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.

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The alkaline hydrolysis of propham, chlorpropham, and swep to carbanilic acids was investigated for sodium hydroxide concentrations ranging from 0.01 to 4.0 N. The dependence of the observed pseudo-first-order rate constants on the hydroxide ion concentration was in agreement with the rate laws of the two reaction mechanisms-ElcB and Bh2-that can be involved in the hydrolysis of carbamates. The values of the ionization constants of propham (pKa = 15.0), chlorpropham (pKa = 14.45), and swep (pKa = 13.9) account for their behavior in aqueous media. The Hammett p value of 0.98 obtained for the hydrolysis of a series of isopropyl carbanilates, the activation entropy of swep and the changeover in reaction mechanism, from ElcB to BA,2, observed in the hydrolysis of alkyl and aryl 3,4-dichlorocarbanilates are decisive arguments supporting the involvement of a B&2 reaction scheme in the hydrolysis of propham, chlorpropham, and swep. The formation of phenyl isocyanate during the degradation of these herbicidal carbanilates can therefore be ruled out.