Oxidizing agents chromium compounds

Low Oxidation State Chromium Compounds. Cr(0) compounds are TT-bonded complexes that require electron-rich donor species such as CO and C H to stabilize the low oxidation state. A direct synthesis of Cr(CO)g, from the metal and CO, is not possible. Normally, the preparation requires an anhydrous Cr(III) salt, a reducing agent, an arene compound, carbon monoxide that may or may not be under high pressure, and an inert atmosphere (see Carbonyls). 

All compounds of chromium are colored the most important are the chromates of sodium and potassium and the dichromates and the potassium and ammonium chrome alums. The dichromates are used as oxidizing agents in quantitative analysis, also in tanning leather. 

In order to circumvent this problem, there has been significant activity directed toward the search for a less environmentally toxic and more selective oxidizing agent than chromium. For example, Hoechst has patented a process which uses organorhenium compounds. At a 75% conversion, a mixture of 86% of 2-methyl-l,4-naphthoquinone and 14% 6-methyl-l,4-naphthoquinone was obtained (60). Ceric sulfate (61) and electrochemistry (62,63) have also been used. 

Ghromium(III) Compounds. Chromium (ITT) is the most stable and most important oxidation state of the element. The E° values (Table 2) show that both the oxidation of Cr(II) to Cr(III) and the reduction of Cr(VI) to Cr(III) are favored in acidic aqueous solutions. The preparation of trivalent chromium compounds from either state presents few difficulties and does not require special conditions. In basic solutions, the oxidation of Cr(II) to Cr(III) is still favored. However, the oxidation of Cr(III) to Cr(VI) by oxidants such as peroxides and hypohaUtes occurs with ease. The preparation of Cr(III) from Cr(VI) ia basic solutions requires the use of powerful reducing agents such as hydra2ine, hydrosulfite, and borohydrides, but Fe(II), thiosulfate, and sugars can be employed in acid solution. Cr(III) compounds having identical counterions but very different chemical and physical properties can be produced by controlling the conditions of synthesis. 

A variety of methods have been described to solve the task in solution.16 Common oxidative agents for this transformation include various heavy-metal reagents such as chromium-or ruthenium-based oxides, pyri-dine-S03, and dimethylsulfoxide (DMSO) in combination with acetic anhydride, carbodiimide, or oxalyl chloride for activation. One of the most prominent methods for the reliable conversion of sensitive compounds is the Dess-Martin reagent or its nonacetylated equivalent, 1-hydroxy-(17/)-benzo-l,2-iodoxol-3-one-l-oxide (2-iodoxybenzoic acid, IBX). 

The preparation of potassium dichromate (Preparation 61) illustrated how chromic oxide, Cr203, can be oxidized to a chromate in which chromium exists as Cr03. For the preparation of chromic alum, it might seem as if chromic oxide or the natural chromite should yield chromic sulphate directly on treatment with sulphuric acid. This is impossible, however, because both of these substances are very resistant to the action of acids. Practically, they yield only to the action of alkaline oxidizing agents, which convert them into a chromate. Therefore potassium, or sodium, dichromates are always the products made directly from the mineral, and these serve as the materials from which other compounds of chromium are prepared. To make chromic alum from potassium dichromate it is necessary to reduce the chromium to the state of oxidation in which it originally existed in the mineral, and to add sufficient sulphuric acid to form the sulphates of potassium and... 

The most stable state of chromium is the +3 state compounds of hexavalent chromium are almost as good oxidizing agents as elemental chlorine, whereas compounds of Cr(II) ( chromous compounds) are potentiometrically more easily oxidized than cadmium metal. Divalent chromium, like Ag(II) and Au(III), may exist in equilibrium with aqueous media only as the cation of a relatively insoluble salt or in a slightly dissociated complex. However, solutions containing the blue Cr24 ion may be... 

The idea that complex formation may be important in the catalytic process can be carried further. It has been found, for example, that bis-arene chromium complexes supported on silica-alumina are active for ethylene polymerization. These catalysts are prepared by activating the support alone in the usual manner and then impregnating with a hydrocarbon solution of the bis-arene compound at room temperature in the absence of air or other oxidizing agent. 

Aluminophosphates (A1P04) were discovered in 198248 and a large amount of research has been directed towards the incorporation of various elements into the framework of these molecular sieves 49 A particular area of study is the oxidation of primary and secondary alcohols to the corresponding carbonyl compounds, which are useful synthetic intermediates. Traditionally, alcohol transformations are performed with stoichiometric chromium(VI) reagents.50 However, due to environmental problems associated with chromium-containing effluent, attention has focused on the use of chromium in conjunction with oxidizing agents such as tert-butyl hydroperoxide.51 Sheldon and co-workers... 

Less than 15% of the ore is transformed into chromium compounds, principally chromates, dichromates, chromium(VI) oxide, chromium(III) oxide, and so on. Alkaline oxidative roasting of chromite in rotary kilns yields sodium chromate (see equation 1), which is leached out with water and typically converted into sodium dichromate with sulfimc acid (equation 2) or carbon dioxide (equations). Fiuther treatment of sodium dichromate with sulfuric acid yields chromium(VI) oxide ( chromic acid ), while its reduction (with carbon, sulfur, or anuuo-nium salts) produces chromium(III) oxide. Finally, basic chromium(III) salts, for example Cr(0H)S04, which are used as tanning agents for animal hides, also result from reduction of sodium dichromate. Heterogeneous chromium catalysts are used for the polymerization of ethylene. 

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