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.
The hydrolysis kinetics of the dicarboximide fungicides procymidone, vinclozolin and chlozolinate in neutral and alkaline solutions of pH 60 to 13·7 at 25°C have been determined conjointly by ultraviolet spectrophotometry and by high performance liquid chromatography. Under alkaline conditions, the fungicides undergo attack by the hydroxide ion on a specific carbonyl group and the rate of hydrolysis increases proportionally to the hydroxide ion concentration. Procymidone gives quantitatively and irreversibly 2‐(3,5‐dichlorophenylcarba‐moyl)‐l,2‐dimethylcyclopropanecarboxylate. The reaction is not subject to general base catalysis and experimental data are in agreement with a rate‐determining attack by the hydroxide ion. After a rapid hydrolytic loss of the ethoxycarbonyl substituent from chlozolinate, the dicarboximide ring cleavage of the two other fungicides leads, by mechanisms which differ with respect to the type of base catalysis and the rate‐determining step, to the corresponding anilides, producing as intermediates the carbamic acids, which undergo loss of carbon dioxide. The hydrolysis of vinclozolin and chlozolinate yields, respectively, N‐(3,5‐dichloro‐phenyt)‐2‐hydroxy‐2‐methylbut‐3‐enanilide and N‐(3,5‐dichlorophenyl)‐2‐hydroxy‐propanilide.
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