AbstractThe 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 andN‐(3,5‐
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