The kinetics of the cerium(IV)-uranium(IV) reaction at low sulfate concentrations

The rate of oxidation of uranium(IV) by cerium(IV) was measured with a stopped-flow spectrophotometer at sulfuric acid concentrations of 2 × 10⁻⁶−0.5 M. At a constant hydrogen ion concentration of 0.5 M, the maximum rate constant was observed for 2 × 10⁻³ M sulfuric acid; at that concentration, two sulfate ions were involved in the activated complex. The dependence of the rate constant on the hydrogen ion concentration showed that the reaction paths involving one or two sulfate ions also involved one hydroxyl ion, whereas one hydrogen ion was involved in the five sulfate dependent path.Spectrophotometric measurements supported the existence of a hydrolyzed monosulfatocomplex of cerium(IV).     

Substituent effects on the kinetics of the cerium(IV) oxidation of mandelic acids in sulfate media. Anomalous kinetic behavior of the methoxy derivative

The kinetics of the cerium(IV) oxidation of p-nitro and p-methoxymandelic acids have been investigated in H₂SO₄-MHSO₄ (M⁺ = Li⁺, Na⁺, K⁺) and H₂SO₄-MClO₄ (M⁺ = H⁺, Na⁺) mixtures at a constant total electrolyte concentration of 2.00 mol/dm³. The oxidation of p+nitromandelic acid proceeds through two [H⁺]-independent paths, as was also observed for some substituted mandelic acids studied previously. The kinetic behavior of the p-methoxy derivative differs from that of the other mandelic acids in that (1) the oxidation occurs via two [H⁺]-dependent paths, (2) the reaction rate is anomalously high, (3) the activation enthalpy and entropy of the overall process are markedly lower. It provides strong support to the suggestion that a different mechanism is operative. The substituent effects and the reaction mechanism are discussed.     

Interfacial behavior of Cyanex 302 and kinetics of lanthanum extraction

In this paper, interfacial tension of Cyanex 302 is measured by a Sigma-701 tensiometer and the adsorption parameters are calculated according to the Gibbs and Szyszkowski adsorption isotherms. The interfacial adsorbed behavior of Cyanex 302 is investigated. The results demonstrate that the dimer is the predominant species in the bulk organic phase; however, the monomer is adsorbed at the interface and more interfacially active. The effects of aqueous pH, ion strength, and temperature on the interfacial activity of Cyanex 302 in heptane are discussed and explained in detail. The lower interfacial activity of Cyanex 302 in aromatic hydrocarbon than in aliphatic hydrocarbon has also been determined. The values of interfacial excess at the saturated interface increase in the order n-heptane>cyclohexane>toluene>benzene, which is consistent with the order of extractability of lanthanum by Cyanex 302 in these diluents. The interfacial activity data are used to discuss the kinetic mechanism of lanthanum(III) extraction. It is shown that an interfacial mechanism is very probable, and the extraction limiting step is the reaction between the Cyanex 302 molecules in the organic phase sublayer and the adsorbed intermediate complex.     

Photochemical nitrooxylation of methylbenzenes by cerium(IV) ammonium nitrate in acetonitrile

The photochemical reaction of substituted toluenes with CAN in CH₃CN leads to good yields of benzyl nitrates under very mild conditions.     

Effect of acrylamide concentration on the kinetics of oxidation of tartaric acid by cerium(IV) in sulfuric-perchloric acid media

The kinetics of oxidation of tartaric acid by Ce(IV) in the absence and presence of acrylamide has been investigated spectrophotometrically in aqueous H₂SO₄–HClO₄ media at a constant ionic strength 2.0M and 25°C. Oxidation of tartaric acid in both cases was first order with respect to Ce(IV). Kinetic data showed that the reaction involves the formation of an unstable complex and an intermediate free radical. The activation parameters were calculated to be E _a =91.3±0.4 kJ-mol^–1, S=20.2±1.0 J-mol^–1-K^–1, H=88.8±0.4 kJ-mol^–1. A polymerization mechanism is discussed.     

