Chromium trioxide / pyridine / water

When pyridine is added to a solution of chromium trioxide in water, it is possible to obtain a precipitate of the pyridinium salt of dichromic acid, that is pyridinium dichromate (PDC).11... 

An alternative mechanism has also been proposied in which oxidation at the double bond leads to a ketol derivative, elimination of water from which then gives the unsaturated ketone (Scheme 18a)." Limited kinetic data are available and suggest that Scheme 17 is obtained for chromic acid oxidations. The discovery of the chromium trioxide-pyridine complex led to the accessibility of allylic oxidatitm under much less harsh conditions, typically room temperature reaction in dichloromethane solution ... 

A common way to change reaction conditions for the oxidation of alcohols is to modify the acid that is added to the medium. Indeed, chromium trioxide will have different oxidizing abilities in different acids. Since most organic compounds are insoluble in water, a cosolvent is usually required to dissolve not only the chromium reagent but also the alcohol substrate. This solvent must be resistant to oxidation, and acetic acid or acetone are commonly used. For the alcohol - carbonyl conversion several Cr(VI) reagents can be used, including chromium trioxide in water or aqueous acetic acid catalyzed by mineral acid, sodium dichromate in aqueous acetone catalyzed by mineral acid, sodium dichromate in acetic acid, the Cr03 pyridine complex, and err-butyl chromate.Both primary and secondary alcohols can be oxidized to the aldehyde or ketone, respectively. Aldehydes may be oxidized to the carboxylic acid under some conditions. 

A 1-liter flask is equipped with a magnetic stirrer, a thermometer immersed in the reaction mixture, and a drying tube. In the flask is placed 100 ml of anhydrous pyridine, and the flask is cooled in an ice-water bath to 15-20° (lower temperatures impede the complex formation). Chromium trioxide (80 g) is added in small portions to the stirred solvent at a rate so as to keep the temperature below 30°. After about one-third of the chromium trioxide has been added, the yellow complex begins to precipitate. At the end of the addition (about 1 hour), a slurry of the yellow complex in pyridine remains. (This form of the complex is apparently a microcrystalline form and is very difficult to handle.)... 

Alumina has been used in a similar manner. Normally, alumina is added to an aqueous solution of PCC in water, prepared by mixing chromium trioxide, hydrochloric acid (6N) and pyridine. Removal of water leads to the formation of alumina particles covered by PCC, described as PCC on alumina, which is commercially available. Alternatively, it has been described that best results are obtained when alumina and PCC are finely ground in a mortar.231 The alumina not only helps in the work-up by allowing an easy filtering of the chromium-containing by-products, but also accelerates the oxidation with PCC.229a... 

Pyridinium dichromate, prepared from chromium trioxide in a minimum amount of water and pyridine, forms a bright-orange solid and is soluble in water, dimethylformamide, dimethyl sulfoxide, and dimethyl-acetamide sparingly soluble in dichloromethane, chloroform, and acetone and almost insoluble in hexane, toluene, ether, and ethyl acetate. Allylic secondary alcohols are oxidized more rapidly than their saturated analogues. Oxidations are carried out in dichloromethane solutions at 25 °C, and ketones are obtained in high yields (equation 251) [603. ... 

HIDROXILAMINA (Spanish) (7803-49-8) A powerful reducing agent. Aqueous solution is a base. Contact with water or steam causes decomposition to ammonium hydroxide, nitrogen, and hydrogen. Contaminants and/or elevated temperatures above (reported at 158°F/70°C and 265°F/129°C) can cause explosive decomposition. Moisture in air or carbon dioxide may cause decomposition. Violent reaction with oxidizers, strong acids, copper(II) sulfate, chromium trioxide, potassium dichromate, phosphorus chlorides, metals calcium, sodium, zinc. Incompatible with carbonyls, pyridine. Forms heat-sensitive explosive mixtures with calcium, zinc powder, and possibly other finely divided metals. Aqueous solution incompatible with organic anhydrides, acrylates, alcohols, aldehydes, alkylene oxides, substituted allyls, carbonyls, cellulose nitrate, cresols, caprolactam solution, epichlorohydrin, ethylene dichloride, glycols, isocyanates, ketones, nitrates, phenols, pyridine, vinyl acetate. Attacks aluminum, copper, tin, and zinc. 

A soln. of a gram of chromium trioxide in 100 c.c. of water is treated with 10 grms. of pyridine, and 25 c.c. of a 3 per cent. soln. of hydrogen dioxide at room temp. When the mixture is allowed to stand for a short time, blue crystals of HCrO. CsHjN separate out. 

Jones s oxidation of a primary alcohol leads to a carboxylic acid in most cases. A mixture of chromium trioxide and pyridine gives a reagent that can oxidize a primary alcohol to an aldehyde. This is called Collins s oxidation. The reaction of chromium trioxide and pyridine in aqueous HCl leads to pyridinium chlorochromate, called PCC. The reaction of chromium trioxide and pyridine in water leads to pyridinium dichromate (PDC). Both PCC and PDC can oxidize a secondary alcohol to a ketone or a primary alcohol to an aldehyde. 

Ethyl sulfate Flammable liquids Fluorine Formamide Freon 113 Glycerol Oxidizing materials, water Ammonium nitrate, chromic acid, the halogens, hydrogen peroxide, nitric acid Isolate from everything only lead and nickel resist prolonged attack Iodine, pyridine, sulfur trioxide Aluminum, barium, lithium, samarium, NaK alloy, titanium Acetic anhydride, hypochlorites, chromium(VI) oxide, perchlorates, alkali peroxides, sodium hydride... 

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