Kinetics and mechanisms of oxidations by metal ions. Part IX(1) Oxidation of phenylphosphinic acid by chromic acid in perchlorate medium

Summary The kinetics of chromic acid oxidation of phenylphosphinic acid to phenylphosphonic acid has indicated the formation of an anhydride between HCrO ₄ ^– and phenylphosphinic acid in its active PhP(OH)₂ and inactive PhPH(O)OH forms. The ambiguity about the reactive form of phenylphosphinic acid arises from the fact that protonation of the anhydride leads to the same transition state which disproportionates in the rate-determining step to phosphonium ion and chromium(IV). These, through different reactions in the fast step, yield phenylphosphonic acid and chromium(III) as the final products. That HCrO ₄ ^– is the reactive species of chromium(VI) is confirmed by the fact that k₀ is independent of the inital [Cr^VI] where k₀ is defined by the Equation k₀=k_obs[Cr^VI]/[HCrO ₄ ^– ]; k_obs is the pseudo first-order rate constant with respect to chromium(VI) ([Phenyl-phosphinic acid][Cr^VI]).The plot between k₀ and [H⁺] passes through the origin indicating that the reaction does not occur in the absence of H⁺-ions. Furthermore, the plot between log k₀ and –H₀, the Hammett acidity function, is linear with a slope value of 1.02±0.02 confirming the protonation of the anhydride prior to its rate-limiting disproportionation.The equilibrium constant for the anhydride formation and the composite rate constant kK, K is the protonation constant of anhydride, are reported. The equilibrium constant is almost independent of temperature.Sen Gupta and Chakaladar⁽³⁾ reported values of 8.5, 9.2, 11, 12 and 13, respectively, at 26°, 30.4°, 36°, 39.4° and 46° C. The uncertainty limits were not reported. Nevertheless it is apparent from the data that the values are not greatly influenced by temperature. The statistical mean is 11±2 dm³ mol^–1, in fair agreement with our values.     

Kinetics and mechanism of oxidations by metal ions. Part 16. Oxidation of 4-oxopentanoic acid by aquomanganese(III) ions

Summary The outer-sphere oxidation of 4-oxopentanoic acid (4-OPA) to MeCO₂H by aquomanganese(III) ions exhibits a first-order dependence on [4-OPA] and [Mn(III)](aq). The observed pseudo first-order rate constant k _obs ([4-OPA] [Mn^III](aq)) is independent of [Mn^II] but decreases with increasing [H⁺]. The retarding effect of [H⁺] on the observed rate could be explained by considering either the reaction between MnOH⁺(aq) and MeCOCH₂— CH₂CO₂H or between Mn³⁺(aq) and MeCOCH₂CH₂-COO^– ions. The rate constant for the latter pair of reactants is much higher than the rate constant for the reaction between the first pair. Since the activation enthalpy for the first pair is about 14 kJ mol^–1 less than that of the second pair, it is concluded that the reactive species are MnOH²⁺(aq) and MeCOCH₂CH₂CO₂H or that the preferred oxidant is MnOH²⁺(aq) ion.     

Oxidation of selenite by alkaline ferricyanide using osmium tetroxide as a catalyst

Summary Oxidation of selenite to selenate by alkaline ferricyanide catalysed by osmium tetroxide was followed by direct and indirect procedures. Either the ferrocyanide was titrated with selenite solution at 8–10% overall alkalinity or vice versa using amperometric or potentiometric end point. In the indirect procedure the excess ferricyanide was determined by amperometric titration with arsenious oxide at 10–15% alkalinity, and the ferrocyanide with ceric sulphate using o-phenanthroline or amperometric indicator. A cerimetric determination of ferricyanide based on this reaction is described.Sincere thanks of the authors are due to Dr. S. S. Joshi, D. Sc. (London), for kind interest in the work.     

Kinetics and mechanism of metal ion catalysis: Part II. Oxidation of PhCHO by osmium(VIII) continuously regenerated by hexacyanoferrate(III) ions

The kinetics of the oxidation of PhCHO by Os^VIII has been studied in 0.01–0.05 M [OH^–] range. Fe(CN)^3– ₆ was used to regenerate Os^VIII. The very low solubility of PhCHO in H₂O restricted the study to the 0.0024–0.0036 M [PhCHO] range. A mechanism involving the PhCHO hydrate has been proposed.     

