S-Oxygenation of Thiocarbamides IV: Kinetics of Oxidation of Tetramethylthiourea by Aqueous Bromine and Acidic Bromate

Compatible Kinetic Model for Quantitative Description of Dual-Clock Behavior of the Complex Thiourea-Iodate Reaction

The thiourea-iodate reaction has been investigated simultaneously by ultraviolet-visible spectroscopy and high-performance liquid chromatography (HPLC). Absorbance-time traces measured at the isosbestic point of the iodine-triiodide system have revealed a special dual-clock behavior. During the first kinetic stage of the title reaction, iodine suddenly appears only after a well-defined time lag when thiourea is totally consumed due to the rapid thiourea-iodine system giving rise to a substrate-depletive clock reaction. After this delay, iodine in the system starts to build up suddenly to a certain level, where the system remains for quite a while. During this period, hydrolysis of formamidine disulfide as well as the formamidine disulfide-iodine system along with the Dushman reaction and subsequent reactions of the intermediates governs the parallel formation and disappearance of iodine, resulting in a fairly constant absorbance. The kinetic phase mentioned above is then followed by a more slowly increasing sigmoidally shaped profile that is characteristic of autocatalysis-driven clock reactions. HPLC studies have clearly shown that the thiourea dioxide-iodate system is responsible mainly for the latter characteristics. Of course, depending on the initial concentration ratio of the reactants, the absorbance-time curve may level off or reach a maximum followed by a declining phase. With an excess of thiourea, iodine may completely disappear from the solution as a result of the thiourea dioxide-iodine reaction. In the opposite case, with an excess of iodate, the final absorbance reaches a finite value, and at the same time, iodide ion will disappear completely from the solution due to the well-known Dushman (iodide-iodate) reaction. In addition, we have also shown that in the case of the formamidine disulfide-iodine reaction, unexpectedly the triiodide ion is more reactive toward formamidine disulfide than iodine. This feature can readily be interpreted by the enhancement of the rate of formation of the transition complex containing oppositely charged reactants. A 25-step kinetic model is proposed with just 10 fitted parameters to fit the 68 kinetic traces measured in the thiourea-iodate system and the second, but slower, kinetic phase of the thiourea-iodine reaction. The comprehensive kinetic model is constituted in such a way as to remain coherent in quantitatively describing all of the most important characteristics of the formamidine disulfide-iodine, thiourea dioxide-iodine, and thiourea dioxide-iodate systems.

Fast and efficient removal of Cr(VI) to ppb level together with Cr(III) sequestration in water using layered double hydroxide interclated with diethyldithiocarbamate

It is still a great challenge to find an eco-friendly, easy-to-synthesize, and cheap adsorbent to rapidly remove Cr(VI) to ppb level in the Cr(VI)-polluted water. Herein, a new layered double hydroxide nanocage intercalated with diethyldithiocarbamate (DDTC-LDH) was fabricated via a facile calcination-rehydration method. The DDTC-LDH rapidly decreased Cr(VI) concentration from 5 to <0.05 mg/L within 35 min, and only a few seconds were required to completely remove it at an initial concentration of 0.5-1 mg/L, primarily attributed to the effective adsorption-reduction of Cr(VI) to Cr(III) by sulfur atoms in CS and CS groups. Attractively, the generated Cr(III) was also quickly removed to below 0.1 mg/L via an opportune Lewis hard-hard interaction with C-SO_x groups produced through CS oxidation. Additionally, Cr(VI) could be removed by DDTC-LDH at a wide pH application range (3.17-10.78) and with weak effects by coexisting anions (Cl^-, NO₃^-, CO₃^2-, SO₄^2-, and PO₄^3-). We systematically analyzed and proposed the mechanisms for Cr(VI) removal by the DDTC-LDH, orderly containing electrostatic attraction, Cr(VI) complexation by sulfur atoms in CS and CS groups, reduction of the Cr(VI) to Cr(III) by the CS and CS groups, and Cr(III) complexation by sulfur atoms in C-SOx groups. Our results provide new insights into the Cr(VI) removal using organosulfur compounds, that is to say, the organosulfur group Lewis hardness increased (from C-S to C-SOx) as the Cr species Lewis hardness increased (from Cr(VI) to Cr(III)), so as to opportunely ensure fast and efficient capture of both Cr(VI) and Cr(III) via Lewis acid-base interactions.

Reactivity of Small Oxoacids of Sulfur

Oxidation of sulfide to sulfate is known to consist of several steps. Key intermediates in this process are the so-called small oxoacids of sulfur (SOS)—sulfenic HSOH (hydrogen thioperoxide, oxadisulfane, or sulfur hydride hydroxide) and sulfoxylic S(OH)₂ acids. Sulfur monoxide can be considered as a dehydrated form of sulfoxylic acid. Although all of these species play an important role in atmospheric chemistry and in organic synthesis, and are also invoked in biochemical processes, they are quite unstable compounds so much so that their physical and chemical properties are still subject to intense studies. It is well-established that sulfoxylic acid has very strong reducing properties, while sulfenic acid is capable of both oxidizing and reducing various substrates. Here, in this review, the mechanisms of sulfide oxidation as well as data on the structure and reactivity of small sulfur-containing oxoacids, sulfur monoxide, and its precursors are discussed.

