Dithionic Acid and Dithionates

The oxidation state of sulfur in H2S206 is formally +5. This acid is not obtained as a pure compound, although a sizable number of dithionate salts are known. The structure of the dithionate ion is 

A bond distance of 145 pm for S-0 is comparable to that in S042- and is indicative of some double-bond character. However, the S-S bond distance is slightly longer than that for a normal single bond. 

At high temperatures, solid metal dithionates disproportionate to give a metal sulfate and S02 with the general reaction being shown as follows  

Toward transition metals, the dithionate ion behaves as a bidentate ligand, and it forms a large number of complexes. 

Peroxydisulfuric acid, H2S20s, is a colorless solid that has a melting point of 65 °C. The acid and its salts are strong oxidizing agents, and the acid is not very stable. The sodium, potassium, and ammonium salts are most commonly used in oxidation reactions. The peroxydisulfate ion is generated by the anodic oxidation of bisulfate as represented by the equation 

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In dithionic acid and dithionates, 8205 , the oxidation state of the 2 8 atoms has been reduced from VI to V by the formation of an 8-8 bond (Table 15.18, p. 705). The free acid has not been obtained pure, but quite concentrated aqueous solutions can be prepared by treatment of the barium salt with the stoichiometric amount of H28O4 ... 

Sulfuric acid, H2SO4, the most important commercial sulfur compound (see Sulfuric acid and sulfur trioxide), and peroxymonosulfuric acid [7722-86-3] (Caro s acid), H2SO, are discussed elsewhere (see Peroxides and peroxide compounds, inorganic). The lower valent sulfur acids are not stable species at ordinary temperatures. Dithionous acid [15959-26-9] H2S2O4, sulfoxyHc acid [20196-46-7] H2SO2, and thiosulfuric acid [13686-28-7] H2S2O2 are unstable species. A discussion of efforts to isolate and characterize the unstable sulfur acids is given (330). 

Alkylthio, arylthio, and thioxo. The thioxo group in pyrimidine-2,4-dithione can be displaced by amines, ammonia, and amine acetates, and this amination is specific for the 4-position in pyrimidines and quinazolines. 2-Substitution fails even when a 5-substituent (cf. 134) sterically prevents reaction of a secondary amine at the 4-position. Acid hydrolysis of pyrimidine-2,4-dithione is selective at the 4-position. 2-Amination of 2-thiobarbituric acid and its /S-methyl derivative has been reported. Under more basic conditions, anionization of thioxo compounds decreases the reactivity 2-thiouracil is less reactive toward hot alkali than is the iS-methyl analog. Hydrazine has been reported to replace (95°, 6 hr, 65% 3deld) the 2-thioxo group in 5-hexyl-6-methyl-2-thiouracil. Ortho and para mercapto- or thio- azines are actually in the thione form. ... 

Redox titrants (mainly in acetic acid) are bromine, iodine monochloride, chlorine dioxide, iodine (for Karl Fischer reagent based on a methanolic solution of iodine and S02 with pyridine, and the alternatives, methyl-Cellosolve instead of methanol, or sodium acetate instead of pyridine (see pp. 204-205), and other oxidants, mostly compounds of metals of high valency such as potassium permanganate, chromic acid, lead(IV) or mercury(II) acetate or cerium(IV) salts reductants include sodium dithionate, pyrocatechol and oxalic acid, and compounds of metals at low valency such as iron(II) perchlorate, tin(II) chloride, vanadyl acetate, arsenic(IV) or titanium(III) chloride and chromium(II) chloride. 

Imidazopyridopyrimidines can be prepared simply by the reaction between anthranilic acid and the imidazolidine-2,4-dithione under basic conditions (Equation 33) <2001MI233, 2001PS(173)105>. 

For the constitution and estimation of dithionic acid and its salts, see later (pp. 221-4.). 

