Thionic acids, salts

Salts of Thionic Acids.—A third class of thio salts are those derived from the various thionic acids of which sodium thiosulfate and barium dithionate may be... 

Properties.—Like the lower members of the series of thionic acids, free pentathionic acid is known only in aqueous solution a solution of the pure acid is obtained by treating an aqueous solution of the potassium salt with the requisite quantity of tartaric acid for the removal of the potassium in the form of hydrogen tartrate. The solution is denser than water 4 it cannot be concentrated beyond a limit of 50 to 60 per cent, acid without decomposition. 

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. 

Dithionic acid appears in a solution of sulphurous acid oxidised by finely divided MnOg. From this, baryta precipitates all the sulphur-containing anions except dithionate, and, when the excess of Ba + has been precipitated by H2SO4, only 112820 is left in the solution. Although conductance measurements show the acid to be dibasic, acid salts are unknown. Dithionates are soluble in water and, unlike salts of the higher thionic acids, are not decomposed by sulphites and sulphides. 

A preparative and X-ray crystallographic survey (ill) of salts of hexa-thionic acid led to unit cell and space group data for two salts, the crystals of which are, in view of the rather unstable nature and poor crystallization power of hexathionates, surprisingly well developed and stable. One is a potassium barium double salt, and the other a cobalt complex salt. 

Numerous esters of the phosphorothioic acids are known. In salts and esters of these oxygen/ sulphur (0-S) acids there may be a preferred location of the multiple bond, but in general this is not known. Thus in principle there are two series of possible acids, each of which might give rise to salts and esters, as indicated in the nomenclature scheme in Table 9.3. The thionic acids contain the group P=S, while the thiolic acids contain the group P-SH. The term thioic is often used when the molecular form is unknown, or when specification is not desired. An analogous nomenclature scheme is applicable to 0/Se acids. 

Aqueous H2[S406] is the most stable thionic acid but has not been isolated. Its dilute solution can be boiled without decomposition. A concentrated solution decomposes into S, SO2 and H2SO4. Its acidity is as great as that of H2[S206]. The salts of H2[S406] are generally soluble. In the solid state they may be kept for a month or more, but they readily decompose in solution, especially when warmed. The alkali salts are more stable than those of Ba Cu etc. 

Isopropenyl esters, conveniently prepared from carboxylic acids and propyne in the presence of the zinc salt, readily effect the acylation of ise-thionic acid and of A-methyltaurine. ... 

This salt, when decomposed by sulphuric acid, yields tetra-thionic arid. 

The simplest method of preparation of bis(phosphinothiolyl)sulphides (Table 9.6), is to heat phosphinodithioic acids to eliminate hydrogen snlphide (9.599). A more general method is to heat salts of phosphinodithioic acids with phosphino thionic halides (9.600). 

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