Exact Concentration Dependence of the Landolt Time in the Thiourea Dioxide-Bromate Substrate-Depletive Clock Reaction

The thiourea dioxide (TDO)-bromate reaction has been reinvestigated spectrophotometrically under acidic conditions using phosphoric acid-dihydrogen-phosphate buffer within the pH range of 1.1-1.8 at 1.0 M ionic strength adjusted by sodium perchlorate and at 25 °C. The title system shows a remarkable resemblance to the classical Landolt reaction, namely, the clock species (bromine) may only appear after the substrate TDO is completely consumed. Thus, the title system can be classified as substrate-depletive clock reaction. Despite the well-known slow rearrangement characteristic of TDO in acidic solution, it is surprisingly found that the Landolt time of the title reaction does not depend at all on the age of TDO solution applied. It is, however, shown experimentally that the inverse of Landolt time linearly depends on the initial bromate concentration as well as on the square of the hydrogen ion concentration. In addition to this, it is also noticed that dihydrogen phosphate markedly affects the Landolt time as well, and this feature may easily be taken into consideration by the H₂PO₄^- dependence of the rate of bromate-bromide reaction quantitatively. Based on the experiments, a simple three-step kinetic model is proposed from which a complex formula is derived to indicate the exact concentration dependence of the Landolt time.

S-oxygenation of thiocarbamides V: oxidation of tetramethylthiourea by chlorite in slightly acidic media

The reaction between tetramethylthiourea (TTTU) and slightly acidic chlorite has been studied. The reaction is much faster than comparable oxidations of the parent thiourea compound as well as other substituted thioureas. The stoichiometry of the reaction in excess oxidant showed a complete desulfurization of the thiocarbamide to yield the corresponding urea and sulfate: 2ClO2(-) + (Me2N)2C ═ S + H2O → (Me2N)2C ═ O + SO4(2-) + 2Cl(-) + 2H(+). The reaction mechanism is unique in that the most stable metabolite before formation of the corresponding urea is the S-oxide. This is one of the rare occasions in which a low-molecular-weight S-oxide has been stabilized without the aid of large steric groups. ESI-MS data show almost quantitative formation of the S-oxide and negligible formation of the sulfinic and sulfonic acids. TTTU, in contrast to other substituted thioureas, can only stabilize intermediate oxoacids, before formation of sulfate, in the form of zwitterions. With a stoichiometric excess of TTTU over oxidant, the TTTU dimer is the predominant product. Chlorine dioxide, which is formed from the reaction of excess chlorite and HOCl, is a very important reactant in the overall mechanism. It reacts rapidly with TTTU to reform ClO2(-). Oxidation of TTTU by chlorite has a complex dependence on acid as a result of chlorous acid dissociation and protonation of the thiol group on TTTU in high-acid conditions, which renders the thiol center a less effective nucleophile.

Oxyhalogen-sulfur chemistry: kinetics and mechanism of oxidation of chemoprotectant, sodium 2-mercaptoethanesulfonate, MESNA, by acidic bromate and aqueous bromine

The oxidation of a well-known chemoprotectant in anticancer therapies, sodium 2-mercaptoethanesulfonate, MESNA, by acidic bromate and aqueous bromine was studied in acidic medium. Stoichiometry of the reaction is: BrO3(-) + HSCH2CH2SO3H → Br(-) + HO3SCH2CH2SO3H. In excess bromate conditions the stoichiometry was deduced to be: 6BrO3(-) + 5HSCH2CH2SO3H + 6H(+) → 3Br2 + 5HO3SCH2CH2SO3H + 3H2O. The direct reaction of bromine and MESNA gave a stoichiometric ratio of 3:1: 3Br2 + HSCH2CH2SO3H + 3H2O → HO3SCH2CH2SO3H + 6Br(-) + 6H(+). This direct reaction is very fast; within limits of the mixing time of the stopped-flow spectrophotometer and with a bimolecular rate constant of 1.95 ± 0.05 × 10(4) M(-1) s(-1). Despite the strong oxidizing agents utilized, there is no cleavage of the C-S bond and no sulfate production was detected. The ESI-MS data show that the reaction proceeds via a predominantly nonradical pathway of three consecutive 2-electron transfers on the sulfur center to obtain the product 1,2-ethanedisulfonic acid, a well-known medium for the delivery of psychotic drugs. Thiyl radicals were detected but the absence of autocatalytic kinetics indicated that the radical pathway was a minor oxidation route. ESI-MS data showed that the S-oxide, contrary to known behavior of organosulfur compounds, is much more stable than the sulfinic acid. In conditions where the oxidizing equivalents are limited to a 4-electron transfer to only the sulfinic acid, the products obtained are a mixture of the S-oxide and the sulfonic acid with negligible amounts of the sulfinic acid. It appears the S-oxide is the preferred conformation over the sulfenic acid since no sulfenic acids have ever been stabilized without bulky substituent groups. The overall reaction scheme could be described and modeled by a minimal network of 18 reactions in which the major oxidants are HOBr and Br2(aq).