Chromium trioxide

Chromium trioxide. CrOj. Red precipitate from [Cr04p plus cone. H2SO4, m.p. 198 C, loses oxygen at 420" C. CrOa is a powerful oxidizing agent and is used as such. Acidic, gives [Cr04] - with water. 

Anthracene is oxidised by chromium trioxide, Cr04, to anthraquinone. As the reaction is carried out in solution, a solvent is required which will dissolve both the anthracene and the chromium trioxide, and at the same time be... 

Required Anthracene, lo g. acetic acid, 150 ml. chromium trioxide, 20 g. 

Required. Anthracene, i g. chromium trioxide, 2 g. glacial acetic acid, 15 ml. 

Dissolve 1 g. of anthracene in 10 ml. of glacial acetic acid and place in 50 ml. bolt head flask fitted with a reflux water-condenser. Dissolve 2 g. of chromium trioxide in 2 ml. of water and add 5 ml. of glacial acetic acid. Pour this solution down the condenser, shake the contents of the flask and boil gently for 10 minutes. Cool and pour the contents of the flask into about 20 ml. of cold water. Filter off the crude anthraquinone at the pump, wash with water, drain well and dry. Yield, 1 g. Purify by re crystallisation from glacial acetic acid or by sublimation using the semi-micro sublimation apparatus (Fig. 35, p. 62, or Fig. 50, p. 70). 

Equip a I litre three-necked flask with a mechanical stirrer and a thermometer, and immerse the flask in a bath of ice and salt. Place 306 g. (283 ml.) of acetic anhydride, 300 g. (285 ml.) of glacial acetic acid and 25 g. of p-nitrotoluene in the flask, and add slowly, with stirring, 42 5 ml. of concentrated sulphuric acid. When the temperature has fallen to 5°, introduce 50 g. of A.R. chromic anhydride in small portions at such a rate that the temperature does not rise above 10° continue the stirring for 10 minutes after all the chromium trioxide has been added. Pour the contents of the flask into a 3 litre beaker two-thirds filled with crushed ice and almost fill the beaker with cold water. Filter the solid at the pump and wash it with cold water until the washings are colourless. Suspend the product in 250 ml. of cold 2 per cent, sodium carbonate solution and stir mechanically for 10-15 minutes filter (1), wash with cold water, and finally with 10 ml. of alcohol. Dry in a vacuum desiccator the yield of crude p-nitrobenzal diacetate is 26 g. (2),... 

Anthraqiiinone may also be prepared by the oxidation of pure anthracene with a solution of chromium trioxide in glacial acetic acid ... 

Tertiary alcohols are usually degraded unselectively by strong oxidants. Anhydrous chromium trioxide leads to oxidative ring opening of tertiary cycloalkanols (L.F. Fieser, 1948). 

We shall describe a specific synthetic example for each protective group given above. Regiosdective proteaion is generally only possible if there are hydroxyl groups of different sterical hindrance (prim < sec < tert equatorial < axial). Acetylation has usually been effected with acetic anhydride. The acetylation of less reactive hydroxyl groups is catalyzed by DMAP (see p.l44f.). Acetates are stable toward oxidation with chromium trioxide in pyridine and have been used, for example, for protection of steroids (H.J.E. Loewenthal, 1959), carbohydrates (M.L. Wolfrom, 1963 J.M. Williams, 1967), and nucleosides (A.M. Micbelson, 1963). The most common deacetylation procedures are ammonolysis with NH in CH OH and methanolysis with KjCO, or sodium methoxide. 

The most abundant natural steroid is cholesterol. It can be obtained in large quantides from wool fat (15%) or from brain or spinal chord tissues of fat stock (2-4%) by extraction with chlorinated hydrocarbons. Its saturated side-chain can be removed by chromium trioxide oxidation, but the yield of such reactions could never be raised above 8% (see page 118f.). 

Nitric acid, fuming Organic matter, nonmetals, most metals, ammonia, chlorosulfonic acid, chromium trioxide, cyanides, dichromates, hydrazines, hydrides, HCN, HI, hydrogen sulflde, sulfur dioxide, sulfur halides, sulfuric acid, flammable liquids and gases... 

Pyridine Chlorosulfonic acid, chromium trioxide, formamide, maleic anhydride, nitric acid, oleum, perchromates, silver perchlorate, sulfuric acid... 

