Tetrathionate anion

Sodium tetrathionate (Na2S406) is a redox compound that under the right conditions can facilitate the formation of disulfide bonds from free sulfhydryls. The tetrathionate anion reacts with a sulfhydryl to create a somewhat stable active intermediate, a sulfenylthiosulfate (Fig. 102). Upon attack of the nucleophilic thiolate anion on this activated species, the thiosulfate (S203 =) leaving group is removed and a disulfide linkage forms (Pihl and Lange, 1962). The reduction of tetrathionate to thiosulfate in vivo was a subject of early study (Theis and Freeland, 1940 Chen et al., 1934). 

Fig. 5 Electropherogram of six sulfur anions illustrating peak fronting [thiosulfate (S2Ol-)] and peak tailing [tetrathionate (S4Ol-)] for better visibility of peak asymmetry, the perpendicular peak axis is drawn as a solid line. Separation in 20 mM chromate run buffer at pH 8.3.

Sodium thiosulfate is a common reducing agent. It reduces iodine to iodide anion forming sodium tetrathionate. This reaction is utilized in the so-called iodometric titration ... 

The observation that in the electrolysis of ammonium thiosulphate solution tetrathionic acid is formed at the anode also belongs to this class, because the process is not one of the coupling of discharged anions, but an anodic oxidation of the thiosulphate.11... 

This is similar to the synthesis of the neutral Roussin esters [Fe2(SR)2-(N0)4] from iron(II), RSH, and nitric oxide [Eq. (1), Section II,B,1], except that in Eq. (1) the iron(II) initially present undergoes disproportionation to Fe(I), which is present in the product [Fe2(SR)2(NO)4], and Fe(III), which is removed as oxide. In Eq. (12), by contrast, the iron(II) is reduced to Fe(I) by excess of thiosulfate, which is itself oxidized to tetrathionate. Apart from its diamagnetism (4), practically nothing is known about this anion. 

The acids themselves have never been isolated, but salts of each have been prepared. Higher polythionates having chains of five or six sulfur atoms per molecule have been made. Tetrathionate, the most important thionic acid anion, results from the oxidation of S2OI with I2. Salts of the remaining thionic acids must be prepared by special methods. 

Ending of names of anions and esters of onic oxoacids, e.g. boronate, phosphonate, tetrathionate. 

The thiosulfate anion reacts rapidly with molecular iodine, bleaching the brown/purple color of the latter to produce colorless iodide and tetrathionate, 8406 . 

The structure of the product, tetrathionate, may not immediately suggest a mechanism. One way of approaching the problem is simply to look for potential nucleophiles and electrophiles. The anionic terminal sulfur of thiosulfate should be a good nucleophile and I2 a reasonably good electrophile. Their interaction may be expected to be as follows ... 

There are a variety of polythionates, [03S-S -S03], where n can vary from 0 (dithionate) to the 20s. The anions with lower n values are named by reference to the total number of sulfur atoms for example, S3OI is trithionate and 840 is tetrathionate. The structure of the latter is given in Table 17.2 as a representative polythionate. None of the corresponding acids are stable. Selenium analogs of the polythionates in which all the sulfur atoms are replaced with the heavier congener do not exist, but compounds with a general formula [03S-Se -S03], = 2—6, do. 

When the alkali concentration is 0.5-1.0M, dithionite can be titrated with standard hexacyanoferrate(IIl) solution. Because of the possibility of oxidation in air, the titration vessel should be provided with an inlet and an outlet for purging and maintaining an oxygen-free nitrogen atmosphere. Under these conditions, the dithionite is oxidised to sulphite. When the alkali concentration is raised to 4-4.5 M and the temperature is raised to 50°-60°C, the sulphite thus produced or already present together with the thiosulphate, the anions are oxidised by the iron(IIl) complex to sulphate. Titration is carried out in presence of osmium tetroxide as a catalyst. A second titration with iodine in acid solution oxidises the thiosulphate to tetrathionate. This can be titrated with the iron(III) complex at 50°C when the alkali concentration is 5 M. The end-point could be detected potentiometrically or by the dead-stop set up. 

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