Raj N. Mehrotra

The kinetics of the oxidation of thiourea by 12-tungstocobaltate(III) ion: evidence for anionic, neutral and protonated thiourea species in acetic acid–acetate buffer and perchloric acid solution

The kinetics of the redox reaction between 12-tungstocobaltate(III) ion, [Co(III)W]5−, and thiourea is studied in acetic acid–acetate buffered solutions (3.4 ≤ pH ≤ 5.6), and dilute perchloric acid solution. The reaction in buffered solution is first order both in [Co(III)W]5− and low [H2NCSNH2]. A simultaneous first- and second order dependence is observed at high [H2NCSNH2]. The rate is independent of pH ≤ 4.6 due to the participation of H2NCSNH2. In solutions of pH ≥ 4.6, the rate increases with the pH due to the dissociation of H2NCSNH2 to the reactive H2NCSNH− ion. In dilute perchloric acid solutions the rate increases with increasing [H+] due to the participation of protonated NH2C+SHNH2 species. The seat of the reaction is thought to be the enolic S atom (not protonated S) rather than the protonated nitrogen of the NH2 group as in the oxidations of NH2OH, H2NNH2 and N3H where the rate is retarded by the increase in [H+]. The acid dissociation constant, Ka, of protonated NH2C+SHNH2 is estimated to be 0.048 mol dm−3 at 40 °C. The Marcus theory is used to estimate the self-exchange rate (k22) of the H2NCSNH2–(H2N)2CSSC(NH2)2 couple. The estimated k22 is 2.44 × 10−10 dm3 mol−1 s−1. The low value is attributed to the stable dimeric (H2N)2CSSC(NH2)2 species.

Kinetics of cerium(IV) oxidation of formaldehyde in aqueous sulphuric acid solution

The oxidation of formaldehyde by cerium(IV) sulphate in aqueous sulphuric acid is an inner sphere reaction involving the intermediate Ce(SO4)2HCHO complex. The formation constant and disproportionation rate of this complex have been calculated at different temperatures. The reaction has a solvent isotope effect, kH2O:kD2O = 1·7. The rate is inversely proportional to (sulphuric acid), but it increases with sulphuric acid at constant (HSO4staggered−). The activation paramters are reported.

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

SummaryThe kinetics of chromic acid oxidation of phenylphosphinic acid to phenylphosphonic acid has indicated the formation of an anhydride between HCrO4− 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 HCrO4− is the reactive species of chromium(VI) is confirmed by the fact that k0 is independent of the inital [CrVI] where k0 is defined by the Equation k0=kobs[CrVI]/[HCrO4−]; kobs is the pseudo first-order rate constant with respect to chromium(VI) ([Phenyl-phosphinic acid]≫[CrVI]).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 β for the anhydride formation and the composite rate constant kK, K is the protonation constant of anhydride, are reported. The equilibrium constant β is almost independent of temperature.

Kinetics and mechanisms of redox reactions in aqueous solution. Part 8. Osmium(VIII) catalysed oxidation of amino acids by alkaline hexacyanoferrate(III)

Amino acids, in the presence of OsVIII as a catalyst, are oxidised by alkaline hexacyanoferrate(III) to aldehydes. There is a zero - order dependence in [Fe(CN)63–], an order less than unity in [amino acid], and a first-order dependence in [OsVIII] and [OH–]. The observed zero - order rate constant was independent of [Fe(CN)64–]. The proposed mechanism envisages the formation of a transient [OsO4(OH)2]2––amino acid complex prior to its rate-limiting interaction with hydroxide ion. The respective values of the equilibrium and rate-limiting constants are reported. The values of the enthalpy and entropy of activation in each case are also estimated.

Kinetics of oxidation of propane-1,3- and butane-1,4-diols by cerium(IV) in sulphuric acid

Abstract Two cerium(IV) species, in sulphuric acid ∼ 1 M, CeSO42+ and Ce(SO4)2, are the reactive species in an outer sphere oxidation of propane-1,3- and butane-1,4-diols. The reaction has a first order dependence both on [Ce(IV)] and [Diol], and a inverse dependence on [H2SO4] although the reaction is catalysed by H+ at constant HSO4−. Two distinct paths of reaction, one involving the bimolecular collision of the reactants and the other involving the formation of an intermediate complex, are suggested in the mechanism and supported by the activation parameters. The rate controlling step is considered to be C-H fission.

Kinetics and mechanism of oxidation of hydroxylamine by tetrachloroaurate(III) ion

AbstractThe oxidation of H2NOH is first-order both in [NH3OH+] and [AuCl4−]. The rate is increased by the increase in [Cl−] and decreased with increase in [H+]. The stoichiometry ratio, Δ[NH3OH+]/Δ[AuCl4−], is ≈1. The mechanism consists of the following reactions.

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

\begin{gathered} {\text{NH}}_{\text{3}} {\text{OH}}^{\text{ + }} \mathop \leftrightharpoons \limits^{K_{\text{a}} } {\text{NH}}_{\text{2}} {\text{OH}} + {\text{H}}^ + {\text{ (i)}} \hfill \\ {\text{AuCl}}_{\text{4}}^ - + {\text{NH}}_{\text{2}} {\text{OH}}\xrightarrow{k}{\text{AuCl}}_{\text{4}}^ - + {\text{HNO}} + {\text{2Cl}}^ - + 2{\text{H}}^ + {\text{ (ii)}} \hfill \\ {\text{AuCl}}_{\text{4}}^ - + {\text{NH}}_{\text{3}} {\text{OH}} + {\text{Cl}}^ - \xrightarrow{{k_1 }}{\text{AuCl}}_{\text{2}}^ - + {\text{HNO}} + 3{\text{Cl}}^ - + 3{\text{H}}^ + {\text{ (iii)}} \hfill \\ {\text{2HNO}}\xrightarrow{{{\text{fast}}}}{\text{N}}_{\text{2}} {\text{O + H}}_{\text{2}} {\text{O (iv) }} \hfill \\\end{gathered}

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

The rate law deduced from the reactions (i)–(iv) is given by Equation (v) considering that [H+] ≫ Ka.