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

Abstract

The kinetics of the chlorine dioxide–thiourea dioxide reaction was investigated by monitoring absorbance–time profiles at λ = 360 nm. Under acidic conditions, the primary carbon‐containing product is cyanamide, not urea as considered previously for many oxidation reactions of thiourea dioxide. Increase of the rate of the reaction by an increase of pH can be readily explained by the slow pH‐dependent formation of a more reactive form of thiourea dioxide (TDO) that is produced steadily and unavoidably as the stock TDO solution ages. We have also found that the absorbance–time profiles of the chlorine dioxide–TDO reaction are sigmoidal with excess TDO. The addition of methionine as a hypochlorous acid scavenging agent inhibits the reaction significantly, whereas the addition of chlorite ions and trace amounts of hydrogen sulfite ions accelerates the decay of chlorine dioxide. On the basis of these experiments, a sixteen‐step kinetic model involving hypochlorous acid, chlorite ions, and hydrogen sulfite ions as key intermediates that provide an autocatalytic cycle is proposed to account for the overall kinetic behavior observed, including the slow rearrangement of TDO.

Autocatalysis-Driven Clock Reaction III: Clarifying the Kinetics and Mechanism of the Thiourea Dioxide–Iodate Reaction in an Acidic Medium

“…We therefore concluded that it is important to use a TDO stock solution at the same age to establish a reliable kinetic model. Thus, except otherwise stated, all of the experimental runs were initiated having the age of the TDO solution to be 270 ± 4 s. At the same time, this series of kinetic runs appears to suggest that the aged form of TDO is significantly more reactive than the fresh one, which is quite consistent with what was observed in the case of other redox reactions of TDO. − …”

Kinetics of the chromium(III)/l‐glutamic acid complexation reaction: Formation, decay, and UV‐vis spectrum of a long‐lived intermediate

“…The absorbance‐time curves showed a sigmoidal profile (Figure ) and the rate‐time curves a bell‐shaped one (Figure ). Although these features are also compatible with autocatalysis, this kind of process is to be expected for reactions involving considerably reactive products or intermediates rather than for the ligand replacement reactions from the notably inert Cr(III) complexes. Thus, the long‐lived intermediate‐based explanation should be considered more probable in the present case.…”

On the Kinetics and Mechanism of the Thiourea Dioxide–Periodate Autocatalysis-Driven Iodine-Clock Reaction

“…Quite recently, in a number of subsequent papers, it has also been demonstrated experimentally that TDO, even under acidic aqueous conditions, ages, and several times, the aged form of TDO is more reactive toward the corresponding oxidizing agents ,, than its original form. The only exception was found to be the TDO–bromate reaction, where no effect of aging was found when the TDO solution was not older than a day.…”

“…It is demonstrated that the title system may be classified as an autocatalysis-driven clock reaction because the clock species iodine may as well appear, although just transiently, in substrate excess after a well-defined and reproducible time lag. It is also highlighted that the Landolt time depends on the age of the stock TDO solution in the case of the TDO–periodate reaction likewise in the TDO–iodine, TDO–chlorine dixoide, , and TDO–iodate reactions. As was shown, the kinetic model of the overall system was gradually built up from those of its subsystems as in the TDO–iodine and the TDO–iodate reactions.…”

“…Despite the widespread practical application in the paper and textile industry, the redox transformation of thiourea dioxide (TDO) in aqueous solution is still not fully understood due to its unusual structure and its complex decomposition or rearrangement reactions. , TDO is fairly stable under acidic conditions; however, it decomposes relatively rapidly into a sulfoxylate ion and urea via a carbon–sulfur bond cleavage depending on the pH under basic conditions. , Xu et al have recently demonstrated that TDO slowly rearranges into a more reactive form, whose reactions may be considerably faster than those of the original form of TDO with the corresponding oxidizing agent. Although it is still in dispute whether this rearrangement belongs to tautomerism , or an oligomer–monomer-, or even an aminoimino–carbenoid slow transformation, it is quite clear that this phenomenon has to be taken into consideration when studying the redox reactions of TDO. Very recently, Shao et al have arrived at a conclusion supported by TOF-MS spectroscopy and quantum chemical calculations that in aqueous solution TDO exists in cyclic clusters containing various numbers of molecules of TDO and in the form of aminoiminomethanesulfinic acid (NH 2 NHCSO 2 H) .…”