Nucleophilic aromatic substitution reactions of chloroazines with bisulfide (HS-) and polysulfides (Sn2-).
Reactions of bisulfide and polysulfides with chloroazines (important constituents of agrochemicals and textile dyes) were examined in aqueous solution at 25 degrees C. For atrazine, rates are first-order in polysulfide concentration, and polysulfide dianions are the principal reactive nucleophiles; no measurable reaction occurs with HS-. Second-order rate constants for reactions of an array of chloroazines with polysulfides are several orders of magnitude greater than for reactions with HS-. Transformation products indicate the substitution of halogen(s) by sulfur. Ring aza nitrogens substantially enhance reactivity through a combination of inductive and mesomeric effects, and electron-withdrawing or electron-donating substituents markedly enhance or diminish reactivity, respectively. The overall second-order nature of the reaction, the products observed, and reactivity trends are all consistent with a nucleophilic aromatic substitution (S(N)Ar) mechanism. Rate constants for reactions with HS- and Sn2- (n = 2-5) correlate only weakly with lowest unoccupied molecular orbital energies, suggesting that the electrophilicity of a chloroazine is not the sole determinant of its reactivity. When second-order rate constants are extrapolated to HS- and Sn2- concentrations reported in salt marsh pore waters, half-lives of minutes to years are obtained. Polysulfides in particular could play an important role in effecting abiotic transformations of chloroazines in hypoxic marine waters.
Lippa
K A
KA
Department of Geography and Environmental Engineering, The Johns Hopkins University, Baltimore, Maryland 21218-2686, USA.
Roberts
A L
AL
D001391:Azo Compounds / Q000737:chemistry*; D017606:Chlorine Compounds / Q000737:chemistry; D013440:Sulfides / Q000737:chemistry*; D014227:Triazines / Q000737:chemistry*; D014874:Water Pollutants, Chemical
D001391:Azo Compounds; D017606:Chlorine Compounds; D013440:Sulfides; D014227:Triazines; D014874:Water Pollutants, Chemical