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/

The enantiomerization of chiral phenoxyalkanoic acid herbicides in soi l under laboratory conditions in the presence of D2O was investigated. The compounds studied were 2-(4-chloro-2-methylphenoxy)propionic acid (MCPP), 2-(2,4-dichlorophenoxy)propionic acid (DCPP) and, for compari son, the achiral 4-chloro-2-methylphenoxyacetic acid (MCPA) and 2,4-di chlorophenoxyacetic acid (2,4-D). Enantiomerization was studied by inc ubating racemic and enantiopure MCPP and DCPP in D2O/soil and followin g the formation of deuterated analogs using enantioselective high-reso lution gas chromatography (HRGC) and mass spectrometry/mass spectromet ry (MS/MS). MS/MS showed less interference from natural C-13 isotopome rs than conventional MS and allowed the exact localization of deuteriu m in the labeled compounds. The results indicated exclusive H-D exchan ge at C-2 (alpha-methin H) of MCPP and DCPP with formation of the 2-de uterio analogs. H-D exchange in MCPP proceeded with retention as well as inversion of configuration, forming both of the labeled enantiomers from each of the native ones at comparable rates. H-D exchange was al so observed for MCPA and 2,4-D with formation of the 2-deuterio and 2, 2-dideuterio analogs. The importance of racemization reactions of chir al compounds is discussed, and general conclusions are drawn on the en vironmental fate of chiral compounds and on consequences for the monit oring of such compounds in the environment using enantioselective tech niques.