Oxidation of tertiary silanes by osmium tetroxide

In the presence of an excess of pyridine ligand L, osmium tetroxide oxidizes tertiary silanes (Et(3)SiH, (i)Pr(3)SiH, Ph(3)SiH, or PhMe(2)SiH) to the corresponding silanols. With L = 4-tert-butylpyridine ((t)Bupy), OsO(4)((t)Bupy) oxidizes Et(3)SiH and PhMe(2)SiH to yield 100 +/- 2% of silanol and the structurally characterized osmium(VI) mu-oxo dimer [OsO(2)((t)Bupy)(2)](2)(mu-O)(2) (1a). With L = pyridine (py), only 40-60% yields of R(3)SiOH are obtained, apparently because of coprecipitation of osmium(VIII) with [Os(O)(2)py(2)](2)(mu-O)(2) (1b). Excess silane in these reactions causes further reduction of the OsVI products, and similar osmium "over-reduction" is observed with PhSiH(3), Bu(3)SnH, and boranes. The pathway for OsO(4)(L) + R(3)SiH involves an intermediate, which forms rapidly at 200 K and decays more slowly to products. NMR and IR spectra indicate that the intermediate is a monomeric Os(VI)-hydroxo-siloxo complex, trans-cis-cis-Os(O)(2)L(2)(OH)(OSiR(3)). Mechanistic studies and density functional theory calculations indicate that the intermediate is formed by the [3 + 2] addition of an Si-H bond across an O=Os=O fragment. This is the first direct observation of a [3 + 2] intermediate in a sigma-bond oxidation, though such species have previously been implicated in reactions of H-H and C-H bonds with OsO(4)(L) and RuO(4).     

Kinetics and mechanism of the oxidation of reducing sugars by osmium tetroxide in alkaline medium

A general mechanism for oxidizing reducing sugars (pentoses, hexoses and disaccharides) by OsO₄ in alkaline medium is proposed. The reactions exhibit pseudounimolecular kinetics with respect to OsO₄, are first order with respect to lower [sugar] and [OH⁻], but tend towards zeroth order with respect to both higher [sugar] and [OH⁻]. These results suggest the formation of an activated complex between the enediol andd osmium tetroxide, which slowly disproportionates to give osmium(VI) species and the intermediate products. The reaction products were identified. When an aldehydo sugar is treated with alkali, structural changes occur and an equilibrium is set up, mainly involving a keto sugar product and the starting aldehydo sugar consumed. This process is reversible. These changes are mainly due to the well-known Lobry-de-Bruyn-Alberda Van Ekenstein transformations. This revised version was published online in June 2006 with corrections to the Cover Date.     

Kinetics and mechanisms of oxidation by metal ions,part XII [1] oxidation of formaldehyde by alkaline osmium(VIII)

The stopped-flow measurements on the disappearance of alkaline osmium(VIII) at 402 nm indicated a first-order dependence each in [Os(VIII)] and [HCHO]. The pseudo first-order rate constant k_obs ([HCHO] ? [Os(VIII)]) decreased with increasing [OH^?]. The ionic strength, however, had no effect on k_obs. The rapid scan spectra of the reaction mixture indicated the formation of an inert complex which absorbs at 319 nm. Therefore the rate determining step is considered to involve the bimolecular collision between OsO₄(OH) and hydrated formaldehyde. The values of the rate limiting constant k and the equilibrium constant K_ha for the formation of the alkoxide ion from the reaction of hydrated formaldehyde with OH^? are evaluated. The equilibrium constant K_ha, within the experimental limits, is independent of temperature. The pK_a value, calculated from K_ha, is 13.62 ± 0.05 which is in good agreement with the pK_a value 13.27 for formaldehyde. The activation parameters, ΔH? = 40 ± 2 kJ mol^?1 and ΔS? = ?51 ± 6 JK^?1 mol^?1, for the rate limiting constant k are determined.     

Oxidation of formate by alkaline ferricyanide

Summary A catalytic oxidation of formate by alkaline ferricyanide has been studied. To the formate solution an excess of ferricyanide and osmium tetroxide as catalyst were added and the excess ferricyanide was back titrated with arsenious oxide using the amperometric dead stop end point method. The method is superior to the oxidative determination of formate by alkaline permanganate.

