title: Mechanisms of the thermal decay of chlorpropham.
abstract: DFT calculations were performed on the thermal reactions of chlorpropham 1, a carbamate pesticide and plant growth regulator frequently used in the storage of potatoes. At the conditions normally used in applying 1 (injection of a methanolic solution of 1 into a hot air stream, T ≈ 500°C), both ester pyrolysis of 1 and a methanol-or water-catalysed isocyanate cleavage are expected to proceed rapidly (lifetime of 1 less than a second). In both reactions, the final reaction product will be toxic and carcinogenic m-chloroaniline 2. Matrix-isolation experiments indicate that 1 undergoes thermal decay at temperatures as low as 250°C. Up to temperatures of ca. 500°C, formation of m-chlorophenylisocyanate 4 and isopropanol was the predominant reaction observed, while formation of propene, CO(2), and m-chloroaniline 2 was the most important reaction channel at higher pyrolysis temperatures. m-Chlorophenyl carbamic acid 3 could not be observed. The results indicate that at lower temperatures, 1 decays exclusively via isocyanate cleavage of 1, provided that traces of catalytic water or other protic compounds are present. At higher temperatures, ester cleavage of 1 becomes competitive and outweighs the isocyanate cleavage by a factor of ca. 10:1.
authors.affiliation: WestCHEM School of Chemistry, University of Glasgow, Joseph-Black-Building, University Avenue, Glasgow G12 8QQ, United Kingdom. margaret.smith@glasgow.ac.uk
authors.forename: Margaret J
authors.initials: MJ
authors.lastname: Smith
authors.forename: Sandra
authors.initials: S
authors.lastname: Müller
authors.forename: Wolfram
authors.initials: W
authors.lastname: Sander
authors.forename: Götz
authors.initials: G
authors.lastname: Bucher
mesh_terms: D002748:Chlorpropham / Q000737:chemistry*; D006540:Herbicides / Q000737:chemistry*; D006358:Hot Temperature; D013058:Mass Spectrometry
chemical_list: D006540:Herbicides; D002748:Chlorpropham