The Key Heterolysis Selectivity of Divalent Sulfur–Sulfur Bonds for a Unified Mechanistic Scheme in the Thiosulfatolysis and Sulfitolysis of the Pentathionate Ion

Abstract

The thiosulfatolysis and sulfitolysis of pentathionate were studied in the pH ranges 4.50-8.0 and 4.0-5.0 at T = (25.0 ± 0.1)°C by HPLC monitoring of the time-dependence and distribution of different sulfur species. The two systems are first order with respect to both reactants. In contrast to tetrathionate, pentathionate and higher polythionates contain inequivalent divalent sulfur atoms; therefore, the complexity of the mechanism increases. The selective heterolytic cleavages of [a]

Kinetics and Mechanism of the Concurrent Reactions of Hexathionate with S(IV) and Thiosulfate in a Slightly Acidic Medium

“…However, our effort to synthesize solid potassium heptathionate with a purity higher than 90% has failed; therefore, peak areas of heptathionate were not converted into concentrations. In addition to this, it has recently been shown that during the course of thiosulfatolysis of pentathionate 25 turbidity of the reaction mixture appears at the beginning stage of the reaction and then it later disappears. In the case of the thiosulfatolysis of hexathionate because of the presence of γ and γ′-sulfur it is expected that this feature becomes more profound at the beginning stage, which may lead to the appearance of significant amounts of sulfur precipitation.…”

“…Step R4 is the thiosulfate−sulfite group displacement equilibrium of pentathionate leading to the formation of hexathionate and sulfite studied first by Ji e al. 25 They were able to determine k R4 and k −R4 values as 0.0165 and 1050 M −1 s −1 , respectively. As it is seen, this reaction has to be a central part of our model as it must certainly be the initiating equilibrium when studying the hexathionate−sulfite reaction.…”

“…As it is seen in Table 1 our recent experiments led to evaluating k R4 and k −R4 to be 0.0509 ± 0.0005 and 584 ± 4 M −1 s −1 , respectively; both are in a fairly good agreement with Ji et al's result. 25 Step R5, that is, the hexathionate−thiosulfate reaction, leading to formation of heptathionate and sulfite, has not been studied directly so far. The kinetic model of thiosulfatolysis and sulfitolysis of shorter polythionates was shown to be built up successfully from a bottom-up construction in a stepwise fashion 25,42 because they cannot be studied individually excluding completely the contributory side effect of other polythionates.…”

“…25 Step R5, that is, the hexathionate−thiosulfate reaction, leading to formation of heptathionate and sulfite, has not been studied directly so far. The kinetic model of thiosulfatolysis and sulfitolysis of shorter polythionates was shown to be built up successfully from a bottom-up construction in a stepwise fashion 25,42 because they cannot be studied individually excluding completely the contributory side effect of other polythionates. Therefore, the most reliable determination of the rate coefficients of the forward and backward processes is expected to be obtained from studying experimentally the most closely related system.…”

“…19,22−24 Heterolysis selectivity of the divalent sulfur−sulfur bond plays a key role in modulating the distribution of sulfur species in the case of thiosulfatolysis and sulfitolysis of pentathionate as the attack of thiosulfate or sulfite may take place on the inequivalent divalent sulfur atoms of hexathionate formed. 25 In the present work, the concurrent reactions of hexathionate with thiosulfate and sulfite were investigated in a slightly acidic medium where decomposition of polythionates does not have any significant contribution to the rearrangement reaction. This study is performed to determine the individual rate coefficients of the polythionate−thiosulfate and polythionate−sulfite reactions in order to establish the relationship between the reactivity and structure of polythionates.…”