Selenium preparation

Preparation and Properties of Selenium(lV) Oxide. Selenium compounds are very poisonous Hand in the prepared substances and residues o/ selenium preparations to the laboratory assistant. Run all experiments with selenium in a fume cupboard After work, thoroughly wash the ware and your hands. 

Selenium prepared by the reduction of selenious acid with sodium or ammonium hydrogen sulphite forms colloidal solutions, that precipitated... 

Crystalline Se6 has been prepared. The synthesis consists of some kind of recrystallization of red amorphous selenium, prepared from highly purified selenium, which is extracted with carbon disulfide. By a special crystallization method, two types of single crystals have been... 

Fig. 5. Low-temperature Raman spectra (-105 5°C) of monoclinic a-Se8 (a) and of red amorphous selenium prepared by reduction of Se02 (b) (14).

Rather different spectra are obtained when the equilibrium vapor above metallic selenium (160°C) is ionized by a field ion source (2.5-8.0 kV), resulting in the ions Se + (n = 2 and 5-8) neither Se+ nor Se3+ and Se4+ have been observed (these species usually result from fragmentation processes), but occasionally traces of Seg+ have been found. Using a Knudsen cell, Se6+ is the most abundant species, followed by Se8+ and Se7+, which all originate from the corresponding neutral, cyclic molecules (56). Field evaporation (20-100°C) of whiskers of selenium (prepared by field condensation of Se vapor) produces mainly Se5+, but small amounts of molecular ions with one, two, and four positive charges up to Se33 have been identified. However, these species may have chainlike structures (56). 

Sato et al. described three types of 1,2,5-benzotrichalcogenepins (130, 131, and 133) containing sulfur and selenium prepared by the reaction of the corresponding 2,2-dimethyl-l,3,2-benzodichalcogenastannoles 129 with thiiranes using -butyllithium in an oxygen atmosphere (Scheme 12 see Chapter 13.15) <1997TL5821>. 

Selenium prepared from the niter slag may contain consider . tellurium, which may be removed by adding sulfuric acid the strongly alkaline solution, thus Na2Te03 + H2SO Na2S04 + Te02 + H20. Separation by electrolysis in a solution of alkali hydroxide or salt has also been proposed.1... 

Research is now underway to find ways to alleviate this danger. For example. Professor Robert Hurt and his colleagues at Brown University have found that selenium prepared as tiny particles has a very high affinity for mercury and can be used in recycling operations to prevent dangerous occupational exposure to mercury. 

Salmond, W. G., M. A. Barta, A. M. Cain, and M. C. Sobala Alternative Modes of Decomposition of Allylic Selenoxides Diastereoisomeric at Selenium. Preparation of A5,7- and 5 (3-Hydroxy-A6-steroids. Tetrahedron Lett. 1977, 1683. 

To meet the needs of the advanced students, preparations have now been included to illustrate, for example, reduction by lithium aluminium hydride and by the Meerwein-Ponndorf-Verley method, oxidation by selenium dioxide and by periodate, the Michael, Hoesch, Leuckart and Doebner-Miller Reactions, the Knorr pyrrole and the Hantzsch collidine syntheses, various Free Radical reactions, the Pinacol-Pinacolone, Beckmann and Arbusov Rearrangements, and the Bart and the Meyer Reactions, together with many others. 

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 ... 

In view of the small difference in cost between selenium and selenium dioxide, the preparation of any quantity of the latter from selenium is hardly worth while. It is better to purify commercial selenium dioxide. 

Ninhydrin (also named 1 2 3-triketoindane or 1 2 3-triketohydrindene hydrate) is prepared most simply from the inexpensive phthahc anhydride (I). The latter is condensed with acetic anhydride In the presence of potassium acetate to give phthalylacetlc acid (II) reaction of the latter with sodium methoxide in methanol yields 1 3-indanedionecarboxyhc acid, which is decomposed upon warming with dilute hydrochloric or sulphuric acid to indane-1 3-dione (or 1 3-diketohydrindene) (HI). Selenium dioxide oxidation of (III) afibrds indane-1 2 3-trione hydrate (ninhydrin) (IV). 

Selenium exists in several allotropic forms. Three are generally recognized, but as many as that have been claimed. Selenium can be prepared with either an amorphous or crystalline structure. The color of amorphous selenium is either red, in powder form, or black, in vitreous form. Crystalline monoclinic selenium is a deep red crystalline hexagonal selenium, the most stable variety, is a metallic gray. 

A solution of 4.8 g of sodium in 300 ml of anhydrous liquid ammonia (note 1) was prepared by introducing the sodium, cut into 1-g pieces, into the ammonia. When the sodium had dissolved (after 10 min), 0.12 mol of selenium was added in 0.5-g portions over a period of 10 min with vigorous stirring. A dropping funnel, containing 250 ml of methanol, combined with a gas inlet was then placed on the neck... 

