Kinetics and mechanism of interaction of dipeptide (glycyl–glycine) with ninhydrin in aqueous micellar media

The rates of reaction between ninhydrin and dipeptide glycyl–glycine (Gly–Gly) have been determined by studying the reaction spectrophotometrically at 70°C and pH 5.0 in aqueous and in aqueous cationic micelles of cetyltrimethylammonium bromide (CTAB). The reaction follows first‐ and fractional‐order kinetics, respectively, in [Gly–Gly] and [ninhydrin]. The observed rate constant is affected by [CTAB] changes and the maximum rate enhancement is ca. three‐fold. As the k_ψ ? [CTAB] profile shape is characteristic of bimolecular reactions catalyzed by micelles, the catalysis is explained in terms of the pseudo‐phase model of the micelles (proposed by Menger and Portnoy and developed by Bunton and Romsted). The presence of inorganic salts (NaCl, NaBr, Na₂SO₄) does not reveal any regular effect but the data with organic salts (NaBenz, NaSal) show an increase in the rate followed by a decrease. The kinetic data have been used to calculate the micellar binding constants K_S for Gly–Gly and K_N for ninhydrin and the respective values are 317 and 69 mol^?1 dm³. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 643–650, 2006     

Effect of Cationic Micelles on the Kinetics of Interaction of [Cr(III)-Gly-Gly] with Ninhydrin

The effect of cationic micelles of cetyltrimethylammonium bromide (CTAB) on the interaction of chromium dipeptide complex ([Cr(III)-Gly-Gly]²⁺) with ninhydrin under varying conditions has been investigated. The rates of the reaction were determined in both water and surfactant micelles in the absence and presence of various organic and inorganic salts at 70 °C and pH 5.0. The reaction followed first- and fractional-order kinetics with respect to [Cr(III)-Gly-Gly²⁺] and [ninhydrin]. Increase in the total concentration of CTAB from 0 to 40×10⁻³ mol·dm⁻³ resulted in an increase in the pseudo-first-order rate constant (kψ) by a factor of ca 3. Quantitative kinetic analysis of kψ−[CTAB] data was performed on the basis of the pseudo-phase model of the micelles. As added salts induce structural changes in micellar systems that may modify the substrate-surfactant interactions, the effect of some inorganic (NaBr, NaCl, Na₂ SO₄) and organic (NaBenz, NaSal, NaTos) salts on the rate was also explored. It was found that the tightly bound counterions (derived from organic salts) were the most effective.     

Catalytic effect of CTAB on the interaction of dipeptide glycyl-tyrosine (Gly-Tyr) with ninhydrin

The effect of cationic micelles of cetyltrimethylammonium bromide (CTAB) on the interaction of dipeptide glycyl-tyrosine (Gly-Tyr) with ninhydrin under varying conditions has been studied spectrophotometrically at 70 °C and pH 5.0. The reaction followed first- and fractional-order kinetics with respect to [Gly-Tyr] and [ninhydrin], respectively. Increase in total concentration of CTAB from 0 to 70 × 10⁻³ mol dm⁻³ resulted in an increase in the pseudo-first-order rate constant (k_ψ) by a factor of ca. 3. Quantitative kinetic analysis of k_ψ − [CTAB] data was performed on the basis of pseudo-phase model of the micelles (proposed by Menger and Portnoy and developed by Bunton) and Piszkiewicz model. A possible mechanism has been proposed and the kinetic data have been used to evaluate the micellar binding constants K_S (268 mol⁻¹ dm³ for Gly-Tyr) and K_N (64 mol⁻¹ dm³ for ninhydrin).     

Sub- and post-micellar catalytic and inhibitory effects of cetlytrimethylammonium bromide in the permanganate oxidation of phenylalanine