In a one-pot reaction, a,a-disubstituted a-halo carbonyl compounds 1 (R, potassium thiocyanate, acetic acid, and monosubstituted hydrazines 3 are transformed into dihydro-lfT-imidazo[l,5- 7][l,2,4]triazole-2,5 (3H,6H)dithiones 8 (Scheme 1) (93TH, 01TH). With R =H, the reaction takes a different course (cf. Section 2.3). 

The criss-cross reaction of azoalkenes is restricted to the addition of thiocyanic acid and could not be extended to cyanic acid, as this is the case in the classical reaction with azines. However, the cycloadduct expected to arise from the reaction of an azoalkene with cyanic acid, the heterobicyclic 2,5-dione 13, can be obtained simply by reacting the 2,5-dithiones 8 with hydrogen peroxide in sodium hydroxide (Scheme 3) (93TH, OITH). 

Crude polysulfanes with x = 7 -12 can be obtained in up to 50% yield at the cathode by reduction of SO2/H2SO4 solutions. The initial reaction product at the cathode is dithionous acid H2S2O4, which decomposes in strong acid to give sulfurous acid and sulfane. Persulfate is obtained at the anode. 

In acidic solutions, dithionic acid decomposes to sulphate and sulphur dioxide at rates which are readily measured (Table 26) . The reaction is of first order at defined acidity and the rate increases, but not proportionately, with acid concentration. The authors have favoured an interpretation in terms of undissociated H2S2O6. Salt effects and solvent deuterium effects are in general agree-... 

The c/s-[aquabis(ethylenediamine)hydroxochromium(III)] dithionate salt is stable for years. It is insoluble in water but very soluble in strong acid and strong base, giving the corresponding m-diaqua and m-dihydroxo species. The crude product is used in the preparation of m-[diaquabis(ethylenediamine)chromium-(III)] bromide (Sec. G) and of di-/a-hydroxobis[bis(ethylenediamine)chromium-(III)] dithionate (Sec. J). 

Crude cis-[tetraammineaquahydroxochromium(III)] dithionate (2.00 g, 0.0060 mole) is heated for 1 hour and 15 minutes at 100° in an oven to give a violet product of very impure di- i-hydroxo-bis[tetraamminechromium(III)] dithionate. This material is added to 8 mL of a saturated (at room temperature) solution of ammonium bromide, and the suspension is cooled in an ice bath, with thorough stirring, for M. hour. The dithionate salt dissolves and red crystals of di-Ai-hydroxo-bis[tetraamminechromium(III)] bromide tetrahydrate precipitate. The sample is filtered, washed with four 5-mL portions of 50% v/v ethanol-water, and dried in the air. This procedure yields 1.00 g (50%) of an almost pure sample. A pure product is obtained after two further reprecipitations. A 1.00-g quantity is dissolved in 10 mL of 0.01 M hydrobromic acid and reprecipitated from the filtered solution by addition of 10 mL of the saturated solution of ammonium bromide with stirring and cooling in an ice bath. The bromide salt is isolated as above in a yield of 0.75 g (75%). Anal. Calcd. for [(NH3)4Cr(OH)2-Cr(NH3)4]Br4-4H20 Cr, 15.62 Br, 48.00 N, 16.83 H, 5.15. Found Cr, 15.50 Br, 48.16 N, 16.88 H, 4.87. 

Crude ds-[aquabis(ethylenediamine)hydroxochromium(III)] dithionate (40.0 g, 0.109 mole) (see Sec. E) is added to 400 mL of acetic anhydride. The suspension is heated to reflux within 20 minutes, kept at reflux for another 20 minutes, and then cooled in an ice bath. The sample is filtered, washed with two 50-mL portions of 96% ethanol, five 50-mL portions of 2 M acetic acid, and four 100-mL portions of 96% ethanol, and then thoroughly washed with diethyl ether. By the washing with acetic acid, a small amount of unreacted ds-[aqua-bis(ethylenediamine)hydroxochromium(III)] dithionate is removed. Drying in air yields 34.5 g (91%) of a nearly pure product. The crude dithionate is used in the syntheses of the chloride and the bromide, as given below. The pure dithionate is obtained from the pure bromide by the following procedure. 

