title: Hydrolysis of mefenpyrdiethyl: an analytical and DFT investigation.
abstract: The hydrolysis of the herbicide safener mefenpyrdiethyl (1-(2, 4-dichlorophenyl)-4, 5-dihydro-5-methyl-1H-pyrazole-3,5-dicarboxylic acid diethyl ester) was investigated in aqueous solutions in the pH range from 2 to 9 and the temperature range from 298 to 323 K. The kinetics of hydrolysis were pseudo first order and were found to be strongly pH and temperature dependent. While near-constant in acidic medium, the hydrolysis rates strongly increased in alkaline pH, and total hydrolysis was observed at pH 11. Two main hydrolysis products, mefenpyrethyl (monoester) and mefenpyr (dicarboxylic acid) were isolated by ultrahigh-pressure liquid chromatography (UHPLC) and characterized using high-resolution Fourier transform ion cyclotron resonance mass spectroscopy (ICR-FT/MS) as well as (1)H, (13)C and 2D NMR analyses. Additionally, a density functional theory (DFT) investigation explained the stability of the pesticide at pH 7 and the high reactivity of the pesticide in alkaline medium. The key nucleophilic reaction partner is hydroxyl ions instead of neutral water molecules. Furthermore, the calculated activation barrier for hydrolysis in alkaline medium is in agreement with the extrapolated and experimentally determined activation barrier at pH 14.
authors.affiliation: Department of BioGeoChemistry and Analytics, Helmholtz Center München, German Research Center for Environmental Health, Institute of Ecological Chemistry, Ingoldstaedter Landstraße 1, 85764 Neuherberg, Germany.
authors.forename: Amina
authors.initials: A
authors.lastname: Chnirheb
authors.forename: Mourad
authors.initials: M
authors.lastname: Harir
authors.forename: Basem
authors.initials: B
authors.lastname: Kanawati
authors.forename: Agnes
authors.initials: A
authors.lastname: Fekete
authors.forename: Mohammed
authors.initials: M
authors.lastname: El Azzouzi
authors.forename: Norbert
authors.initials: N
authors.lastname: Hertkorn
authors.forename: Philippe
authors.initials: P
authors.lastname: Schmitt-Kopplin
mesh_terms: D006540:Herbicides / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D006868:Hydrolysis; D010573:Pesticide Residues / Q000737:chemistry; D011720:Pyrazoles / Q000737:chemistry / Q000378:metabolism*; D011789:Quantum Theory
chemical_list: C555731:1-(2, 4-dichlorophenyl)-4, 5-dihydro-5-methyl-1H-pyrazole-3,5-dicarboxylic acid diethyl ester; D006540:Herbicides; D010573:Pesticide Residues; D011720:Pyrazoles