Kinetic Evidence of Tautomerism of Thiourea Dioxide in Aqueous Acidic Solutions

Abstract

Indirect experimental evidence of the existence of a sluggish tautomerism of thiourea dioxide in acidic aqueous solution was discovered while studying its oxidation by chlorine dioxide. Thiourea dioxide slowly rearranges into aminoiminomethanesulfinic acid, but the reactivity of these two species towards chlorine dioxide marginally differs. It was clearly demonstrated that thiourea dioxide reacts with chlorine dioxide in a relatively slow reaction in an autocatalytic fashion with respect to the carbon(IV) species. In contrast, reaction of aminoiminomethanesulfinic acid with chlorine dioxide is orders of magnitude faster but proceeds through a normal second‐order process.

Mechanism Involving Hydrogen Sulfite Ions, Chlorite Ions, and Hypochlorous Acid as Key Intermediates of the Autocatalytic Chlorine Dioxide–Thiourea Dioxide Reaction

“…Surprisingly, the 13 C NMR spectrum clearly shows that the carbon-containing end product is not urea or hydrogen carbonate but is cyanamide. As we did not find the peak of cyanamide in the 13 C NMR spectrum in our previous report, [19] this apparent contradiction deserves a conceivable explanation. The main difference during the measurement of the NMR spectrum was the acquisition time.…”

“…These results also imply that the majority of the initial amount of sulfur forms sulfate ions; therefore, in addition to TTO, other nitrogenand carbon-containing species must form. As many oxidation reactions of TDO with different oxidizing agents were supposed to form urea under acidic conditions [3,16,17] and as carbonate and ammonium ions were considered in our recent article, [19] we performed 13 C NMR spectroscopy measurements to identify the major carbon-and nitrogencontaining products. The 13 C NMR spectrum of the end products after the chlorine dioxide completely disappeared from the solution is shown in Figure 2.…”

“…In our previous study, [19] we observed for the first time that autocatalysis also occurs in the reaction between ClO 2 and TDO, though at that time we focused our interest on an explanation for the aging effect of TDO in acidic medium on the basis of the remarkable reactivity difference between the tautomeric forms of TDO without a direct insight into the mechanistic details of the given reaction. We also found that the addition of hydrogen carbonate ions decreases the length of the induction period of autocatalysis; therefore, the otherwise inert hydrogen carbonate ions may also have an important effect on the autocatalytic cycle.…”

“…[13] Some oxidation reactions of TDO have been controlled in acidic solutions. [14][15][16] The oxidations of TDO with chlorite ions, iodine, and bromine have been performed by Simoyi et al [15][16][17][18] Regrettably, the tautomerization of TDO, which seems to occur relatively slowly from (NH 2 ) 2 CSO 2 to NH 2 NHCSO 2 H [19,20] at low pH values, has so far not been taken into account in these reactions, though the two tautomers may have quite different reactivity towards chlorine dioxide and other chlorine-containing intermediates.…”

“…We also found that the addition of hydrogen carbonate ions decreases the length of the induction period of autocatalysis; therefore, the otherwise inert hydrogen carbonate ions may also have an important effect on the autocatalytic cycle. [19] On the basis of a general interest on the application of TDO, we report here a detailed, systematic study on the kinetics of the ClO 2 -TDO reaction to obtain better insights into the mechanism of the fascinating chloritethiourea reaction.…”