The crude dithionate (10.0 g, 0.0143 mole) is added to 50 mLof a saturated solution of ammonium chloride and the suspension is stirred at room temperature for hour. Violet crystals of the chloride salt are filtered, washed with two 20-mL portions of 50% v/v ethanol-water, and dried in air. The sample is dissolved in 130 mL of 0.01 M hydrochloric acid at room temperature and filtered. Then 160 mL of a saturated solution of ammonium chloride is added portion-wise during 10 minutes to the stirred solution, which is then cooled slowly in an ice bath. After it is cooled for 1 hour, the sample if filtered, washed with 15 mL of 50% v/v ethanol-water and two 15-mL portions of 96% v/v ethanol-water and dried over 5.4 Af sulfuric acid. This yields 5.0 g (63%) of the almost pure di-fi-hydroxo-bis[bis(ethylenediamine)chromium(III)] chloride dihydrate. A 2.00-g, quantity is dissolved in 35 mL of 0.01 M hydrochloric acid and reprecipitated with 40 mL of a saturated solution of ammonium chloride, as above. The yield is 1.55 g (78%) of pure salt. Anal. Calcd. for [(en)2Cr(OH)2Cr(en)2]CU 2H20 Cr, 18.70 C, 17.27 N, 20.15 H, 6.89 Cl, 25.49. Found Cr, 18.46 C, 17.28 N, 20.13 H, 6.75 Cl, 25.56. 

Fanghaenel, E., Kordts, B., Richter, A. M., Dutschmann, K. Lewis-acid and photochemically induced Dimroth rearrangement of3H,6H-2,5-bis(p-N,N-dimethylaminophenyl)1,2-thiazolino[5,4-d]1,2-thiazoline-3,6-dithione. J. Prakt. Chem. 1990, 332, 387-393. 

Systematic use of the prefix hypo would give the names hypodisulfuric acid for dithionic acid and hypodisulfurous acid for dithionous acid. 

Pyrazolin-5-ones form complexes with both inorganic and organic compounds much more readily than do the 2-pyrazolin-5-ones. The most extensive series of complexes is that formed with a variety of metallic salts. Antipyrine (2,3-dimethyl-l-phenyl-3-pyrazolin-5-one) forms a series of complexes with salts of divalent, trivalent and tetra-valent metals. Two molecules of antipyrine form a complex with one molecule of copper, cadmium, cobalt and zinc salts.266,866,1116 Complexes prepared from metallic nitrates are usually hydrated.1322 There also exists a series of complexes in which three molecules of antipyrine form a complex with one or two molecules of metallic salts. Such complexes form with two molecules of simple ferric salts272 or with one of complex iron cyanides.608 Nitrates of thorium, lanthanum, cerium and samarium also give such complexes.841 This ratio also occurs in some antipyrine complexes with cadmium and zinc thiocyanate.266 A number of salts of rare earths and iron which have complex anions such as thiosulfate, thiocyanate, dithionic acid and complex iron cyanides form complexes in which six molecules of antipyrine are present.405,408 608,841,950 Stannic chloride forms salts containing three or four molecules of antipyrine and hydrochloric acid.46... 

The product with m.p. 83,5—84.5° C which Jacobson isolated appears to have been 3,4,4-trialkyl thiazolidin-5-one-2-thione rather than l,5,5-trimethylimidazolidine-2,4-dithione. The former can be synthesized independently from a-(methylamino) isobutyric acid and carbon disulfide (see also V.3). Other a-(methylamino) nitriles when refluxed with carbon disulfide, also yield 3,4,4-trialkyl-5-imino thiazol-idine-2-thiones (CLXII) (listed in Table 23) which can be hydrolyzed... 

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