CH3I (methyl iodide) principal axes, 103 If rotation, 113 CH2NH (methanimine) interstellar, 120 Cr203 (chromium trioxide) in alexandrite laser, 347ff in ruby laser, 346ff HC3N (cyanoacetylene) interstellar, 120 HCOOH (formic acid) interstellar, 120 NH2CN (cyanamide) interstellar, 120... 

Usually, organoboranes are sensitive to oxygen. Simple trialkylboranes are spontaneously flammable in contact with air. Nevertheless, under carefully controlled conditions the reaction of organoboranes with oxygen can be used for the preparation of alcohols or alkyl hydroperoxides (228,229). Aldehydes are produced by oxidation of primary alkylboranes with pyridinium chi orochrom ate (188). Chromic acid at pH < 3 transforms secondary alkyl and cycloalkylboranes into ketones pyridinium chi orochrom ate can also be used (230,231). A convenient procedure for the direct conversion of terminal alkenes into carboxyUc acids employs hydroboration with dibromoborane—dimethyl sulfide and oxidation of the intermediate alkyldibromoborane with chromium trioxide in 90% aqueous acetic acid (232,233). 

Concentration Effects. The reactivity of ethyl alcohol—water mixtures has been correlated with three distinct alcohol concentration ranges (35,36). For example, the chromium trioxide oxidation of ethyl alcohol (37), the catalytic decomposition of hydrogen peroxide (38), and the sensitivities of coUoidal particles to coagulation (39) are characteristic for ethyl alcohol concentrations of 25—30%, 40—60%, and above 60% alcohol, respectively. The effect of various catalysts also differs for different alcohol concentrations (35). 

There are many synthetic routes to alloxan. Probably the best is direct oxidation of barbituric acid (1004 R = H) with chromium trioxide (5208(32)6) but it may be made from barbituric acid via its benzylidene derivative by direct or indirect oxidation of uric acid from 5-chlorobarbituric acid (1004 R = C1) by nitration or from 5-nitrobarbituric acid (1004 R = N02) by chlorination, both via the intermediate (1005) (64M1057) or by permanganate oxidation of uracil (1006) under carefully controlled conditions (73BSF1167). 

Appropriate pyrido[2,3-d]pyrimidin-5-ones with formyl groups in the 6-position have been oxiized to piromidic (68) and pipemidic (69) acids, or to intermediates for these, using moist silver oxide, chromium trioxide (potassium dichromate), potassium permanganate or, alternatively, sodium chlorite/hydroxylamine-O-sulfonic acid. 6-Acetyl groups have been similarly oxidized using sodium hypobromite in aqueous dioxane, whilst 2-acetyl groups give dimethylaminomethylene derivatives en route to 2-pyrazolylpyrido[2,3-d]pyrimidines. 

Again, as with pyridopyrimidines, the main reaction is oxidation of di- or poly-hydro derivatives to fully aromatic structures, often merely by air or oxygen. In some cases the reagent of choice is mercury(II) oxide, whilst other reagents used include sulfur, bromine, chloranil, chromium trioxide-acetic acid, hydrogen peroxide, and potassium ferricyanide, which also caused oxidative removal of a benzyl group in the transformation (306) (307)... 

Chromium trioxide (chromic anhydride) [1333-82-0] M 100.0, m 197°, dec at 250° to Cr203, d 2.70 (pK 0.74, pK 6.49, for H2Cr04, chromic acid). Red crystals from water (0.5mL/g) between 100° and -5°, or from water/conc HNO3 (1 5). It separates when potassium or sodium dichromate are dissolved in cone H2SO4. Dried in a vacuum desiccator over NaOH pellets hygroscopic, powerful oxidant, can ignite with organic compounds. It is a skin and pulmonary IRRITANT. 

The chromium trioxide was of 97% purity. Cooling is necessary on dissolving chromium trioxide in acetic anhydride. Caution. Addition of acetic anhydride to solid chromium trioxide has resulted in explosive decompositions. The trioxide should be added in small portions to the cooled anhydride. 

A similar procedure may be used for the preparation of /)-cyanobenzaldiacetate from -tolunitrile. Information submitted by Rorig and Nicholson, of G. D. Searle and Company, indicates that the critical step in this preparation is to maintain the reaction temperature below 10° throughout the process. Exposure of -cyanobenzaldiacetate to excess chromic, acetic, and sulfuric acids causes a reduction in yield. During the oxidation care should be taken to prevent chromium trioxide from adhering to the walls of the flask above the reaction mixture and then dropping in large amounts into the solution. 

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