posted on 2002-04-04, 00:00authored byK. A. Lippa, A. L. Roberts
Reactions of bisulfide and polysulfides with chloroazines
(important constituents of agrochemicals and textile dyes)
were examined in aqueous solution at 25 °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 (SNAr) 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 porewaters, 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.