Oxidation cerium -catalysed

Chromium(III) catalyses the cerium(IV) oxidation of primary and secondary alcohols in a mixture of H2SO4 and HC104. Kinetic results have been interpreted in terms of the formation of chromium(IV) in a reversible equilibrium, which forms a complex with the alcohol. Internal oxidation-reduction occurs in a rate-determining step to give aldehyde or ketone and regenerate the catalyst in the +3 state. The oxidation of ethanol under similar conditions has also been studied. ... 

Cerium(IV) oxidations of organic substrates are often catalysed by transition metal ions. The oxidation of formaldehyde to formic acid by cerium(IV) has been shown to be catalysed by iridium(III). The observed kinetics can be explained in terms of an outer-sphere association of the oxidant, substrate, and catalyst in a pre-equilibrium, followed by electron transfer, to generate Ce "(S)Ir", where S is the hydrated form of formaldehyde H2C(OH)2- This is followed by electron transfer from S to Ir(IV) and loss of H+ to generate the H2C(0H)0 radical, which is then oxidized by Ce(IV) in a fast step to the products. Ir(III) catalyses the A -bromobenzamide oxidation of mandelic acid and A -bromosuccinimide oxidation of cycloheptanol in acidic solutions. ... 

The kinetics of chromium(III)-catalysed oxidation of formic acid by Ce(IV) in aqueous H2SO4 can be rationalized in terms of initial formation of an outer-sphere complex involving oxidant, catalyst, and substrate (S), Ce(IV)(S)Cr(III), followed by an inner-sphere complex Ce(III)(S)Cr(IV). It is proposed that electron transfer occurs within this complex from substrate to Cr(TV) (with elimination of H+) followed by fast reaction to give C02 (again with elimination of H+).54 In contrast, there was no kinetic evidence for the accumulation of a corresponding inner-sphere intermediate in the osmium(Vin)-catalysed Ce(IV) oxidation of DMSO to dimethyl sulfone here, the observed rate law was rationalized in terms of rate-determining bimolecular electron transfer from DMSO to Os(VIII) in an outer-sphere step.55 The kinetics of oxidation of 2-hydroxy- 1-naphthalidene anil by cerium(IV) in aqueous sulfuric acid have been... 

Ce(S04)2 is found to be the reactive species in the iridium(III)-catalysed oxidation of f-butyl alcohol,63 cyclohexanol,64 and methanol65 with cerium(IV) in sulfuric acid. The reaction is first order in cerium(IV) but of fractional order in the reduc-tants. Induced polymerization of acrylonitrile indicated the generation of free radicals. Mechanisms have been suggested on the basis of experimental results. 

The oxidation of benzoin with cerium(IV) in perchloric acid solution is proposed to involve an interaction between Ce4+(aq.) ions and the keto alcohol, resulting in the formation of free radicals. The final product is benzoic acid.66 The rate of oxidation of crotyl alcohol with cerium(IV) is independent of the concentration of Ce(IV). The reaction induced polymerization of acrylonitrile indicating the formation of free radicals. The kinetics and activation parameters for the reaction have been determined.67 For the Ir(III)-catalysed oxidation of methyl ketones68 and cyclic ketones69 with Ce(IV) perchlorate, successive formation of complex between the reductant and Ce(IV) and then with the catalyst has been proposed. Results showed that in acidic solutions, iridium(III) is a more efficient catalyst than osmium and ruthenium compounds. 

In flow-reactor experiments, the effect of NO in the gas phase and a supported platinum catalyst mixed with the soot is twice as large for cerium when compared to that of copper, iron, and Printex-U. All other conditions are similar and, therefore, it is concluded that cerium catalyses the oxidation of soot with NO2. Because there is... 

A study of cerium(iv) oxidation of the diketone CFs CO-CHa-CO-CFs in aqueous sulphuric acid has shown that the kinetics are first order with respect to the diketone and cerium, and that the reaction proceeds as an inner-sphere process via an intermediate CeS04-diketone complex thermodynamic studies of the acid dissociations of various diketones (including CFa CO CHg-CO-CFs and PhCO-CH2 CO CF3) have also been made. Increasing CFs substitution in the series of diketones RICO CH2 COR (Ri = R = Me Ri = Me, R = CFs Ri = R2 = CFs) increases the acidity (pKe, = 8.9, 6.79, and 5.3, respectively), whereas the rates of HgO-catalysed detritiations are reduced by a factor of ca. 2 for each CFs group introduced this anomalous behaviour is attributed to CFs substitution increasing the tendency for hydrate formation. ... 

Induced electron transfer has been investigated in the action of one-electron oxidants on pyridinemethanolpentammine-cobalt(iii) complexes. The direct [and silver(i)- and cobalt(ii)-catalysed] oxidations of pentammine-(pyridine-4-methanol)cobalt(iii) (G) and the corresponding pyridine-3-methanol complex by cerium(iv) yield aldehydic complexes of cobalt(iii)... 

An outer-sphere mechanism is suggested for the oxidation of acetylacetone giving acetic acid by Ce(C104)4 solution. The reaction is first order each in substrate and Ce(IV) and is not catalysed by H+. The oxidations of pentane-l,5-diol, octane-1,8-diol, and fra 5-cyclohexane-l,2-diol by cerium(IV) have been studied in perchloric acid solution intermediate and final products of oxidation were identified and plausible reaction mechanisms were proposed. A mechanism has been proposed for the ruthenium(Vni)-catalysed oxidation of methoxyethanol by Ce(C104)4 in HCIO4 medium, which is zero order in Ce(IV) and H+ and first order each in substrate and Ru(VIII). In oxidations by cerium(IV) perchlorate, fV,fV,-disubstituted anilines gave... 

