Reductive Cleavage of Halides in Decomposition of Substituted Benzonitrile Pesticides

The reduction mechanism of chloroxynil (3,5-dichloro-4-hydroxybenzonitrile), bromoxynil (3,5-dibromo-4-hydroxybenzonitrile) and ioxynil (3,5-diiodo-4-hydroxybenzonitrile) was studied in dimethylsulfoxide by the electrochemical methods combined with spectroelectrochemical and GC/MS identification of the products. DC polarography shows two irreversible reduction waves for ioxynil and only one reduction wave for bromoxynil and chloroxynil. Cyclic voltammetry at different scan rates and at various concentrations of pesticides is consistent with the radical anion formation followed by the subsequent chemical reactions. The main decomposition pathway includes the cleavage of the halogen atom and the dimerization of the dehalogenated intermediate. The differences in the mechanism of ioxynil, bromoxynil and chloroxynil are caused by the mutual differences in the rates of the bond cleavage and the dimerization process.