S-Oxygenation of thiobencarb in tap water processed by chlorination

Thiobencarb, an herbicide, was incubated in tap water containing 0.7 mg/L residual chlorine at 30 degrees C, and the solution was analyzed by HPLC. Thiobencarb could not be detected after 3 h, and only a byproduct was detected. The increase in concentration of the byproduct; correlated well with the decrease in that of thiobencarb. In a high free chlorine medium (more than 10 mg/L), the byproduct degraded to p-chlorobenzyl alcohol, p-chlorobenzaldehyde, and p-chlorobenzyl chloride, but not in a low free chlorine medium (0.1-10 mg/L). The byproduct was identified to be thiobencarb sulfoxide by LC/MS, infrared, and NMR spectra, and a reduction technique using 2-mercaptoethanol. Chiral HPLC analysis made it clear that the sulfoxide was a racemic compound. The sulfoxide was mutagenic regardless of the presence or absence of S9mix in the Ames Salmonella typhimurium TA100 assay. It was suggested that the management and control of thiobencarb in tap water processed by chlorination should include monitoring of thiobencarb sulfoxide, using HPLC.