Preparation of naphthaldehydes by cerium(IV)ammonium nitrate oxidation of methylnaphthalenes

Naphthalenes with an Me group in the 1-position gave aldehydes in good to excellent yields when oxidized with cerium(IV) ammonium nitrate (CAN) in 50% HOAc at 85°. Under the same conditions methylnaphthalenes with no Me group in a peri position gave aldehydes in fair yields but also significant amounts of 1,4-naphthoquinone.     

Oxidative modification of native protein residues using cerium(IV) ammonium nitrate

A new protein modification strategy has been developed that is based on an oxidative coupling reaction that targets electron-rich amino acids. This strategy relies on cerium(IV) ammonium nitrate (CAN) as an oxidation reagent and results in the coupling of tyrosine and tryptophan residues to phenylene diamine and anisidine derivatives. The methodology was first identified and characterized on peptides and small molecules, and was subsequently adapted for protein modification by determining appropriate buffer conditions. Using the optimized procedure, native and introduced solvent-accessible residues on proteins were selectively modified with polyethylene glycol (PEG) and small peptides. This unprecedented bioconjugation strategy targets these under-utilized amino acids with excellent chemoselectivity and affords good-to-high yields using low concentrations of the oxidant and coupling partners, short reaction times, and mild conditions.     

Complexing of cerium(IV) with nitrilotriacetic acid in the presence of sulfate and nitrate ions

Conclusions The authors have made a spectrophotometric investigation of the composition of the complexes formed in the system Ce(IV)-nitrilotriacetic acid in 1 M (NH₄)₂SO₄ and 1 M NH₄NO₃ solutions and have calculated the instability constants of CeX⁺ and CeX ₂ ^2– complexes.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1448–1451, July, 1976.     

Formation of monoaldehydes by cerium(IV) ammonium nitrate oxidation of unsymmetric dimethylnaphthalenes

When 1,2-, 1,3-and 1,6-dimethylnaphthalene are oxidized by Ce⁴⁺ in acetic acid the corresponding monoaldehydes are formed in better than 80% yield. In each case aldehyde formation takes place with a high degree of selectivity as the methyl-1- to methyl-2-naphthaldehyde ratio is better than 11:1. The selectivity may be explained from differences in reactivity as calculated within the frontier orbital method.     

Interfacial behavior of ACORGA CLX-50 and surface kinetics of copper extraction

Interfacial tension isotherms were determined and interpreted for ACORGA CLX-50. The hydration of extractant molecules in aqueous solution and at hydrocarbon/water interfaces was studied by molecular modelling. The usefulness of this technique to interpret the adsorption behavior was demonstrated. The interfacial kinetics was considered and relationships for various models of interfacial mechanism were derived and discussed. Despite its high hydrophobicity, ACORGA CLX 50 strongly adsorbs at the hydrocarbon/water interfaces and thus decreases effectively the interfacial tension. This high interfacial activity of ACORGA CLX 50 can be explained by the formation of hydrates. The interfacial tension isotherm can be well matched with the Szyszkowski equation. Molecular modelling suggests that ACORGA CLX 50 adsorbs at the hydrocarbon/water interface probably as a tetrahydrate containing two water molecules bonded to the same carbonyl oxygen atom (e.g., at position 3), one water molecule bonded to the oxygen atom of the second alkoxyl group (i.e., at position 5 when the hydration of carbonyl oxygen at position 3 is previously considered) and, finally, one water molecule bonded with the pyridine nitrogen atom. Positions 3 and 5 are equivalent. It is also shown that when the extraction of copper takes place in the kinetic regime, the reaction order with respect to ACORGA CLX 50 can change depending on the limiting step and the range of extractant concentration considered. Thus, a decrease of the extractant concentration from 10^?5M to 3·10^?3M causes a fall of the order with respect to ACORGA CLX 50 from 1 to 0 and 2 to 1 when the formation of the intermediate 1∶1 and final 2∶1 complexes are considered to be the limiting step, respectively.     

