Photochemistry of halogenated benzene derivatives. X. Effects of sodium chloride on the aquatic photodegradation of bromoxynil (3,5-Dibromo-4-hydroxybenzonitrile) herbicide

Abstract

Aqueous solution phase photochemistry of the herbicide bromoxynil (3,5-dibromo-4-hydroxybenzonitrile) (1) in the presence of various concentrations of sodium chloride was extensively investigated with ultraviolet radiation near 313 nm. In the presence of 0.5 to 25.0x10⁻³ M NaCl, the quantam yields for the phototransformation of the herbicide bromoxynil (1) amounted to 0.045±0.005 to 0.017±0.007 vs. 0.052±0.004 in the absence of sodium chloride. These quantum yield data for the photolysis of1 followed the Stern-Volmer equation. The photolysis of the 7.8x10⁻⁶ M aqueous solution of the herbicide1 in the presence of sodium chloride (10.0x10⁻³ M) gave rise to the formation of 3-bromo-4-hydroxybenzonitrile (2), 3-bromo-5-chloro-4-hydroxybenzonitrile (3), 3-chloro-4-hydroxybenzonitrile (4), and 4-hydroxybenzonitrile (4A). The products3 and4 were formed via the photoincorporation of chloride ions into bromoxynil (1) and into the primary photoproduct2, respectively. In the case of this photoreaction of the chemical1, the percentages of maximum concentrations of the photoproducts3, 2, 4, and4A were obtained after 10.5, 20.0, 30.0, and 44.0-min exposures to UV light, respectively, the percentage disappearance of the starting material1 being 90% for 44-min photolysis. Photoproducts 2,3, 4, and4A were identified with the aid of GC-MS. The formation of 4-hydroxybenzonitrile (4A) decreased with the rise in concentration of NaCl. When a mixture of aqueous solutions of 3.0 mL bromoxynil (2.0x10⁻⁴M) plus 0.5 mL NaCl (0.5 M) was exposed to UV light for up to 3 hr, the photoproduct4A could not be observed; however, other products, namely, phenols2–4 were produced.