Chromate catalysis

Some protagonists of these two views have had a tendency to account for all catalyses in terms of the one idea to the exclusion of the other. In actual fact it appears from the available data that with the possible exception of catalysis by molybdate, which appears to involve only the formation and decomposition of permolybdates, there is not one case which can be unequivocally accounted for in terms of one view only. Thus the chromate catalysis, which on the face of it is an example of the intermediate product mechanism, is more complex than the simple theory implies, and it is probable that in certain circumstances the reduction CrVI —> CrUI and the reverse oxidation also occur, suggesting that compensating reactions are also important. On the other hand, the kinetics of the halide catalyses, which have been the main basis for the theory of. compensating reactions, appear from more recent work to indicate the participation of intermediates probably of a peroxidic nature. 

Fig. 4. Chromate catalysis at different acidities. [K2Cr20 ] = 0.00192 M in each experiment.

Such oxidation reactions may be responsible in part for the enhancement of the chromate catalysis which is produced by Mn++, Co++, Cu++, Ce+++, Ni++ (120,121). Alternatively this promotion may arise from the reaction of these ions with perchromate compounds, and it is possible that chain reactions may occur similar to those in the ferric ion catalysis with the perchromate replacing the peroxide. Uri has suggested such a scheme for promotion in the molybdate and tungstate catalyses (see Sec. IX.3). However the data are too fragmentary for any definite conclusions to be drawn. 

Chromium compounds decompose primary and secondary hydroperoxides to the corresponding carbonyl compounds, both homogeneously and heterogeneously (187—191). The mechanism of chromium catalyst interaction with hydroperoxides may involve generation of hexavalent chromium in the form of an alkyl chromate, which decomposes heterolyticaHy to give ketone (192). The oxidation of alcohol intermediates may also proceed through chromate ester intermediates (193). Therefore, chromium catalysis tends to increase the ketone alcohol ratio in the product (194,195). 

The mechanism and rate of hydrogen peroxide decomposition depend on many factors, including temperature, pH, presence or absence of a catalyst (7—10), such as metal ions, oxides, and hydroxides etc. Some common metal ions that actively support homogeneous catalysis of the decomposition include ferrous, ferric, cuprous, cupric, chromate, dichromate, molybdate, tungstate, and vanadate. For combinations, such as iron and... 

There is brief mention of some iodide-induced exchange of bromate oxygen If correct, this would imply that the rate is limited by (13) or (16). Since, in the reaction with sulphite"the sulphate formed takes oxygen from bromate, that reaction is related to (14), not to (11). In addition to catalysis by chromate , vanadium(V) has been shown to be effective . When oxalate is also present, radicals appear to be produced , as shown by polymerization of acrylonitrile. Vanadium(VI) is suggested as an intermediate . This is of some interest since most authors have regarded one-electron oxidation steps in this whole class of reactions as highly improbable. 

BENSULFOID (7704-34-9) Combustible solid (flash point 405°F/207°C). Finely divided dry materia forms explosive mixture with air. The vapor reacts violently with lithium carbide. Reacts violently with many substances, including strong oxidizers, aluminum powders, boron, bromine pentafluoride, bromine trifluoride, calcium hypochlorite, carbides, cesium, chlorates, chlorine dioxide, chlorine trifluoride, chromic acid, chromyl chloride, dichlorine oxide, diethylzinc, fluorine, halogen compounds, hexalithium disilicide, lampblack, lead chlorite, lead dioxide, lithium, powdered nickel, nickel catalysis, red phosphorus, phosphorus trioxide, potassium, potassium chlorite, potassium iodate, potassium peroxoferrate, rubidium acetylide, ruthenium tetraoxide, sodium, sodium chlorite, sodium peroxide, tin, uranium, zinc, zinc(II) nitrate, hexahydrate. Forms heat-, friction-, impact-, and shock-sensitive explosive or pyrophoric mixtures with ammonia, ammonium nitrate, barium bromate, bromates, calcium carbide, charcoal, hydrocarbons, iodates, iodine pentafluoride, iodine penloxide, iron, lead chromate, mercurous oxide, mercury nitrate, mercury oxide, nitryl fluoride, nitrogen dioxide, inorganic perchlorates, potassium bromate, potassium nitride, potassium perchlorate, silver nitrate, sodium hydride, sulfur dichloride. Incompatible with barium carbide, calcium, calcium carbide, calcium phosphide, chromates, chromic acid, chromic... 

As judged from our ecological model compound sea water, the bioavailability of molybdenum (104 nM) is higher than that of chromium (962pM) (see Table 1.1). While chromium is insoluble as Cr(III) in the Earth s crust, the reduction of molybdate is not as easy as chromate reduction, which leads to a factor of 10 000 when the release of chromium and molybdenum from the Earth s crust into sea water is compared. Together with its low toxicity (Nies 1999), this makes molybdate the prime choice for biochemical reactions requiring oxyanion catalysis (Williams and da Silva 2002). 

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