The kinetics of phenylalanine (phe) oxidation by permanganate has been investigated in absence and presence of cetlytrimethylammonium bromide (CTAB) using conventional spectrophotometric technique. The rate shows first- and fractional-order dependence on [MnO₄⁻] and [phe] in presence of CTAB. At lower values of [CTAB] (≤10.0 × 10⁻⁴ mol dm⁻³), the catalytic ability of CTAB aggregates are strong. In contrast, at higher values of [CTAB] (≥10.0 × 10⁻⁴ mol dm⁻³), the inhibitory effect was observed in absence of H₂SO₄. We find that anions (Br⁻, Cl⁻ and NO₃⁻) in the form of sodium salts are strong inhibitors for the CTAB catalyzed oxidation. Kinetic and spectrophotometric evidences for the formation of an intermediate complex and an ion-pair complex between phe and MnO₄⁻, CTAB and MnO₄⁻, respectively, are presented. A mechanism consistent with kinetic results has been discussed. Complex formation constant (K_c) and micellar binding constant (K_s) were calculated at 30 °C and found to be K_c = 319 mol⁻¹ dm⁻³ and K_s = 1127 mol⁻¹ dm⁻³, respectively.     

Kinetic and Mechanistic Studies on [Zn(II)-Gly-Phe]+–Ninhydrin Reaction in Aqueous and Cationic CTAB Surfactant Micelles

The rates of reaction between metal-dipeptide complex ([Zn(II)-Gly-Phe]⁺) and ninhydrin have been determined in aqueous and aqueous–cationic micelles of cetyltrimethylammonium bromide (CTAB) at 70°C and pH 5.0. The rate data indicate that the reaction follows the template reaction mechanism in both the media. The reaction followed a first-order and fractional-order kinetics with respect to [Zn(II)-Gly-Phe]⁺ and [ninhydrin], respectively, in the excess of ninhydrin over [Zn(II)-Gly-Phe]⁺. The rate constant is affected by [CTAB] changes and maximum rate enhancement is approximately three-fold. CTAB micelles decrease the activation enthalpy and make the activation entropy less negative. Quantitative kinetic analysis of rate constant (k _ψ)–[CTAB] data was performed on the basis of pseudophase model of the micelles (proposed by Menger and Portnoy and developed by Bunton). The values of binding constants K _S for [Zn(II)-Gly-Phe]⁺ and K _N for ninhydrin with micelles are calculated with the help of observed kinetic data. The results obtained in micellar medium are treated quantitatively on the basis of pseudophase model.     

IONIC STRENGTH EFFECTS ON THE GROUND STATE COMPLEXATION and TRIPLET STATE ELECTRON TRANSFER REACTION BETWEEN ROSE BENGAL and METHYL VIOLOGEN

Abstract— The equilibrium constants, K_c, for complexation between methyl viologen dication (MV²+) and Rose Bengal, or Eosin Y, decrease with increasing ionic strength. At zero ionic strength K_c is 6500 (± 500) mol^?1 dm³ for Rose Bengal and 3200 (± 200) mol^?1 dm³ for Eosin Y, and these values decrease to 1500 (± 100) and 680 (± 40) mol^?1 dm³, respectively, at an ionic strength of 0.1 mol dm^?3. K_c is independent of pH between 4.5 and 10. ΔH is -25 (± 1) kJ mol^?1 for complexation with either dye, whereas ΔS is -15 (± 3) J K^?1 mol^?1 for Rose Bengal, and - 23 (± 3) J K^?1 mol^?1 for Eosin Y. The complexation constant for Rose Bengal and the neutral viologen, 4,4'-bipyridinium-N, N'-di(propylsulphonate), (4,4'-BPS), is 420 (± 35) mol^?1 dm³, and independent of ionic strength. No complexation could be observed for either Rose Bengal or Eosin with another neutral viologen, 2,2'-bipyridinium-N,N'-di(propylsulphonate), (2,2'-BPS). MV²+ quenches the triplet state of Rose Bengal with a rate constant of 7 × 10⁹ mol^?1 dm³ s^?1, and this rate constant decreases slightly as ionic strength increases. The cage escape yield following quenching, Φ_cc is very low (Φ_cc= 0.02 (± 0.005), and independent of ionic strength. 4,4'-BPS quenches the triplet state of Rose Bengal with a rate constant of 2.2 (± 0.1) × 10⁹ mol^?1 dm³ s^?1, and gives a cage escape yield of 0.033 (± 0.006). 2,2'-BPS quenches the Rose Bengal triplet with a rate constant of 6 (± 1) × 10⁸ mol^?1 dm³ s^?1 and gives a cage escape yield of 0.07 (± 0.01). Conductivity measurements indicate that MV²+(Cl^?)₂ is completely dissociated at concentrations below 2 × 10^?2 mol dm^?3.     

