Oxidations at Sulfur Centers by Aqueous Hypochlorous Acid and Hypochlorite: Cl+ Versus O Atom Transfer

Sulfur-containing compounds are known to be susceptible to oxidation by aqueous HOCl, but the factors affecting the rates of these reactions are not well-established. Here we report on the kinetics of oxidation of thiosulfate, thiourea, thioglycolate, (methylthio)acetate, tetrathionate, dithiodiglycolate, and dithiodipropionate at 25 °C and 0.4 M ionic strength. These reactions obey the general rate law -d[OCl^-]/dt = (k_OCl^-[OCl^-] + k_HOCl[HOCl])[substrate] with some exceptions: tetrathionate and the two disulfides undergo rate-limiting hydrolysis at high pH, and dithiodiglycolate has an additional term in the rate law that is second order in [substrate]. The reactions of HOCl are believed to have a Cl⁺ transfer mechanism, and in the case of thiosulfate the rate of hydrolysis of the ClS₂O₃^- intermediate was determined. In the case of thiourea evidence was obtained for thiourea monoxide as a long-lived product. It is shown that sulfite and species with terminal sulfur atoms have k_HOCl values in the vicinity of 1 × 10⁹ M^-1 s^-1, while SCN^- and thioethers react somewhat more slowly; tetrathionate, trithionate, and disulfides react much more slowly. Comparison of the rate constants with those for oxidation of these sulfur substrates by H₂O₂ and [Pt(CN)₄Cl₂]^2- shows that HOCl reacts a few orders of magnitude more rapidly than [Pt(CN)₄Cl₂]^2- and ∼9 orders of magnitude more rapidly than H₂O₂. Many of the k_HOCl values are leveled by the high electrophilicity of HOCl. It is proposed that the k_OCl^- values correspond to oxygen-atom transfer mechanisms, as supported by LFERS (linear free energy relationships) relating these rate constants to those for reactions of H₂O₂ and [Pt(CN)₄Cl₂]^2-.