---

Zusammenfassung Formiat kann durch katalytische Oxydation mit alkalischer Hexacyanoferrat(III)-lösung bestimmt werden, indem man die Probelösung mit einem Überschuß des Reagenses sowie Osmiumtetroxid als Katalysator versetzt und den Überschuß mit Arsen(III)-lösung zurücktitriert.Der Endpunkt wird nach der Dead stop-Methode bestimmt. Das Verfahren ist der Oxydation mit alkalischer Permanganatlösung überlegen.

---

---

Sincere thanks of the authors are due to Prof. G. B. Singh for providing the necessary facilities. The award of a U.G.C. Scholarship to one of them (A.L.J.R.) is also acknowledged.     

Kinetics and mechanism of oxidation of some alcohols by osmium tetroxide

Spectrophotometric studies of the kinetics of oxidation of 2-methylpropan-1-ol and 2-butanol by an alkaline solution of osmium tetroxide have been reported. A first-order dependence to osmium tetroxide was observed. A first-order dependence to both 2-methylpropan-1-ol and alkali at low concentration tends to zero order at higher concentrations. In the case of 2-butanol, first-order kinetics is exhibited with respect to 2-butanol but first-order kinetics observed at lower concentrations of alkali decrease at higher concentrations. A negligible ionic strength effect of the medium was observed. Activation parameters have been computed. A suitable mechanism in conformity with our kinetic observations has been suggested.     

Kinetics and mechanisms of oxidation by metal ions. Part XIII. Osmium(VIII)-catalysed oxidation of cycloalkanols by alkaline hexacyanoferrate(III)

Summary The kinetics of OsO₄-catalysed oxidation of cyclopentanol, cyclohexanol and cyclooctanol by alkaline hexacyanoferrate(III) have been studied at low [OH^–] so that the equilibrium between alcohol and alkoxide ion is not unduly shifted towards the latter. The reaction shows a first-order dependence in [OH^–]. The order of the reaction with respect to cycloalcohol is fractional, indicating the formation of an intermediate complex with Os^VIII since the order with respect to hexacyanoferrate(III) ion is zero. The order with respect to Os^VIII may be expressed by the equation k_obs=a+b[Os^VIII]. The analysis of the rate data indicates a significant degree of complex formation between [OsO₃(OH)₃]^– and ROH. It was found that the bimolecular rate constant k for the redox reaction between complex and OH^–k₁, the forward rate constant for the formation of alkoxide ion. The activation parameters of these rate constants are reported.     

Kinetics and mechanisms of oxidation by metal ions. Part XIV(1). Osmium(VIII)-catalysed oxidation of cycloalkanones by alkaline hexacyanoferrate(III)

Transition Metal Chemistry - The OsVIII-catalysed oxidation of cyclopentanone and cyclohexanone by alkaline hexacyanoferrate(III) ions is zeroth-order with respect to $ {? F_{e}(CN)_{6}^{3-}...     

Oxidative determination of thiosulphate by alkaline ferricyanide using osmium tetroxide as a catalyst

Summary The catalytic action of osmium tetroxide has been utilised in the determination of thiosulphate by oxidation to sulphate with alkaline ferricyanide. Both direct and indirect procedures were developed. In the former the titration was carried out either ways using amperometric detection. In the latter, both the excess ferricyanide and ferrocyanide formed, were determined by arsenious oxide or iodometrically with thiosulphate, and by ceric sulphate respectively. The high conversion factor, availability of alternate procedures, use of potassium ferricyanide as a standard oxidant with a high molecular weight are characteristic features of this redox procedure.     

Kinetics and mechanisms of oxidation by metalions. Part XI(1). Kinetics of the osmium(VIII)-catalysed oxidation of glycols by alkaline hexacyanoferrate(III)

Summary The kinetics of the Os^VIII-catalysed oxidation of glycols by alkaline hexacyanoferrate(III) ion exhibits zerothorder dependence in [Fe(CN) ₆ ^3– ] and first-order dependence in [OsO₄]. The order with respect to glycols is less than unity, whereas the rate dependence on [OH^–] is a combination of two rate constants; one independent of and the other first-order in [OH^–]. These observations are commensurate with a mechanism in which two complexes, [OsO₄(H₂O)G]^– and [OsO₄(OH)G]^2–, are formed either from [OsO₄(H₂O)(OH)]^– or [OsO₄(OH)₂]^2– and the glycol GH, or by [OsO₄(H₂O)₂] and [OsO₄(H₂O)(OH)]^– and the glycolate ion, G^–, which is in equilibrium with the glycol GH through the reaction between GH and OH^–. Hence there is an ambiguity about the true path for the formation of the two Os^VIII-glycol complexes. A reversal in the reactivity order of glycols in the two rate-determining steps, despite the common attack of OH^– ion on the two species of Os^VIII-complexes, indicates that the two complexes are structurally different because S changes from the negative (corresponding to k₁₁) to positive (related to k₂).     

