Selenious acid oxidant

Pyrazinecarboxylic acid has been obtained by selenious acid oxidation in pyridine of methylpyrazine or aqueous permanganate oxidation of ethylpyrazine, in yields of 64 and 48%, respectively.171,218 It has also been obtained in 70% yield by partial decarboxylation of pyrazine-2,3-dicarboxylic acid on heating in vacuo at 210°.219 Aqueous permanganate oxidation of 2,5-distyrylpyrazine gives the 2,5-dicarboxylic acid.220 Pyrazine-2,5-dicarboxylic acid has also been prepared in 45% yield by direct carboxylation of pyrazine with carbon dioxide at 50 atm pressure at 250° for 3 hours in the presence of a potassium carbonate and calcium fluoride catalyst.221 Pyrazine-tricarboxylic acid (57), obtainable in only very poor yields by oxidation of 2,5-dimethyl-3-ethylpyrazine, is prepared in 87% yield by alkaline permanganate oxidation of 2-(D-arabo)tetrahydroxybutyl-quinoxaline (56).222 Decarboxylation of the tricarboxylic acid by... 

Selenious acid oxidizes pyrrole to a blue dyestuff of unknown composition ( pyrrole blue ). Iron salts accelerate the reaction when it is carried out in phosphoric acid solution. Selenic, tellurous and telluric acids do not react under the conditions given below the test therefore provides a method of distinguishing selenites and selenates. 

Under suitable conditions, selenious acid oxidizes p3m ole to pyrrole blue", a dyestuff of unknown constitution. This reaction can serve as a test for selenious acid, since selenic, tellurous, and telluric acids remain unaltered. Hence, the test can be used to distinguish between selenious and selenic acids. 

Selenious acid, H2Se03. Formed water on Se02. More stable than H2SO3, oxidized to H2Se04, reduced to Se by SO2. Gives selen-ates(IV) with oxides, hydroxides or carbonates. 

Selenium dioxide. The compound is conveniently prepared by the oxidation of selenium with nitric acid, followed by dehydration of the selenious acid thus formed by cautious heating ... 

The selenious acid does not need to be freshly prepared. A larger amount does not increase the yield. The submitters specified selenium dioxide as the oxidizing agent, but the checkers prepared their material by evaporating an aqueous solution to dryness on the water bath. They, therefore, have considered the oxidizing agent to be selenious acid and have calculated the... 

Selenious acid, 24, 61, 62 Selenium, 24, 63 powdered, black, 24, 90 Selenium compounds, ease of oxidation, 24,91... 

Following a report60 that D-fructose, but not D-glucose, is oxidized by selenious acid, Dixon and Harrison61 used this reagent to prepare D-glucosone from D-fructose in aqueous solution isolation and purification were carried out after the manner of Fischer,4 but no yield was given. By this... 

In the preparation of D-glucosone by the direct oxidation of D-glucose, D-fructose, or D-mannose by such reagents as that of Fenton,37 cupric acetate,16- 46- 46 selenious acid,16-61 etc., the degree of oxidation must be carefully controlled if the osone, which is the first product, is to be the main product of the reaction. The nature and mechanism of formation of the products of further oxidation of D-glucosone are discussed on p. 68. 

The pyrazine ring is stable toward permanganate oxidation, and this explains a variety of pyrazinecarboxylic acids that have been prepared from quinoxalines or benzo-fused quinoxalines. In contrast, alkyl side chains on pyrazines are effectively oxidized by permanganate, selenious acid, selenium dioxide, or dichromate to afford the corresponding carboxylic acids (Section 8.03.7.1). Oxidation of pyrazines with hydrogen peroxide or percarboxylic acids gives pyrazine A -oxides and/or A, A -dioxides (Section 8.03.5.2). 

Compound 31, the synthesis of which is described in Section 10.09.9.2.3, undergoes a substantial rearrangement upon oxidation with selenious acid, giving 1,2,4-triazole 32 (Equation 6). A plausible mechanism for this transformation is presented in the original report <2003RCB1740>. 