This chapter is an attempt to present the important results of studies of the synthesis, reactivity, and physicochemical properties of this series of compounds. The subject was surveyed by Bulka (3) in 1963 and by Klayman and Gunther (4) in 1973. Unlike the oxazoles and thiazoles. there are few convenient preparative routes to the selenazoles. Furthermore, the selenium intermediates are difficult to synthesize and are often extremely toxic selenoamides tend to decompose rapidly depositing metallic selenium. This inconvenience can be alleviated by choice of suitable reaction conditions. Finally, the use of selenium compounds in preparative reactions is often complicated by the fragility of the cycle and the deposition of metallic selenium. 

The first compounds with a Aj selenazoline structure were mentioned in the literature in 1892 by Michels (55). He prepared 2-methyl-A -selenazoline in the course of his investigations into sulfur and selenium... 

Selenium heterocycles receive far less mention in the literature than do such homologs as oxazole, thiazole, or imidazole. In fact, preparative methods of selenium heterocycles are much more limited than for the other series, mainly because of manipulatory difficulties arising from the toxicity of selenium (hydrogen selenide is even more toxic) that can produce severe damage to the skin, lungs, kidneys, and eyes. Another source of difficulty is the reactivity of the heterocycle itself, which can easily undergo fission, depending on the reaction medium and the nature of the substituents. 

Mocimycin was prepared from dihydromocimycin (3) by oxidation with selenium dioxide (14,56). 4-Amino-4-dehydroxyefrotomycins were obtained by aminolysis of efrotomycin-4-0-phenylchlorophosphate (57). 

Soda. Ash Roasting. Some of the first processes to recover selenium on a commercial basis were based on roasting of copper slimes with soda ash to convert both selenium and tellurium to the +6 oxidation state. Eigure 1 shows flow sheets for two such processes. Slimes are intensively mixed with sodium carbonate, a binder such as bentonite, and water to form a stiff paste. The paste is extmded or peUetized and allowed to dry. Care in the preparation of the extmdates or pellets is required to ensure that they have sufficient porosity to allow adequate access to the air required for oxidation. 

Inorganic Compounds. Inorganic selenium compounds are similar to those of sulfur and tellurium. The most important inorganic compounds are the selenides, haUdes, oxides, and oxyacids. Selenium oxidation states are —2, 0, +1, +2, +4, and +6. Detailed descriptions of the compounds, techniques, and methods of preparation, and references to original work are available (1—3,5,6—10, 51—54). Some important physical properties of inorganic selenium compounds are Hsted in Table 3. 

Selenides. Selenium forms compounds with most elements. Biaary compounds of selenium with 58 metals and 8 nonmetals, and alloys with three other elements have been described (55). Most of the selenides can be prepared by a direct reaction. This reaction varies from very vigorous with alkah metals to sluggish and requiring high temperature with hydrogen. 

Selenium trioxide, SeO, is white, crystalline, and hygroscopic. It can be prepared by the action of sulfur trioxide on potassium selenate or of phosphorous pentoxide on selenic acid. It forms selenic acid when dissolved in water. The pure trioxide is soluble in a number of organic solvents. A solution in Hquid sulfur dioxide is a selenonating agent. It is stable in very dry atmospheres at room temperature and on heating it decomposes first to selenium pentoxide [12293-89-9] and then to selenium dioxide. 

Some of the other selenium oxyacids are permonoselenic acid [81256-77-17, H2SeO perdiselenic acid [81256-78-2] H2Se20g and pyroselenic acid [14998-61-9], H2Se20y. Selenosulfuric acid, H2SeS02, is not known but its alkah metal salts have been prepared. 

Other inorganic selenium compounds include sodium selenocyanate [4768-87-0], NaSeCN, which is prepared by melting together selenium and sodium cyanide selenocyanogen [27151 -67-3] (SeCN)2 sodium selenosulfate [25468-09-1], Na2SeS02, which is prepared by dissolving selenium in aqueous sodium sulfite (acidification decomposes this compound) and selenate alums, eg, Al2(Se0 2 I SeO [13530-59-1]. 

Organ oselenium compounds, such as phosphine selenides, are being evaluated in solvent extraction systems for silver and gold (63). Also, potential pharmaceuticals containing selenium have been prepared (64). 

Medicine and Nutrition. A stabilized buffered suspension of selenium sulfide has been marketed for many years as Selsun Blue (Abbott Laboratories) for control of seborrheic dermatitis of the scalp. A similar sulfur or selenium sulfide shampoo containing a metallic cation complex has been prepared (122). Topical appHcation of selenium sulfide controls dermatitis, pmritis, and mange iu dogs (see Cosmetics Veterinarydrugs). 

Selenium deficiency has been identified iu humans iu a broad area of China, stretching from the northeast to the southwest. Two syndromes are evident Keshan disease, an endemic cardiomyopathy which affects children, and Kashin-Beck disease, an edemic osteoarthropathy (big-joiat), also mainly involving children, which occurs iu eastern Siberia and parts of China. As a result of these observations, a protective effect for selenium has been proposed, and various selenium-containing preparations have been appHed to preserve animal and human health. 

Silver Selenate. Silver selenate, Ag2Se04, is prepared from silver carbonate and sodium selenate (see Seleniumand selenium compounds). 

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