Dimerisation and complexation of 6-(4′-t-butylphenylamino)naphthalene-2-sulphonate by β-cyclodextrin and linked β-cyclodextrin dimers

This study shows that stereochemical factors largely determine the extent to which 6-(4′-t-butylphenylamino)-naphthalene-2-sulphonate, BNS^?  and its dimer, (BNS^? )₂, are complexed by β-cyclodextrin, βCD, and a range of linked βCD dimers. Fluorescence and ¹H NMR studies, respectively, show that BNS^?  and (BNS^? )₂ form host–guest complexes with βCD of the stoichiometry βCD.BNS^?  (10^? 4 K ₁ = 4.67 dm³ mol^? 1) and βCD.BNS₂ ^2 ?  (10^? 2 K ₂′ = 2.31 dm³ mol^? 1), where the complexation constant K ₁ = [βCD.BNS^? ]/([βCD][BNS^? ]) and K ₂′ = [βCD. (BNS^? )₂]/([βCD.BNS^? ][BNS^? ]) in aqueous phosphate buffer at pH 7.0, I = 0.10 mol dm³ at 298.2 K. (The dimerisation of BNS^?  is characterised by 10^? 2 K _d = 2.65 dm³ mol^? 1.) For N,N-bis((2^AS,3^AS)-3^A-deoxy-3^A-β-cyclodextrin)succinamide, 33βCD₂su, N-((2^AS,3^AS)-3^A-deoxy-3^A-β-cyclodextrin)-N′-(6^A-deoxy-6^A-β-cyclodextrin)urea, 36βCD₂su, N,N-bis(6^A-deoxy-6^A-β-cyclodextrin)succinamide, 66βCD₂su, N-((2^AS,3^AS)-3^A-deoxy-3^A-β-cyclodextrin)-N′-(6^A-deoxy-6^A-β-cyclodextrin)urea, 36βCD₂ur, and N,N-bis(6^A-deoxy-6^A-β-cyclodextrin)urea, 66βCD₂ur, the analogous 10^? 4 K ₁ = 11.0, 101, 330, 29.6 and 435 dm³ mol^? 1 and 10^? 2 K ₂′ = 2.56, 2.31, 2.59, 1.82 and 1.72 dm³ mol^? 1, respectively. A similar variation occurs in K ₁ derived by UV–vis methods. The factors causing the variations in K ₁ and K ₂ are discussed in conjunction with ¹H ROESY NMR and molecular modelling studies.     

Mechanism of the oxidation of hydrazoic acid by tetrachloroaurate(III) ion

The oxidation of hydrazoic acid in perchloric acid in the absence of added chloride under pseudo first-order conditions ([HN₃] » [AuCl ₄ ^? ]) is first order in [Au(III)]. Michaelis–Menten type of dependence (linear plots of k _obs ^?1 vs [HN₃]^?1) is observed with respect to [HN₃]. The k _obs is independent of ionic strength and the plot between k _obs ^?1 and [H⁺] is linear. The inner-sphere mechanism is consistent with the formation of an axial complex (K = 25 dm³ mol^?1) between AuCl₃(HO)^? ion and HN₃ prior to its rate determining decomposition (k = 0.0182 s^?1). It is inferred that the free radicals N ₃ ^? do not oxidise Au(II). The reaction becomes outer-sphere in the presence of added Cl^? ions which are inferred to form a cage around the hydronium ion surrounding the AuCl ₄ ^? ions. The penetration of N ₃ ^? through the cage is rate controlling and within the cage, the electron transfer from N ₃ ^? ion to AuCl ₄ ^? is fast. The value of the rate determining constant k ₂ is 0.547 dm³ mol^?1 s^?1 and the equilibrium constant K _Cl for the cage formation is 5 dm³ mol^?1 at 25 °C. It is calculated that the minimum HN₃ concentration required before the reaction exhibits zero-order dependence in HN₃ is 0.31 mol dm^?3 when [H⁺] = 0.18 mol dm^?3 at 25 °C.     