Kinetics of the labeling reactions of thymine, cytosine and uracil with osmium tetroxide bipyridine

The electroactive complex osmium tetroxide bipyridine holds great promise as a covalent label for biosensor applications regarding nucleic acids and protein detection. Labeling can easily be performed in the laboratory. Until now, almost only DNA species have been investigated using this label. Thymine (which occurs exclusively in DNA) is known to react much faster than cytosine and uracil. In order to explore the possibilities to modify and detect also RNA species in a timely fashion, we have investigated the kinetics of reactions of osmium tetroxide bipyridine with the pyrimidine bases in the micromolar concentration range at different temperatures by means of spectrophotometry. Results were confirmed using voltammetric detection for the determination of labeled oligonucleotides. The modification reaction can be easily completed at room temperature within 7 h, even in case of cytosine and uracil. At 60 °C, 3 h are sufficient for complete modification of all pyrimidine bases that are found in natural nucleic acids. These findings will be important for future biosensor applications with RNA species as target molecules.     

Permanganate ion oxidations X. Kinetics and mechanisms of the initial stage in the oxidation of 4-hydroxy-2-pyrimidinethiols

Permanganate ion rapidly oxidizes the thiol group in 2-thiouracils (2,3-dihydro-2-thioxo-4(1H)-pyrimidinones, 4-hydroxy-2-pyrimidinethiols) to the corresponding uracil-2-sulfonates in phosphate-buffered solutions. The oxidations, which are first order in oxidant and first order in reductants, are characterized by relatively small energies of activation (E_a ? 4.3–6.7kcal/mol) and relatively large negative entropies of activation (ΔS? = ?24.9–?34.4 eu). The relative rates of oxidation of 2-thiouracil (2), 6-methyl- (4), 6-n-propyl- (6), 1,6-dimethyl-(8), and 3,6-dimethyl-2-thiouracil (9) are 1.00:1.23:0.98:7.32:2.54. The rate of oxidation of 1,3,6-trimethyl-2-thiouracil (12) is several orders of magnitude slower than that of the other thiouracils. Although the thiol group or thiolate anion is probably involved in the rate-determining step, several other possible mechanisms are also considered.     

Kinetics and mechanism of metal ion catalysis: Part I. Kinetics of electron transfer from cycloheptanone in aqueous alkali

The kinetics of the electron transfer from cycloheptanone to OsO₄ in alkaline medium has been studied spectrophotometrically. The oxidation of cycloheptanone by Os^VIII, continuously regenerated by Fe(CN)^3– ₆ in alkaline medium in the 0.00123–0.01 M range, is zeroth order with respect to Fe(CN)^3– ₆ and first order with respect to Os^VIII. A suitable mechanism, based on rate data analysis, is proposed.     

Kinetics of metal ion binding by polysaccharide colloids

The dynamics of metal sorption by a gel-like polysaccharide is investigated by means of the electrochemical technique of stripping chronopotentiometry (SCP). The measured response reflects the diffusive flux properties of the metallic species in the dispersion. The colloidal ligand studied here is a functionalized carboxymethyldextran. Its complexation with Pb(II) reveals a time dependence that identifies strong differences in the dynamic nature of the successive metal complexes formed. Apparently, the formation of intramolecular bidentate complexes requires a slow conformational reorganization of the macromolecule that becomes the rate-limiting step in the complexation reaction. The relevant parameters for metal binding and release kinetics are computed and thus provide knowledge of the time-dependent stability and lability of metal polysaccharide complexes.     

Kinetics and mechanism of metal ion catalysis. Part III. Osmium(VIII) catalysed oxidation of m-hydroxybenzaldehyde by Fe(CN)6 3− in alkaline medium

Os^VIII-catalysed oxidation of m-hydroxybenzaldehyde by alkaline Fe(CN)₆ ^3– has been studied in the 0.01–0.05 M [OH^–] range. Higher [OH^–] concentrations were not possible as the substrate turned yellow at [OH^–] > 0.05 M. The very low solubility of the substrate in H₂O restricted the kinetic study to [OH^–] < 0.01 M. A mechanism, consistent with the results is proposed.