Rapid extraction and photometric determination of cerium(IV) with N-p-tolylbenzohydroxamic acid (p-TBHA)

Summary A method for rapid extraction and spectrophotometric determination of cerium (IV) is described. The cerium-N-p-tolylbenzo-hydroxamic acid complex is extracted into chloroform at the pH 8.4 to 9.8. Maximum absorbance occurs between 460 and 470 nm. The values are reproducible at 465 nm and Beer's law is obeyed at this wavelength over the range of 0.03–40, per ml of the cerium(VI). The molar absorptivity of the cerium-p-TBHA complex is 4.6×10³ l · mole^–1 · cm^–2 at 465. An attempt has been made to determine the cerium in sea water.

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Zusammenfassung Eine Methode zur schnellen Extraktion und spektrophotometrischen Bestimmung von Cer(IV) wurde beschrieben. Der Cer-Komplex mit N-p-Tolylbenzohydroxamsäure wird bei pH 8,4–9,8 mit Chloroform extrahiert. Das Absorptionsmaximum liegt zwischen 460 und 470 nm. Bei 465 nm sind die Ablesungen reproduzierbar und wird das Beersche Gesetz zwischen 0,03 und 40g Cer/ml erfüllt. Die molare Extinktion des genannten Komplexes beträgt bei 465 nm 4,6 · 10³ l · mol^–1 · cm^–2. Es wurde versucht, den Cergehalt von Meerwasser zu bestimmen.

     

Mild and chemoselective catalytic deprotection of ketals and acetals using cerium(IV) ammonium nitrate

Cerium(IV) ammonium nitrate (CAN) is a powerful, though mild, reagent for the efficient and selective removal of a range of ketals and acetals. This novel deprotection method requires only catalytic amounts of CAN and tolerates a variety of functional and protecting groups. Mechanistic insights suggest that the Ce(IV) salts act as unique Lewis acids and not as redox active species.     

The role of nitrate free radicals in the photochemical side-chain nitrooxylation of alkylbenzenes by cerium(IV) ammonium nitrate in acetonitrile

It is suggested that the reactive species in the CAN-induced photochemical side-chain nitrooxylation of alkylbenzenes is the nitrate radical, which probably acts as one-electron oxidant.     

Selective extraction of cerium(IV) on tri-n-butyl phosphate and thenoyltrifluoroacetone loaded polyurethane foam

A method for the selective extraction of cerium (IV) on tri-n-butyl phosphate and thenoyltrifluoroacetone loaded foams is described. Parameters affecting the extraction of the metal ion, such as pH, reagent concentration and equilibration time have been investigated. Effects of ten diverse cations have also been studied. The method can be applied to preconcentrate and determine cerium in glasses.This work was performed at the University of Veszprém, Department of Radiochemistry, by A.R. acting as guest research fellow.     

Thermodynamics of the formation of cerium(IV) malonate complex and the kinetics of its redox decomposition

Thermodynamic and kinetic characteristics of cerium(IV) malonate complex formed in the first stage of cerium(IV) oxidation by malonic acid H₂Mal are studied using a spectrophotometer, a photometer, and a pH-meter at a ionic strength of I = 2 in the pH region of 0.3–1.6 in a sulfuric acid medium at a temperature of 296.8 K. Its composition is found to be CeOHMal⁺. The form of organic ligand is Mal^2?; the thermodynamic parameters of its formation and kinetic parameters of its intramolecular redox decomposition are determined. The most likely scheme of the initial stages of redox proceeding in the Ce⁴⁺–SO ₄ ^2- –H₂Mal system is discussed, and a quantitative model of it is proposed.     

Facile synthesis of 1,2,3-tricarbonyls from 1,3-dicarbonyls mediated by cerium(IV) ammonium nitrate

A mild and efficient protocol for the synthesis of vicinal tricarbonyl compounds from β-dicarbonyls in a single step using cerium(IV) ammonium nitrate as a catalytic oxidant is described. Ease of execution, wide substrate scope and the suitability for the synthesis of commercially important compounds like ninhydrin, alloxan and oxoline make this reaction particularly noteworthy.     