Effect of surfactant micelles on the kinetics of oxidation of D‐fructose by cerium(IV) in sulfuric acid medium

Kinetics of the oxidation of D ‐fructose by cerium(IV) has been investigated both in the absence and presence of surfactants (cetyltrimethylammonium bromide, CTAB, and sodium dodecyl sulfate, SDS) in sulfuric acid medium. The reaction exhibits first‐order kinetics each in [cerium(IV)] and [D ‐fructose] and inverse first order in [H₂SO₄]. The Arrhenius equation is found to be valid for the reaction between 30–50°C. A detailed mechanism with the associated reaction kinetics is presented and discussed. While SDS has no effect, CTAB increases the reaction rate with the same kinetic behavior in its presence. The catalytic role of CTAB micelles is discussed in terms of the pseudophase model proposed by Menger and Portnoy. The association constant K_s that equals to 286 mol^?1 dm³ is found for the association of cerium(IV) with the positive head group of CTAB micelles. The effect of inorganic electrolytes (Na₂SO₄, NaNO₃, NaCl) has also been studied and discussed. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 38: 18–25, 2006     

Kinetics and Mechanism of the Interaction of Di-μ-hydroxobis(1,10-phenanthroline)dipalladium(II) Perchlorate with Thioglycolic Acid and Glutathione in Aqueous Solution

The kinetics of interaction between di-μ-hydroxobis(1,10-phenanthroline)dipalladium(II) perchlorate and thioglycolic acid and with glutathione has been studied spectrophotometrically in aqueous medium as a function of the complex concentration as well as the ligand concentrations, pH, and temperature at constant ionic strength. The observed pseudo-first-order rate constants k _obs (s^?1) obeyed the equation k _obs = k ₁[Nu] (Nu = nucleophile). At pH = 6.5, the interaction with thioglycolic acid shows two distinct consecutive steps and both steps are dependent on the concentration of thioglycolic acid. The rate constants for the process are: k ₁ ≈ 10^?5 s^?1 and k ₂ ≈ 10^?3 dm³ · mol^?1 · s^?1. The association equilibrium constant (K _E) for the outer sphere complex formation has been evaluated together with the rate constants for the two subsequent steps. The other bio-active ligand, glutathione, showed a single step reaction depending on [ligand] with a second-order anation rate constant: the 10² (k ₂) values are (61.72, 79.20, 109.24 and 154.33) dm³ · mol^?1 · s^?1 at 20, 25, 30 and 35 °C, respectively. On the basis of the kinetic observations and evaluated activation parameters, plausible associative mechanisms are proposed for both interaction processes.     

Effect of dicationic gemini surfactants 16–s‐16 (s = 4, 5, 6) on the ninhydrin‐dipeptide (glycyl‐tyrosine) reaction

The effect of dicationic gemini surfactants H₃₃C₁₆(CH₃)₂N⁺‐(CH₂)_s‐N⁺(CH₃)₂ C₁₆H₃₃, 2Br^? (s= 4, 5, 6) on the reaction of a dipeptide glycyl–tyrosine (Gly–Tyr) with ninhydrin has been studied spectrophotometrically at 70°C and pH 5.0. The reaction follows first‐ and fractional‐order kinetics, respectively, in [Gly–Tyr] and [ninhydrin]. The gemini surfactant micellar media are comparatively more effective than their single chain–single head counterpart cetyltrimethylammonium bromide (CTAB) micelles. Whereas typical rate constant (k_Ψ) increase and leveling‐off regions, just like CTAB, are observed with geminis, the latter produces a third region of increasing k_Ψ at higher concentrations. This subsequent increase is ascribed to the change in the micellar morphology of the geminis. The pseudophase model of micelles was used to quantitatively analyze the k_Ψ ? [gemini] data, wherein the micellar‐binding constants K_S for [Gly–Tyr] and K_N for ninhydrin were evaluated. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 800–809, 2012     

Kinetics and mechanism of the oxidation of tris(2,2′-bipyridine)iron(II) and tris(1,10-phenanthroline)iron(II) complexes by nitropentacyanocobaltate(III) in acidic aqueous medium