Two accounts have been presented of the mechanisms of chemical oscillators. The cerium(iv)-catalysed oxidation of malonic acid by bromate serves as a model for a conceptual approach and in the second article other examples involving both homogeneous and heterogeneous processes are described. Two reviews have been published of radiation chemistry of metal ions in aqueous solution. - In one article, details are presented of reactions of main-group and first-, second-, and third-row transition metals and lanthanides and actinides. Meyerstein covers somewhat similar ground but deals with complexes in low, intermediate, and high oxidation states. The pulse radiolysis technique has recently been used to provide... 

Cerium(iv).— The acid-promoted redox decomposition and cerium(iv) oxidation of the tris(oxalato)cobaltate(in) ion have been studied in aqueous acid media. In IM sulphuric acid, in the absence of oxidant, there occurs an induction period prior to the internal redox decomposition of the anion. On addition of the cerium(iv), however, there results an increased rate of reduction of the cobalt(ni) centre in contrast to the behaviour of this oxidant to M(C20 ) complexes where M = Cr, Rh, or Ir. The induction in the add-catalysed decomposition is consistent with the formation of a unidentate oxalato-complex-ion which may be the main route towards the stepwise reduction to yield Co and COg. From spectral studies on the total expected absorbance values on mixing, it would appear that the cerium(iv) ion is involved in the pre-equilibrium formation of a dinuclear species which might undergo internal electron transfer with reduction to cerium(in). A possible mechanism in this system may then be written as shown in Scheme 5 (ox = C2O4 -). The variations in rate of the one-electron redox reactions of this type are dependent on the nature of the activated complex, which may differ from one metal centre to another in respect of the number of protons and sulphate anions incorporated. 

Scheme 17.9 Heterogeneous cerium(rv) oxide catalysed transamidation.

Scheme 17.29 Heterogeneous cerium(iv) oxide and Au7ri02 catalysed nitrile hydration.

A mechanistic interpretation of the kinetics of iridium(III) chloride-catalysed oxidation of pentanones by cerium(IV) sulfate in water has accounted for the zero-order dependence on oxidant and change from first to inverse-square order dependence on [H+] with increasing [HS04]. ... 

Kinetic and activation parameters for the Ir(III)-catalysed oxidation of pentane-3-one and 4-methylpentane-2-one by cerium(IV) sulfate have been determined and a mechanism has been suggested. Ag(l)-catalysed oxidation of L-alanine with cerium(IV) in sulfuric acid is first order in Ce(lV) and L-alanine. A mechanism involving formation of free radicals has been suggested. Silver(I)-catalysed oxidation of L-tyrosine and A-acetyl L-tyrosine by Ce(lV) in sulfuric acid medium is proposed to proceed via an Ag(l)-reductant complex, which reacts with Ce(lV) to decompose in a rate-determining step. The active oxidizing species has been identified as Ce(S04)2. Kinetics of the oxidation of fiimaric acid with cerium(lV) in acid medium has been obtained and a mechanism has been suggested. ... 

Y.K. (1982) Silver(I) catalysed oxidation of water with cerium(IV) in aqueous perchloric acid solutions and the reaction of cerium(IV) with silver(I) in the presence of bipyridyl. J. Chem. Soc, Dalton Trans., 549 -553. 

Other examples are the use of osmium(VIII) oxide (osmium tetroxide) as catalyst in the titration of solutions of arsenic(III) oxide with cerium(IV) sulphate solution, and the use of molybdate(VI) ions to catalyse the formation of iodine by the reaction of iodide ions with hydrogen peroxide. Certain reactions of various organic compounds are catalysed by several naturally occurring proteins known as enzymes. 

Discussion. Molybdates [Mo(VI)] are quantitatively reduced in 2M hydrochloric acid solution at 60-80 °C by the silver reductor to Mo(V). The reduced molybdenum solution is sufficiently stable over short periods of time in air to be titrated with standard cerium(IV) sulphate solution using ferroin or /V-phenylanthranilic acid as indicator. Nitric acid must be completely absent the presence of a little phosphoric(V) acid during the reduction of the molybdenum(VI) is not harmful and, indeed, appears to increase the rapidity of the subsequent oxidation with cerium(IV) sulphate. Elements such as iron, copper, and vanadium interfere nitrate interferes, since its reduction is catalysed by the presence of molybdates. 

The oxidation of D-fructose with cerium(IV) in sulfuric acid medium is inhibited by an increase in the acidity. A cationic surfactant, CTAB, catalyses the reaction, whereas SDS has no effect. The catalytic role of CTAB has been explained using the pseudophase model of Menger and Portnoy. A mechanism involving the formation of an intermediate complex between /3-D-fructopyranose and Ce(S04)32- has been proposed.61 The oxidation of cycloalkanones with cerium(IV) in sulfuric acid medium showed a negligible effect of acidity. Formation of an intermediate complex, which decomposes in the rate-determining step, has been suggested.62... 

A combination of a fuel additive and a CR-DPF could possibly cover the temperature window from 575 K and higher and preferably at temperatures lower than 575 K. The combination of a platinum fuel additive for NO oxidation with a cerium additive for catalysed soot oxidation with O2 was studied as a possible application in which the above mentioned two mechanisms are incorporated. 

The oxidation of soot with NO2 is catalysed by cerium present in the activated soot, and not by copper- or iron-activated soot. 

An oxidation cycle can be achieved in which NO is used as an oxygen carrying intermediate. The reactions in this cycle are kinetically coupled, so that fast oxidation of soot with NO2 (catalysed by cerium) leads to fast (re)oxidation of NO (catalysed by platinum). 

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