Colloidal selenium oxide

Selenium dioxide oxidations may be accomplished by heating with neat substrates [5f9], but more often, they are carried out in solvents such as water, tert-butyl alcohol, ethanol, dioxane, acetic acid, and acetic anhydride. The solvent used often affects the outcome of the reaction. The use of acetic acid or acetic anhydride favors oxidation to acetates (and thence to alcohols), whereas in water or dioxane, carbonyl compounds are usually formed. An unpleasant feature of oxidations with selenium dioxide is the formation of red colloidal selenium, which cannot always be separated by distillation but can be removed by treatment of the product with potassium cyanide [522], mercury [523], or deactivated Raney nickel [524]. 

Removal of selenium and selenious acid. Use of selenium dioxide or selenious acid as an oxidant is attended with the difficulty of freeing the reaction mixture of colloidal selenium and of excess oxidant. A procedure for the oxidation of paraldehyde to glyoxal and isolation of the product as the bis-bisulfite addition compound specifies use of lead acetate as more satisfactory for the removal of selenious acid than sulfur dioxide, provided that the solution is kept cool and a large excess is avoided. A... 

Allylic oxidation. Selenium dioxide has been widely used for allylic oxidation of olefins, but this reaction is often troublesome because of formation of selenium-containing by-products and colloidal selenium, which are not easily eliminated. The combination of Se02 with H2O2 to reoxidize selenium species to Set) , has been used to convert olefins into diols and epoxides (2, 362), but this iiicIIuhI is not general for allylic oxidations. Umbreit and Sharpless have effected allylic oxidations with an excess of /-butyl hydroperoxide (90"i.) in combination... 

Removal of colloidal selenium (1, 992-993). The red colloidal form of Se, often obtained in oxidations with SeOa, when heated in DMF precipitates as black granules, which are removable by filtration. ... 

Hayes, KF. Papelis, C. Leckie, J.O. (1988) Modeling ionic strength Effects on anion adsorption at hydrous oxide/solution interfaces. J. Colloid Interface Sd. 78 717—726 Hayes, KF. Roe, A.L. Brown, G.E. Hodgson, KO. Leckie, J.O. Parks, G.A. (1987) In-situ X-ray absorption study of surface complexes Selenium oxyanions on a-FeOOH. Sdence 238 783-786... 

Stable selenium sols may be obtained by the reducing action of quadrivalent titanium. If a solution of titanium trichloride (1-5 per cent.) is boiled for some time, hydrolysis and oxidation occur on addition of this solution to one of selenium dioxide (0-2 per cent.) reduction to selenium occurs and any unchanged titanic acid, Ti(OII)4, remains in colloidal solution and exerts a protective action.4... 

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. 

Methyl and methylene groups adjacent to carbonyl groups are easily oxidized to carbonyls to yield a-keto aldehydes or a-diketones. The reagent of choice is selenium dioxide or selenious acid. The reaction is catalyzed by acids and by acetate ion and proceeds through transition states involving enols of the carbonyl compounds [518]. The oxidation is carried out by refluxing the ketone with about 1.1 mol of selenium dioxide in water, dilute acetic acid, dioxane, or aqueous dioxane [517]. The byproduct, black selenium, is filtered off, but small amounts of red selenium sometimes remain in a colloidal form and cannot be removed even by distillation of the product. Shaking the product with mercury [523] or Raney nickel [524] takes care of the residual selenium. The a-dicarbonyl compounds are yellow oils that avidly react with water to form white crystalline hydrates (equations 407 and 408). 

Selenium dioxide, Seienious acid, SeOj, HsSeOa. Mol. wts. 110.96,128.98. Suppliers Fairmont Chemical Co. K and K Laboratories Matheson, Coleman and Bell (SeOJ Fisher (HjSeO.,). Review and procedures for conversion of recovered Se into SeOa, In oxidations and dehydrogenations with either preformed seienious acid or selenium dioxide and water, usually in acetic acid, ethanol, or dioxane, the solution turns yellow and then red, and later some of the selenium separates in a red form difficult to Alter, some is retained in colloidal solution, and some is bound to an oraanic lubitrita. Elimination of all loleniuin fVom a reaction mixture often oreienti... 

A somewhat similar theory10 postulates the formation of colloidal lead by the decomposition of lead tetraethyl, which deposits on sharp points, edges, and projections in the cylinder which would otherwise aid reaction to an extent that a detonation wave would result. The theory fails to explain the action of organic amines, of di- and tetravalent. selenium, of the colloidal metal sols, and fails to account for the immediate recurrence of knocking when the antiknock dope is discontinued in the gasoline feed. However, tubes coated with lead oxide have been found to reduce the oxidation of hexane more than lead tetraethyl vapors.1158 On the basis of this finding the hypothesis was advanced that oxidation occurs to some extent prior to compression and at the first contact of the gaseous mixture with the hot walls, aud that in the presence of the lead... 

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