The kinetics of the oxidation of tris(2,2′-bipyridyl)iron(II) and tris(1,10-phenanthroline)iron(II) complexes ([Fe(LL)₃]²⁺, LL = bipy, phen) by nitropentacyanocobaltate(III) complex [Co(CN)₅NO₂]^3? was investigated in acidic aqueous solutions at ionic strength of I = 0.1 mol dm^?3 (HCl/NaCl). The reactions were carried out at fixed acid concentration ([H⁺] = 0.01 mol dm^?3) and the temperature maintained at 35.0 ± 0.1 °C. Spectroscopic evidence is presented for the protonated oxidant. Protonation constants of 360.43 and 563.82 dm³ mol^?1 were obtained for the monoprotonated and diprotonated Co(III) complexes respectively. Electron transfer rates were generally faster for [Fe(bipy)₃]²⁺ than [Fe(phen)₃]²⁺. The redox complexes formed ion-pairs with the oxidant with increasing concentration of the oxidant over that of the reductant. Ion-pair constants for these reaction were 160.31 and 131.9 dm³ mol^?1 for [Fe(bipy)₃]²⁺ and [Fe(phen)₃]^2+, respectively. The activation parameters measured for these systems have values as follows: ?H ^≠ (kJ K^?1 mol^?1) = +113.4 ± 0.4 and +119 ± 0.3; ?S ^≠ (J K^?1) = +107.6 ± 1.3 and 125.0 ± 1.6; ?G ^≠ (kJ K^?1) = +81 ± 0.4 and +82.4 ± 0.4; and E _a (kJ mol^?1) = 115.9 ± 0.5 and 122.3 ± 0.6 for LL = bipy and phen, respectively. Effect of added anions (Cl^?, $ {\text{SO}}_{4}^{2 - } $ and $ {\text{ClO}}_{4}^{ - } $ ) on the systems showed decrease in the electron transfer rate constant. An outer-sphere mechanism is proposed for the reaction.     

A thermodynamic study of α-, β-, and γ-cyclodextrin-complexed m-methyl red in alkaline solutions

The UV/Visible spectra of m-methyl red (m-MR) ({3-[4-(dimethyl-amino) phenylazo] benzoic acid}) were examined in basic, acidic and strongly acidic aqueous solutions. The observed spectra of m-MR were analyzed and compared with the tautomeric and resonance structures that suggested theoretically. Three isosbestic points in the spectra were observed around 508, 464 and 443 nm representing three different equilibriums between four different species of m-MR. The inclusion constant (K_f) for the inclusion of basic form of m-MR with alpha-, Beta-, and gamma-Cyclodextrin (α-, β- and γ-CD) was evaluated at different temperatures using Benesi-Hildebrand method. The values of K_f at 25 °C were found to be 8.70 × 10³, 4.93 × 10³ mol^?1 dm³ and 2.95 × 10⁷ mol^?2 dm⁶ basis on the inclusion complex ratios (m-MR:CD) of 1:1, 1:1, and 2:1 respectively. The values of the thermodynamic quantities ΔH°, ΔS°, ΔG° for the different inclusion processes were calculated by using Van’t Hoff plot. For all cases of the studied inclusion processes, these inclusions were favored through entropy and enthalpy changes.     

Thermodynamic and FTIR studies of the interactions between sodium dodecyl sulphate and strong acids – atypical conductivity pattern

Micellisation of sodium dodecyl sulphate (SDS) was studied in the presence of hydrochloric acid (HCl) and perchloric acid (HClO₄) using conductometry method. The conductivity-[SDS] plots showed abnormal profile pattern at [HCl] > 0.002 mol dm^?3 and [HClO₄] > 0.001 mol dm^?3. Below these acid concentrations, conductivity pattern was normal, and the critical micelle concentration (CMC) values of SDS were lower in both acids than in water. At high acid concentrations, post-micellar slopes were negative. Fourier transform infrared (FTIR) analysis showed significant shifts in the bands suggesting the formation of dodecyl hydrogen sulphate by SDS at high acid concentrations. Thermodynamic parameters for SDS micellisation at low acid concentrations ([HCl] = 0.002 mol dm^?3 and [HClO₄] = 0.001 mol dm^?3) were determined in the temperature range 15–40°C. As temperature increases, the change in enthalpy and entropy of micellisation becomes less positive, and the change in free energy of micellisation becomes increasingly negative.     

