarbamic acids to sulfonamides and carbon dioxide. The complete lack of saccharin and of o‐sulfamoyl benzoic acid proves that the ester function of the methoxycarbonyl group is stable.The lack of general acid‐base catalysis and a solvent deuterium isotope effect less than unity are consistent with a rate‐determining cleavage of the protonated substrate.In the basic pH range (10–14) a single reaction occurs, the nucleophilic substitution of the methoxy group on the triazine by a hydroxide group.