Determination of cerium(III) and cerium(IV) in nanodisperse ceria by chemical methods

Percentages of different valence cerium species have been determined in powdery samples, redispersible compositions, and aqueous sols of nanodisperse ceria prepared from cerium(IV) and cerium(III) salts by various methods with or without organic stabilizers. Cerium(III) is shown to be virtually absent in nearly all of the CeO₂ samples studied. Organic stabilizers are shown to be capable of reducing cerium(IV) in aqueous CeO₂ sols.     

Electron transfer reactions of 1-phenyl-4-vinylpyrazole mediated by cerium(IV) ammonium nitrate

Summary The title compound is converted into the cyclic ethers5 and the alcohol6 upon treatment with ceric ammonium nitrate in acetone. Using methanol as a solvent the dimethoxy derivative9 and the nitrate ester10 are formed. No cross cycloaddition is observed in the presence of olefins such as ethyl vinyl ether,DMAD, or indene; however, with cyclopentadiene as co-reagent a mixture of the exo-endo Diels-Alder adducts14 involving the vinylic system of 1 as 2 component is obtained. The results are rationalized through the interrnediacy of the radical cation1 ^·+, generated by single electron transfer (SET) from the neutral precursor1 to Ce(IV).

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Elektrontransfer-Reaktionen von 1-Phenyl-4-vinylpyrazol mittels Cer(IV)-Ammonnitrat

Zusammenfassung Die Titelverbindung wurde mittels Cer(IV)-Ammonnitrat in Aceton in die cyclischen Ether5 und den Alkohol6 umgewandelt. Mit Methanol als Lösungsmittel wurde das Dimethoxyderivat9 und der Nitratester10 gebildet. Bei Anwesenheit von Olefinen wie etwa Ethylvinylether,DMAD oder Inden wurden keine Cross-Cycloadditionen beobachtet. Hingegen wurde mit Cyclopentadien als Mitreagens eine Mischung vonexo-endo-Diels-Alder-Produkten14 erhalten, wobei das vinylische System von1 als 2-Komponente fungiert. Die Ergebnisse werden mittels eines intermediär auftretenden Radikalkation1 ^·+ als Produkt eines Einelektronentransfers (SET) des neutralen Vorläufers1 zu Ce(IV) rationalisiert.

     

The oxidative degradation of dibenzoazepine derivatives by cerium(IV) complexes in acidic sulfate media

The kinetics of the oxidation of imipramine and desipramine using cerium(IV) complexes were studied in the presence of a large excess of azepine derivative (TCA) in acidic sulfate media using UV-Vis spectroscopy. The reaction proceeds via dibenzoazepine radical formation, identified by EPR measurements. The kinetics of the first degradation step were studied independently of the further slower degradation reactions. Linear dependences, with zero intercept, of the pseudo-first-order rate constants (k(obs)) on [TCA] were established for both dibenzoazepine radical formation processes. Rates of reactions decreased with increasing concentration of the H(+) ion indicating that cerium(IV) as well as both reductants exist in an equilibrium with their protolytic forms. The activation parameters for the degradation of dibenzoazepine derivatives in the first oxidation stage were as follows: ΔH(≠) = 39 ± 2 kJ mol(-1), ΔS(≠) = -28 ± 8 J K(-1) mol(-1) for imipramine and ΔH(≠) = 39 ± 2 kJ mol(-1), ΔS(≠) = -28 ± 6 J K(-1) mol(-1) for desipramine, respectively. Imipramine and desipramine radicals dimerized leading to an intermediate radical dimer, which decayed in a first-order consecutive decay process. These two further reactions proceed with rates which are characterized by non-linear dependences of the pseudo-first-order rate constants (k(obs)) on [TCA]. The degradation reaction of the intermediate radical dimer leads to an uncharged dimer as a final product. Mechanistic consequences of all the results are discussed.