Résumé:
The 3,5 -dihalogeno-4-hydroxybenzonitriles (ioxynil, bromoxynil and chloroxynil) have a
similar ohotochemical behaviour in pure aqueous solution irradiated in artificial UV light or
sunligb. The main pathway is a heterolytic photohyrolysis leading to the monohalogenated
dihydrOxybenzonitrile as observed with 3-chlorophenol, 3,4- and 3,5-dichlorophenols, but
differently from 2-halogeno phenols.
Photoitduction was also observed specially when irradiation is carried out in natural water or
in the presence of organic matter.
In a set ond stage similar reactions occur with the other C-halogen bond. The photochemical
behavirur is not significantly affected by the nature of the halogen Cl, Br or I.
Methal enzthiazuron was very slowly photolyzed when irradiated at A. > 290 nm, but could be
successfully oxidized into 6-hydroxymethabenzthiazuron by Aspergillus niger ATCC 9142,
as sho'vn by *H-15N HMBC NMR experiments. The toxicity of this metabolite, determined
by the standardized Microtox® test, was six times lower than that of the parent molecule. 6-
hydrox /methabenzthiazuron was not further metabolized by Aspergillus niger, but was found
to be photooxidized with ring cleavage of the aromatic ring and photodimerized upon
irradia ion at X > 290 nm. In the presence of humic substances, the photodegradation was
slower The transformations of methabenzthiazuron observed either with the fungus
Aspergillus niger or by the action of solar light, do not proceed via the urea chain Ndealky ation as usually reported, but via hydroxylation or cleavage of the benzene ring.