Host–guest interaction of hemicucurbiturils with phenazine hydrochloride salt

The host–guest interactions between phenazine hydrochloride salt (PheH⁺) and hemicucurbit[n]uril (n = 6 or 12) (HemiQ[6 or 12]) have been studied by ¹H NMR, UV–vis, IR, mass spectrometry (MS) and quantum chemistry. In ¹H NMR spectra, the broadening of proton resonances of the hosts suggests the interactions of PheH⁺ with HemiQs. The quantitative stabilities of the host–guest systems have been obtained by UV–vis titration experiments, that is, the stoichiometric interactions of PheH⁺ with HemiQ[6] have been observed with an association constant of K_a = (2.5 ± 1.2) × 10⁶ L mol^? 1, while the 2:1 ratio complexes of PheH⁺ with HemiQ[12] are formed with stepwise association constants of K₁ = (9.2 ± 2.8) × 10⁴ L mol^? 1 and K₂ = (6.4 ± 0.9) × 10⁵ L mol^? 1, respectively, which induce a total association constant of K_a = 5.9 × 10¹⁰ L² mol^? 2. Both the 1:1 and 2:1 complexes have been detected by MS. Quantum chemistry calculations have been used to understand the static structures and thermodynamic stabilities of the supramolecular assemblies.     

Properties and Extraction for [Ni(NH3)6]2+ of ATPS-a Formed by Aqueous Cationic–Anionic Surfactant Mixtures

The properties and extraction for [Ni(NH₃)₆]²⁺ of anionic aqueous two-phase systems (ATPS-a) that formed in mixtures of cetyltrimethylammonium bromide (CTAB) and excess sodium dodecyl sulfate (SDS) aqueous solutions were investigated. The results showed that the properties and extraction effects were strongly affected by the surfactant concentration, the temperature of system, and the mole fraction of surfactants. The increase of temperature induces narrower phase region and larger phase volume ratio. In addition, [Ni(NH₃)₆]²⁺ was extracted into the surfactant-rich phase with higher distribution coefficient when the liquid crystal had the birefringent properties. Moreover, the distribution coefficient can be improved through reducing the concentration of surfactant from 0.15 to 0.05 mol · L^?1 or increasing mole fraction of CTAB from 21.9% to 23.1%. The results showed that ATPS of cationic–anionic surfactants was efficient for [Ni(NH₃)₆]²⁺ extraction with distribution coefficients of 13.5 when the total surfactant concentration was 0.05 mol · L^?1, mole fraction of CTAB was 21%, and temperature was 34°C.     

Alginate polyelectrolyte ionotropic gels. XVIII. Oxidation of alginate polysaccharide by potassium permanganate in alkaline solutions: Kinetics of decomposition of intermediate complex

The kinetics of decomposition of [Alg · Mn ^VIO₄^2?] intermediate complex have been investigated spectrophotometrically at a constant ionic strength of 0.5 mol dm^?3. The decomposition reaction was found to be first-order in the intermediate concentration. The results showed that the rate of reaction was base-catalyzed. The kinetic parameters have been evaluated and found to be ΔS^? = ?103.88±6.18 J mol^?1 K^?1, ΔH^? = 51.61 ± 1.02 kJ mol^?1, and ΔG^? = 82.57 ± 2.86 kJ mol^?1, respectively. A reaction mechanism consistent with the results is discussed. © 1993 John Wiley & Sons, Inc.     

Large inclusion constants of ��-cyclodextrin with carborane derivatives

Complexation of some o-, m- and p-carborane derivatives with ??-cyclodextrin was investigated using phenolphthalein in pH 10.5 (0.05 mol dm^?3) borate buffer. Some carborane derivatives indicated large inclusion constants K_ass > 1 × 10⁵ dm³ mol^?1.     

Inner-sphere oxidation of a ternary dipicolinatochromium(III) complex involving a malonic acid co-ligand

