Chromic acid on silica

The original recipe involved the aqueous impregnation of chromic acid on silica, although nowadays less-poisonous chromium(III) salts are used. Over the years, a family of Phillips-type catalysts has emerged producing no less than 50 different types of polyethylene, and this versatility is the reason for the commercial success of the Phillips ethylene polymerization process. The properties of the desired polymer product can be tailored by varying parameters such as calcination temperature, polymerization temperature and pressure, by adding titania as... 

Several references have appeared on the use of solid-phase oxidants. Solid potassium permanganate-copper sulphate mixtures oxidize secondary alcohols to ketones in high yield, and pyridinium chromate or chromic acid on silica gel are described as convenient off-the-shelf reagents for oxidation of both primary and secondary alcohols. Anhydrous chromium trioxide-celite effects similar transformations only when ether is present as co-solvent. An excellent review, with over 400 references, on supported oxidants covers the use of silver carbonate-celite, chromium trioxide-pyridine-celite, ozone-silica, chromyl chloride-silica, chromium trioxide-graphite, manganese dioxide-carbon, and potassium permanganate-molecular sieve. 

Chromic acid is a very strong oxidizing agent, but its reactivity may be tempered by adsorption onto a support. Chromic acid on alumina was found to be inactive,but on silica it gives instantaneous oxidation, in diethyl ether at room temperature, of primary and secondary alcohols to aldehydes and ketones in good to excellent yield. This reagent can also be conveniently used in the form of a column. 

Chromic acid adsorbed on silica gel (Cr03 Si02 = 3 10) is a mild and safe reagent to oxidise saturated or unsaturated alcohols in carbon tetrachloride to the corresponding aldehydes. 

The oxidation of sulfides to the corresponding sulfoxides and sulfones proceeds under rather strenuous conditions requiring strong oxidants such as nitric acid, hydrogen peroxide, chromic acid, peracids, and periodate. Using MW irradiation, this oxidation is achievable under solvent-free conditions and with desired selectivity to either sulfoxides or sulfones using 10% sodium periodate on silica (Scheme 6.34)... 

Compounds containing susceptible C—H bonds can be oxidized to alcohols.134 Nearly always, the C—H bond involved is tertiary, so the product is a tertiary alcohol. This is partly because tertiary C—H bonds are more susceptible to free-radical attack than primary and secondary bonds and partly because the reagents involved would oxidize primary and secondary alcohols further. In the best method the reagent is ozone and the substrate is absorbed on silica gel.135 Yields as high as 99% have been obtained by this method. Other reagents, which often give much lower yields, are chromic acid,136 alkaline permanganate,137 potassium... 

Finally, the use of some chromic acid species deposited on silica particles must be mentioned.357... 

Low temperature controlled hydrolysis of the -Cr02Cl surface groups is used for the synthesis of chromium oxide layers on the silica surface. The mechanisms of the chemical conversions have been studied in great detail and it has been found that grafted chromium oxide associates (clusters) are formed, based on the condensation of chromic acid molecules with the surface silanol groups.38,39,40 Figure 11.4 shows the monodentate and bidentate grafted structures. 

Chromic acid deposited on silica gel from the anhydride CrOs in aqueous (or in aqueous acidified) solutions affords a useful oxidant. 

Oxidation of Alcohols to Aldehydes and Ketones with Chromic Acid Adsorbed on Silica Gel [5J7]... 

Unfortunately, the catalyst can also become deactivated during the calcination, by several processes. Bulk hexavalent chromium oxide, CrC>3, or chromic acid, is unstable at temperatures above approximately 200 °C and begins to decompose into the trivalent oxide Cr2C>3 [39,40,42], On the catalyst, it is only the esterification with silica that stabilizes chromium in the hexavalent form at temperatures up to 900 °C. However, the chromate or dichromate ester can be hydrolyzed by water vapor present in the air used for the catalyst activation, as shown in Scheme 53. When this happens at elevated temperatures, decomposition to Cr(III) occurs. In the presence of water vapor and traces of Cr(VI), large crystallites of a-chromia are formed [74,75,134,135,731-733], which can be very difficult to reoxidize and disperse. 

Oxidation. Chromic acid adsorbed on silica gel is useful for oxidation of primary and secondary alcohols to carbonyl compounds. Yields are generally 80-907o- In contrast, H2CrOi adsorbed on alumina is ineffective. ... 

For monitoring the hydrolysis, 1— 5 jd of the reaction mixture is applied to silica gel G layers on narrow plates and developed with petrol ether (60—70° C)-diethyl ether-acetic acid (70 + 30 + 2). The lipid fractions are visualised by spraying the chromatogram with the chromic acid-sulphuric acid reagent (No. 46) and heating. At the beginning of saponification, spots of substances of low polarity are visible ahead of the fatty acid spot. After complete hydrolysis, only the spots of the acids, of the non-polar compounds (hydrocarbons) ahead of them and of the strongly polar compoimds (mono- and polyalcohols) behind them are visible. 

Fig. USB. Two-dimensional fractionation of human serum lipids on a layer partly impregnated with silver nitrate [182]. Adsorbents silica gel G and silica gel G-silver nitrate (5%) solvents 1 direction petrol ether (BP 40—60° C)-diethyl ether-acetic acid (85 + 15 + 1), 2 direction petrol ether (BP 40—60° C)-diethyl ether-acetic acid (75 + 25 + 1) visualisation charring by heating with chromic acid/ sulphuric acid amount 300 fxg 26 Thin-Layer Chromatography, 2nd Edition...

Methods of Detection. Ozonides may often be detected as white zones on silica gel G layers without using any indicator. 2, 7 -Dichlorofluorescein solution (Rgt. No. 64) is a convenient spray reagent chromic acid-sulphuric acid (Hgt. No. 46) can be used for charring ozonides [170]. 

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