Chromic oxide, preparation

The colloidal soln. of hydrated chromic oxide prepared as just indicated was shown by W. Biltz, and W. Herz to be a positive h drosol because it migrates to the cathode under the influence of an electrical stress. On the other hand, the... 

Chromium carbide can be best prepared from pure chromic oxide [1308-38-9] (see Chromium compounds). Compacts containing 74 wt %... 

Methyl ketones are important intermediates for the synthesis of methyl alkyl carbinols, annulation reagents, and cyclic compounds. A common synthetic method for the preparation of methyl ketones is the alkylation of acetone derivatives, but the method suffers limitations such as low yields and lack of regioselectivity. Preparation of methyl ketones from olefins and acetylenes using mercury compounds is a better method. For example, hydration of terminal acetylenes using HgSO gives methyl ketones cleanly. Oxymercuration of 1-olefins and subsequent oxidation with chromic oxide is... 

Chromic Perchlorate. Cr(C104)3, mw 350.30, OB to CrO HCI +28.5% only the hexahydrate has been prepd as blue-green delq crysts, mp 94° (Refs 1, 5 6) CA Registry No 13537-21-8 Preparation. By dissolving hydrated chromic oxide in 20% perchloric ac followed by-evapn of the soln (Ref 4)... 

Chromium carbide can be best prepared from pure chromic oxide [1308-38-9], Cr203 (see Chromium compounds). Compacts containing 74 wt % Cr203 and 26 wt % carbon black can be heated in carbon-tube furnaces at 1600°C in the presence of hydrogen, giving a carbide containing 13—13.3 wt % total C and 0.1—0.3 wt % free C. 

Laboratory oxidation of alcohols most often is carried out with chromic acid (H2Cr04), which usually is prepared as required from chromic oxide (Cr03) or from sodium dichromate (Na2Cr207) in combination with sulfuric acid. Ethanoic (acetic) acid is a useful solvent for such reactions ... 

On account of the difficulty of carrying out the industrial process on a laboratory scale, the following less economical procedure is recommended The raw material, chromic oxide, Cr203, is first treated with fused KOH. The oxidizing agent, KN03, is then added and it works more effectively than if it had been added at the outset. The aqueous extract of this fusion is allowed to crystallize to obtain a preparation of potassium chromate. The mother liquor from these crystals is acidified With acetic acid to obtain a preparation of potassium dichromate. Potassium compounds are prepared because they are less soluble and crystallize better than the corresponding sodium compounds. 

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... 

Acetyl-n-valeric acid has been prepared by the oxidation of 1-methylcyclohexene with potassium permanganate 5 by the oxidation of 2-methylcyclohexanone with chromic oxide and sulfuric acid 6 by the reaction of methylzinc iodide on the ethyl ester of adipic acid chloride and saponification of the ethyl ester of 5-acetyl-w-valeric acid so obtained 7 by the saponification of the ethyl ester of diacetylvaleric acid 2 and through the hydrolysis of ethyl a-acetyl-6-cyanovalerate with boiling 20% hydrochloric acid.3... 

At an early date it was already recognized that the ketone (IX) derived from an oxidation of the C-18 carbinol function of methyl reserpate could be of considerable utility for further transformation of the reserpine pentacyclic ring system, but early attempts at the preparation of the desired compound by conventional oxidation, e.g., by Oppenauer s method, AAchlorosuceinimide, sodium dichromate, or chromic oxide in pyridine, were unsuccessful with both methyl reserpate and methyl 18-epireserpate. The ketone was finally obtained by heating methyl reserpate p-bromobenzene sulfonate with dimethyl sulfoxide in the presence of triethylamine (162), a method successfully used for simpler compounds (163). Subsequently, it was found that this oxidation could also be realized with other benzene sulfonate esters of methyl reserpate and 18-epireserpate. That the stereochemistry of the molecule was unaffected was proved by sodium borohydride reduction of the ketone, which gave equal amounts of methyl reserpate and its 18-epimer. This and other simple reactions of the ketone are sketched in Chart III, and additional observations will be given. 

As the imidazole nucleus does not undergo Friedel-Crafts acylation, ketone substituents must be introduced indirectly, either before the ring is formed34,41,210,275,435 or by modification of existing substituents.210 Roe210 prepared 2-acetylimidazole (92) by oxidation of l-(imidazol-2 -yl)ethanol (91) with chromic oxide in pyridine. The acyl-substituted imidazoles have distinctive infrared and ultraviolet... 

Metallic Chromium. The metal can be prepared by reducing chromic oxide with metallic aluminum. The two reactants in the form of fine powder are mixed, and the mixture is ignited. The heat liberated by the reaction is great enough to produce molten chromium ... 

Chromic oxide jellies may be formed by adding sodium or potassium hydroxide or ammonia to a solution of chromic sulphate or chloride containing sodium acetate or by adding sodium or potassium hydroxide, hut not ammonia, to a solution of chrome alum. The jelly is violet if prepared by the addition of ammonia or of a slight excess of the alkali metal hydroxide if the latter is added in larger quantity the jelly is green. The jellies dissolve in hydrochloric acid, but re-form on neutralising the solution if sufficient sodium acetate is present. 

The pyrazine ring is relatively stable to oxidation, and many pyrazine-carboxylic acids have been prepared from quinoxalines, phenazines, and other fused pyrazines by oxidation with potassium permanganate. This reaction has been most used for the oxidation of quinoxalines, for example, 77 -> 78. The oxidations are usually carried out with potassium permanganate in alkali (397), but may also be effected without added base (398). Crippa and Perroncito (399) have also used chromic oxide in acetic acid-acetic anhydride to oxidize benzo(/]quinoxalines (79). Pyrazines prepared by this method are summarized in Table 11.9 (397-419). 

The main applications of oxidation with chromium trioxide are transformations of primary alcohols into aldehydes [184, 537, 538, 543, 570, 571, 572, 573] or, rarely, into carboxylic acids [184, 574], and of secondary alcohols into ketones [406, 536, 542, 543, 575, 576, 577, 578, 579, 580, 581, 582, 583, 584]. Jones reagent is especially successful for such oxidations. It is prepared by diluting with water a solution of 267 g of chromium trioxide in a mixture of 230 mL of concentrated sulfuric acid and 400 mL of water to 1 L to form an 8 N CrOj solution [565, 572, 579, 581, 585, 556]. Other oxidations with chromic oxide include the cleavage of carbon-carbon bonds to give carbonyl compounds or carboxylic acids [482, 566, 567, 569, 580, 587, 555], the conversion of sulfides into sulfoxides [541] and sulfones [559], and the transformation of alkyl silyl ethers into ketones or carboxylic acids [590]. 

For verification, dibenzenechromium bichromate [(C6H6)2Cr]2 Cr207 was prepared from the reaction of dibenzenechromium hydroxide and chromic oxide (CeHo)o CrOH CrOa. The resultant catalyst showed catalytic activity analogous to the original catalyst. 

In aqueous solution. Use of a solution of chromic acid prepared from dilute sulfuric acid and sodium dichromate dihydrate is illustrated by the oxidation of hydroquinone to quinone, isolated by a tedious process in 86-92% yieid. ... 

The complexity of chromic oxide oxidation of car-3-ene has already been mentioned. The reaction of p-toluenesulphonylhydrazones with organolithium compounds has been used to prepare cis-car-4-ene (285) from the corresponding carone (284) if car-5-one (286) is used, the w-menthadiene (287) is also obtained. 

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