Conversion Reactions of Various Phenoxyalkanoic Acid Herbicides in Soil. 2. Elucidation of the Enantiomerization Process of Chiral Phenoxy Acids from Incubation in a D₂O/Soil System

Abstract

The enantiomerization of chiral phenoxyalkanoic acid herbicides in soil under laboratory conditions in the presence of D 2 O was investigated. The compounds studied were 2-(4-chloro-2-methylphenoxy)propionic acid (MCPP), 2-(2,4-dichlorophenoxy)propionic acid (DCPP) and, for comparison, the achiral 4-chloro-2-methylphenoxyacetic acid (MCPA) and 2,4-dichlorophenoxyacetic acid (2,4-D). Enantiomerization was studied by incubating racemic and enantiopure MCPP and DCPP in D 2 O/soil and following the formation of deuterated analogs using enantioselective high-resolution gas chromatography (HRGC) and mass spectrometry/mass spectrometry (MS/MS). MS/MS showed less interference from natural 13 C isotopomers than conventional MS and allowed the exact localization of deuterium in the labeled compounds. The results indicated exclusive H-D exchange at C-2 (α-methin H) of MCPP and DCPP with formation of the 2-deuterio analogs. H-D exchange in MCPP proceeded with retention as well as inversion of config uration, forming both of the labeled enantiomers from each of the native ones at comparable rates. H-D exchange was also observed for MCPA and 2,4-D with formation of the 2-deuterio and 2,2-dideuterio analogs. The importance of racemization reactions of chiral compounds is discussed, and general conclusions are drawn on the environmental fate of chiral compounds and on consequences for the monitoring of such compounds in the environment using enantioselective techniques.