Oxidation by Chromium VI

The stoichiometric equation for oxidation of vanadium(IV) by chromium(Vl) in acid perchlorate solutions is essentially 

That HCr04, VO and VO2 are the predominant species under the conditions of Espenson s kinetic study originates from evidence cited by Tong and King, and Rossotti and Rossotti, respectively. No binuclear species are detectable in the Cr(III) product. Vanadium(V) retards the reaction and the full form of the rate law is 

If mechanism (A) applied the Cr(VI)+V(IV) system would be anomalous when compared with the Cr(VI) + Fe(II) and Ce(IV) + Cr(III) reactions which have similar rate laws and Cr(V) - Cr(IV) transformations as rate-controlling steps. Apart from this there are other good reasons for rejecting mechanism (+). At 25 °C, K is 10 ° and k is 0.56 l.mole sec , allowing At2 to be calculated as 

The equilibrium constant has been measured spectrophotometrically as 98 l.mole at 25 °C and fi = I M. 

consisting of two Cr atoms and one V atom, of composition (HCr207V /M HjO). A reasonable mechanism, similar to B), would have CT20 as a reactant in the first step and a dimer of Cr(V) and Cr(VI) as the intermediate in the slow stage. 

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Primary and secondary amines, double bonds, aldehydes, sulfides and certain aromatic and dihydroaroraatic systems are also oxidized by chromium VI reagents under standard hydroxyl oxidizing conditions. Amines are commonly protected by salt formation or by conversion to amides. Aldehydes and... 

Kinetics and Mechanism of Paracetamol Oxidation by Chromium(VI) in Absence and Presence of Manganese(II) and Sodium dodecyl Sulphate... 

V02+) by chromium(VI) (HCrO ), when carried out in the presence of iodide ions, results in formation of triiodide ions, I3,4 This reaction occurs rather rapidly, whereas both HCrO and vanadium(V) (VOJ, a product) oxidize I- so slowly that these reactions can be ignored. The net reactions with and without I" are... 

Kinetic studies of the oxidation of sulphoxides to sulphones by chromium(VI) species have been carried out131-133. The reaction has been found to be first order with respect to the chromium(VI) species and the sulphoxide and second order with respect to acid. At high sulphoxide concentrations the order with respect to sulphoxide is two. The proposed mechanism involves an electron transfer from the sulphoxide to the active chromium(VI) species (HCr03+ in strong acidic media) in the rate-determining step producing a sulphoxide radical cation which further reacts to give the sulphone. 

Although the fate of Cr(IV) is uncertain, (cf. the alcohol oxidation), some characteristics of the intermediate chromium species have been obtained by Wiberg and Richardson from a study of competitions between benzaldehyde and each of several substituted benzaldehydes. The competition between the two aldehydes for Cr(VI) is measured simply by their separate reactivities that for the Cr(V) or Cr(IV) is obtained from estimation of residual aldehyde by a C-labelling technique. If Cr(V) is involved then p values for oxidation by Cr(VI) and Cr(V) are 0.77 and 0.45, respectively. An isotope effect of 4.1 for oxidation of benzaldehyde by Cr(V) was obtained likewise. 

Oxidation of aldehydes and organic acids by chromium VI) a. Benzaldehyde... 

The salt, prepared in carbon tetrachloride solution, exploded feebly several times as the reaction proceeded. Oxidation of thiocyanate by chromium(VI) is postulated. 

Various saccharin derivatives 260 have been prepared by chromium (VI) oxide catalyzed H5IO6 oxidation of substituted ort/ro-toluenesulfonamides 259 <06T7902>. The reaction presumably proceeds through a benzylic radical intermediate 261 generated from the... 

Persson A process for making chlorine dioxide by reducing sodium chlorate with chromium (III) in the presence of sulfuric acid. The chromium (III) becomes oxidized to chromium (VI) and is then reduced back to chromium (III) with sulfur dioxide. This cyclic redox process with chromium avoids complications that would occur if sulfur dioxide itself were used as the reductant. Installed at the Stora Kopparbergs paper mill, Sweden, in 1946. Sheltmire, W. H., in Chlorine, its Manufacture, Properties and Uses, Sconce, J. S., Ed., Reinhold Publishing, New York, 1962,275,538. 

The synthesis of the smaller fragment methyl esters 144-147 was completed as depicted in Scheme 20. Chelation-controlled allylation of aldehydes 135-138 prepared by chromium(VI) oxidation of alcohols 122,123,133, and 134 with allyltrimethylsilane (139) in the presence of titanium(IV) chloride proceeded... 

Chromium produces some of the most interesting and varied chemistry of the transition elements. Chromium(O) and chromium(I) are stabilized in organometallics (Prob. 8). There have been extensive studies of the redox chemistry of Cr(II), Cr(III) and Cr(VI). Generally the Cr(IV) and Cr(V) oxidation states are unstable in solution (see below, however). These species play an important role in the mechanism of oxidation by Cr(VI) of inorganic and organic substrates and in certain oxidation reactions of Cr(II) and Cr(III). Examination of the substitution reactions of Cr(III) has provided important information on octahedral substitution (Chap. 4). 

Chromium(IV) and (V) are important intermediates in oxidation by Cr(VI), see next section. Chromium(V) is generally more reactive than Cr(VI). It is believed that Cr(V) is most effective for C —H rupture whereas Cr(IV) best breaks C —C bonds. 

Eary LE, Rai. 1987. Kinetics of chromium (III) oxidation to chromium (VI) by reaction with manganese oxide. Environmental Science and Technology 21 1187-1193. 

S. Signorella, C. Palopoli, M. Santoro, S. Garcia, V. Daier, J. C. Gonzalez, V. Roldan, M. I. Frascaroli, M. Rizzotto, and L. F. Sala, Kinetics and mechanistic studies on the chromic oxidation of carbohydrates. Implications on environmental contamination by chromium(VI) in soils (review), Res. Trends, 7 (2001) 197-207. 

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