The oxidation of a ternary complex of chromium(III), [Cr^III(DPA)(Mal)(H₂O)₂]^?, involving dipicolinic acid (DPA) as primary ligand and malonic acid (Mal) as co-ligand, was investigated in aqueous acidic medium. The periodate oxidation kinetics of [Cr^III(DPA)(Mal)(H₂O)₂]^? to give Cr(VI) under pseudo-first-order conditions were studied at various pH, ionic strength and temperature values. The kinetic equation was found to be as follows: \( {\text{Rate}} = {{\left[ {{\text{IO}}_{4}^{ - } } \right]\left[ {{\text{Cr}}^{\text{III}} } \right]_{\text{T}} \left( {{{k_{5} K_{5} + k_{6} K_{4} K_{6} } \mathord{\left/ {\vphantom {{k_{5} K_{5} + k_{6} K_{4} K_{6} } {\left[ {{\text{H}}^{ + } } \right]}}} \right. \kern-0pt} {\left[ {{\text{H}}^{ + } } \right]}}} \right)} \mathord{\left/ {\vphantom {{\left[ {{\text{IO}}_{4}^{ - } } \right]\left[ {{\text{Cr}}^{\text{III}} } \right]_{\text{T}} \left( {{{k_{5} K_{5} + k_{6} K_{4} K_{6} } \mathord{\left/ {\vphantom {{k_{5} K_{5} + k_{6} K_{4} K_{6} } {\left[ {{\text{H}}^{ + } } \right]}}} \right. \kern-0pt} {\left[ {{\text{H}}^{ + } } \right]}}} \right)} {\left\{ {\left( {\left[ {{\text{H}}^{ + } } \right] + K_{4} } \right) + \left( {K_{5} \left[ {{\text{H}}^{ + } } \right] + K_{6} K_{4} } \right)\left[ {{\text{IO}}_{4}^{ - } } \right]} \right\}}}} \right. \kern-0pt} {\left\{ {\left( {\left[ {{\text{H}}^{ + } } \right] + K_{4} } \right) + \left( {K_{5} \left[ {{\text{H}}^{ + } } \right] + K_{6} K_{4} } \right)\left[ {{\text{IO}}_{4}^{ - } } \right]} \right\}}} \) where k ₆ (3.65 × 10^?3 s^?1) represents the electron transfer reaction rate constant and K ₄ (4.60 × 10^?4 mol dm^?3) represents the dissociation constant for the reaction \( \left[ {{\text{Cr}}^{\text{III}} \left( {\text{DPA}} \right)\left( {\text{Mal}} \right)\left( {{\text{H}}_{2} {\text{O}}} \right)_{2} } \right]^{ - } \rightleftharpoons \left[ {{\text{Cr}}^{\text{III}} \left( {\text{DPA}} \right)\left( {\text{Mal}} \right)\left( {{\text{H}}_{2} {\text{O}}} \right)\left( {\text{OH}} \right)} \right]^{2 - } + {\text{H}}^{ + } \) and K ₅ (1.87 mol^?1 dm³) and K ₆ (22.83 mol^?1 dm³) represent the pre-equilibrium formation constants at 30 °C and I = 0.2 mol dm^?3. Hexadecyltrimethylammonium bromide (CTAB) was found to enhance the reaction rate, whereas sodium dodecyl sulfate (SDS) had no effect. The thermodynamic activation parameters were estimated, and the oxidation is proposed to proceed via an inner-sphere mechanism involving the coordination of IO₄ ^? to Cr(III).     

Kinetics and mechanism of oxidation of a ternary complex involving dipicolinatochromium(III) and DL‐aspartic acid by N‐bromosuccinimide

The kinetics of oxidation of [Cr^III(Dpc)(Asp)(H₂O)₂] (Dpc = dipicolinic acid and Asp = DL ‐aspartic acid) by N‐bromosuccinimide (NBS) in aqueous solution have been found to obey the equation: where k₂ is the rate constant for the electron transfer process, K₁ is the equilibrium constant for deprotonation of [Cr^III(Dpc)(Asp)(H₂O)₂], K₂ and K₃ are the pre‐equilibrium formation constants of precursor complexes [Cr^III(Dpc)(Asp)(H₂O)(NBS)] and [Cr^III(Dpc)(Asp)(H₂O)(OH)(NBS)]^?. Values of k₂ = 4.85 × 10^?2 s^?1, K₁ = 1.85 × 10^?7 mol dm^?3, and K₂ = 78.2 mol^?1 dm³ have been obtained at 30°C and I = 0.1 mol dm^?3. The experimental rate law is consistent with a mechanism in which the deprotonated [Cr^III(Dpc)(Asp)(H₂O)(OH)]^? is considered to be the most reactive species compared to its conjugate acid. It is assumed that electron transfer takes place via an inner‐sphere mechanism. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 394–400, 2004