Purification may also be effected by oxidation to selenious acid, e.g. by heating with dilute nitric acid. On evaporation the solid selenium dioxide may be obtained, and this can be purified by repeated sublimation in a current of dust-free dry air.3 It may then be redissolved in water, the solution acidified with hydrochloric acid, and the selenium precipitated by passing in sulphur dioxide.4 For further purification the element can be sublimed in a current of carbon dioxide, and after heating for some time at 100° C. to convert it into the crystalline condition, it may be heated with carbon disulphide to extract any traces of residual sulphur. 

Hypophosphorous acid in slightly alkaline solution reduces selenie acid quantitatively to the element, which can be separated, collected, dried and weighed. If the selenium is originally in a high stage of oxidation it should first be reduced by boiling with hydrochloric acid until no more chlorine is evolved.6... 

Selenious acid readily decomposes potassium permanganate, but analytical results are untrustworthy in the presence of more than a limited quantity of sulphuric acid.1 The oxidation should be carried out at 50° C., a known quantity of 0-lN potassium permanganate being used and the excess determined either by means of standard oxalic acid solution or by electrometric titration with ferrous sulphate. In the presence of tellurium, the latter is also oxidised and should be determined in a separate sample by oxidation with potassium di-chromate, which does not oxidise the selenium, and the necessary deduction can then be made.2... 

In the absence of the halogens the selenium may be oxidised to selenious acid by heating with nitric acid in a Carius tube at 240° to 300° C. The resulting liquid is nearly neutralised, excess of zinc oxide added, and the mixture titrated with silver nitrate using chromate as external indicator 3... 

The Selenites.—Selenious acid is a weak acid but neutralises hydroxides and carbonates witli the production of selenites. Ammonium selenite is formed by dissolving selenious acid in aqueous ammonia.3 The selenites can be obtained in an anhydrous condition by heating the metal oxides in a current of selenium dioxide vapour or with selenium dioxide in a sealed tube in the latter case the products can be obtained crystalline if the selenium dioxide is used in excess.4... 

Selenic Acid, HaSe04.—This acid was first prepared by Mitscher-lich in 1827, who suspended lead selenate in water and precipitated the lead by means of hydrogen sulphide. By the oxidation of aqueous solutions of selenious acid with suitable reagents, e.g. chlorine or bromine, selenic acid may also be obtained.6 When chlorine is used, any hydrogen chloride must be removed as soon as it is formed, for selenic acid is reduced by hot hydrogen chloride to selenious acid, with liberation of chlorine the process is best carried out6 by passing a... 

When selenious acid or a selenite is refluxed for about three hours with 30 per cent, hydrogen peroxide, oxidation proceeds to the extent of about 90 per cent.6... 

W. G. Whitman and L. Evans found that the removal of the lower oxides of nitrogen from nitric acid by a current of air follows the equation —dC/dt— C, where 0 denotes the cone, of the nitrogen oxides, and / is a constant depending on the temp, and rate of flow of air. H. N. Warren found traces of selenious acid in a sample of commercial nitric acid. This impurity was probably derived from the sulphuric acid. F. A. F. Crawford examined the organic impurities in nitric acid ... 

Selenic Acid.—Selenic Acid, H2Se04, is also a il colourless syrupy liquid it can be produced by direct oxidation of selenium by chlorine water, but on concentra-I tion the resulting hydrochloric acid reduces the selenic acid I to selenious acid, as hydriodic acid reduces sulphuric acid. It is best prepared by addition of copper carbonate to the mixture of selenic and hydrochloric acids obtained in that way selenate and chloride of copper are formed the 1 mixture is evaporated to dryness, and the copper chloride is dissolved out with alcohol, leaving the insoluble selenate 1 behind. The selenate is dissolved in water, and on treatment with sulphuretted hydrogen, copper sulphide is precipitated, and removed by filtration the selenic acid is then concentrated if it contains a trace of water, it is a heavy liquid 5 but if quite anhydrous, it forms a solid, melting at 58°. 

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