Selenium powdered, black

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

Sebacamic acid, methyl ESTER, 25, 71 Sebacamide, 25, 95 Sebacic acid, 25, 95 Sebaconitrile, 25, 95 27, 18 Selenious acid, 24, 61, 62 Selenium, 24, 63 27, 33 powdered, black, 24, 90 Selenium compounds, ease of oxidation, 24,91... 

A procedure similar to that described for Section 9-A may be used to prepare this compound from lithium (0.28 g, 40 mmole), benzenethiol (4.1 mL, 40 mmole), iron(III) chloride (1.62 g, 10 mmole), selenium powder (0.79 g, 10 mmole), and tetrabutylammonium iodide (2.77 g, 7.5 mmole). The black precipitate should be collected by filtration, washed copiously with methanol, and dried in vacuo. This solid can be recrystallized by dissolution in the minimum volume of acetonitrile at 45°, adding methanol to incipient crystallization at this temperature and allowing the solution to cool slowly to room temperature before leaving it overnight at ca. -5°. The compound (2.9 g, 80%) crystallizes as large, black prisms. 

For the preparation of the diselenide, the monoselenide [e.g., made from (9H-9-BBN)2 and selenium powder (see above)] is heated at 120° with an equivalent amount of black selenium powder until the elemental selenium is dissolved. The one-step synthesis of the diselenide from (9H-9-BBN)2 and selenium is also possible. However, the product is not as pure as in the two-step process, as it is normally contaminated with some selenol and the products thereof (see the preparation of the CgHi4B sulfides). - ... 

In a 250-mL flask (with magnetic stirrer, inside thermometer tube, and reflux condenser with inert-gas bypass and bubbler) a stirred mixture of bis(l,5-cyclooctanediylboryl)monoselenide (BSeB) (10.65 g, 33.2 mmol) and black selenium powder (2.62 g, 33.2 mmol) in mesitylene (120 mL) is heated 2 h at... 

Finely divided free selenium reacts with mercury vapor to produce black mercury selenide. Suitable reagent paper is prepared by soaking filter paper in a solution of potassium selenocyanate (KCNSe), draining, and exposing to an atmosphere of hydrogen chloride (hood ). The potassium selenocyanate solution is readily prepared by vigorously shaking potassium cyanide solution with an excess of pure selenium powder in a stoppered flask. 

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. 

In a 500-cc. beaker are placed 360 g. (6.4 moles) of powdered potassium hydroxide and 240 g. (3 gram atoms) of black powdered selenium, which have been previously ground together in a mortar. This mixture is heated (Note 1) in an oil bath at 140-150° until a thick, dark red liquid is formed (Note 2), and then it is added (Note 3) in small portions to 400 cc. of ice water in a 5-I. flask. The solution is kept in an ice bath until used. 

Amorphous forms exhibit two colors, occurring as a red powder of density 4.26g/cm3 that has a hexagonal crystal structure and a black vitreous solid of density 4.28g/cm3. The red amorphous selenium converts to the black form on standing. Amorphous selenium melts at 60 to 80°C insoluble in water reacts with water at 50°C when freshly precipitated soluble in sulfuric acid, benzene and carbon disulfide. 

Extraction.—(1) From Pyrites.—In the oxidation of the pyrites (or other sulphur mineral) for the formation of sulphur dioxide in the manufacture of sulphuric acid, foreign elements like arsenic and selenium also undergo oxidation and pass ofC as vapours with the sulphur dioxide. The selenium dioxide produced in this manner their suffers more or less complete reduction by the sulphur dioxide, when finely divided selenium separates, mainly in the lead chambers, as a red, amorphous powder, accompanied possibly by some of the greyish-black form a portion of the dioxide is also found in the Glover tower acid. The amount of selenium in the chamber mud depends, of course, on the nature of the pyrites relatively large quantities of compounds of arsenic, zinc, tin, lead, iron, copper or mercury are always present, arising almost entirely from impurities in the pyrites. 

Aluminium and magnesium selenides are very similar light brown powders, unstable in air. Zinc and iron (ferrous) selenides are more stable in air, the zinc compound being citron-yellow and the iron compound black and metallic in appearance.8 The latter becomes brown in air owing to oxidation. Ferric selenide is difficult to obtain pure. Cadmium selenide, which is dark brown, is very stable in colour and is used as a pigment. With thallium, selenium is said to form three distinct compounds,9 but analyses of these compounds have led to discordant results. The selenides of aluminium, chromium and uranium cannot be prepared in the wet way. Nickel selenide, unlike the sulphide, shows no tendency to form a colloidal solution. 

Dimethyl selenide, (CH3)2Se.—An intimate mixture of selenium and amorphous phosphorus (5 2) is heated, and the black mass powdered and heated in a retort with concentrated sodium hydroxide and potassium methyl sulphate. The mixture foams and a yellow liquid distils over. This is separated from the water and fractionated to remove dimethyl diselenide, which is present in small quantity. 

AlkylSe and alkylTe groups can be introduced by reaction with diselenides and ditellurides, but the desired products can be prepared in a more practical and economical way, by successive addition of the elements and the alkyl halide. The insertion of the elements proceeds most easily in liquid ammonia, provided that grey Te powder and red Se powder is used (black Te powder, obtained by precipitation is unreactive, probably because of the presence of an oxide coating, while the black modification of Se,"selenium nigrum" is also less reactive). In Et20 or THF, temperatures in the range 0-20 C are necessary for the dissolution of the elements. 

VII.12 TELLURIUM, Te (Ar 127 60) - TELLURITES, TeOf Tellurium is less widely distributed in nature than selenium both elements belong to Group VI of the periodic system. When fused with potassium cyanide, it is converted into potassium telluride, K2Te, which dissolves in water to yield a red solution. If air is passed through the solution, the tellurium is precipitated as a black powder (difference and method of separation from selenium). Selenium under similar conditions yields the stable potassium selenocyanide, KSeCN the selenium may be precipitated by the addition of dilute hydrochloric acid to its aqueous solution. 

Sulphur dioxide complete precipitation of tellurium from dilute (1-5m) hydrochloric acid solutions as a black powder. In the presence of much concentrated hydrochloric acid, no precipitate is formed (difference and method of separation from selenium). 

As in the procedure in Section A, powdered gray selenium (1.38 g, 15.0 mmol) is treated with Super Hydride (6.0 mL of a 1 M solution of Li[BEt3H] in THF, 6.0 mmol) to give a brown solution of lithium polyselenides. A suspension of TiCp2Cl2 (0.75 g, 3.0 mmol) in dry THF (50 mL) is purged with N2 and added dropwise to the lithium polyselenides. The reaction mixture becomes purple it is stirred under N2 at ambient temperature for 4.75 h. The volatile components are removed in vacuo (0.01 torr). The black residue is extracted under N2 with CH2CI2 (6 x 50 mL), and the solution is filtered under N2 through Celite (5 mL). The purple filtrate is concentrated in vacuo to 50 mL... 

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