Kinetics and mechanism of oxidation of ferrocyanide by N-bromosuccinimide in aqueous acidic solution

The kinetics of oxidation of ferrocyanide by N-bromosuccinimide (NBS) has been studied spectrophotometrically in aqueous acidic medium over temperature range 20–35 °C, pH = 2.8–4.3, and ionic strength = 0.10–0.50 mol dm⁻³ over a range of [Fe²⁺] and [NBS]. The reaction exhibited first order dependence on both reactants and increased with increasing pH, [NBS], and [Fe²⁺]. The rate of oxidation obeys the rate law: d[Fe³⁺]/dt = [Fe(CN)₆]^4–[HNBS⁺]/(k ₂ + k ₃/[H⁺]). An outer-sphere mechanism has been proposed for the oxidation pathway of both protonated and deprotonated ferrocyanide species. Addition of both succinimide and mercuric acetate to the reaction mixture has no effect on the reaction rate under the experimental conditions. Mercuric acetate was added to the reaction mixture to act as scavenger for any bromide formed to ensure that the oxidation is entirely due to NBS oxidation.     

Theoretical investigation of the intermolecular H-bonding and proton transfer in cytosine assisted by water and methanol

Abstract  

This investigation is devoted to study of the electronic structure of several H-bonded complexes of cytosine tautomers with water and methanol. The stability of these “supersystems” (aggregates) was estimated by calculating the bonding energies ΔE and ΔE _b. The energy barriers of water/methanol-assisted tautomeric conversions were calculated (intermolecular proton transfer), and the electronic structures of the transition states were studied. Each transition state of the proton-transfer reaction was determined as a first-order saddle point on the potential energy surface (full coordinate hyperspace). The crystal structure of the tetramer of cytosine monohydrate was also investigated. It was found that this structure is close to a conical intersection.     

Olefin Epoxidation by Methyltrioxorhenium: A Density Functional Study on Energetics and Mechanisms

A spiro attack on a peroxo group is calculated to be the preferred reaction pathway for olefin epoxidation with the catalytic system CH₃ReO₃/H₂O₂ (see picture). This finding is supported by density functional calculations on more than ten transition states for the most probable mechanisms. Hydration has significant effects on various reaction species: it stabilizes the intermediates and destabilizes, with one exception, the transition states.     

Kinetics of oxidation of N,N-bis(salicylaldehyde-1,2-diaminoethane) cobalt(II) complex by periodate. Evidence for the inhibiting effect of copper(II)

A kinetic study of the oxidation of [Co(H₂L)(H₂O)₂]²⁺ (H₂L = N,N-bis (salicylaldehyde-1,2-diaminoethane) Schiff base) by periodate in aqueous solution was performed over pH (2.3–3.4) range, (0.1–0.5) mol dm⁻³ ionic strength and temperatures 20–35 °C for a range of periodate and complex concentrations. The reaction rate showed a first-order dependence on both reactants and increased with pH over the range studied. The effects of Cu(II) and Fe(II) on the reaction rate were investigated over the (1.0–9.0) × 10⁻⁵ mol dm⁻³ range. The reaction was inhibited as the concentration of Cu(II) increased, and it was independent on Fe(II) concentrations over the ranges studied. An inner-sphere mechanism is proposed for the oxidation pathways of both the protonated and deprotonated Co^II complex species.     

Research on the rupture of the covalent bond of the dichlorine molecule

Abstract  

Photoinduced reaction of gas-phase dichlorine molecules on the short wavelength side of the A-band gives a negative value for the asymmetry parameter of Cl (² P _1/2) fragments, which conflicts with the intrinsic electronic transition mechanism. In this paper, the dissociation process of the dichlorine molecule has been investigated at numerous excitation wavelengths in the range 310–470 nm using the numerical method and the frontier molecular orbital maps are drawn to obtain insight into the character of the relevant molecular orbitals. The possibilities of radial nonadiabatic transition from the C¹Π_u to the third Ω = 1_u (σ _u* ← σ _g) electronic state are also examined, and found to cause large variations for the angular distribution functions. The wavelength dependence of the beta parameter β ₂(Cl*), which is computed from the partial cross-section with the RZD transition mechanism, agrees with the experimental behavior and further justifies the conclusion that the decrease of beta in the Cl + Cl* channel is because of the radial nonadiabatic interaction between the C and 1_u(III) excited states and this interaction is a key mechanism decreasing the beta parameter value. At last, the kinetic energy distributions of fragments are obtained in the asymptotic region.     

Structural characterization of alachlor complexes with transition metal ions by electrospray ionization tandem mass spectrometry

Electrospray ionization mass (ESI-MS) spectrometry was used to investigate the nature of metal complexes of alachlor and their dissociations on activation. Ions of the first row transition metal series were employed to react with alachlor and the products were subjected to collision-induced dissociation (CID) for further structural characterization. The formation of diverse complex ions including doubly charged metal/alachlor complexes; [3L + M]²⁺ and [4L + M]²⁺ (L: alachlor and M: transition metal ions) were observed depending on the experimental conditions including the tube lens offset voltage (TLOV) and relative concentrations of alachlor and transition metal ions. It is clear that complexation with transition metal ions alters the reactive site of alachlor, promoting the loss of chlorine over the loss of CH₃OH that is the major reaction pathway in uncomplexed system. Direct elimination of chlorine from alachlor molecule was confirmed by the use of MnBr₂ instead of MnCl₂. These evidences clearly illustrate the catalytic activities of the metal ions through insertion mechanism. The function of transition metal ions in complexation was emphasized comparing the fragmentation patterns with those of protonated molecule. A change in the oxidation state of copper from + 2 to + 1 during the dissociation of metal complex was observed in company with elimination of radicals which is specific for the copper complex ions.     

Electrochemical activation of the alkylation of GeI2 byn-butyl bromide and methylcobalt(iii) bisdimethylglyoximate

The mechanism of the alkylation of GeI₂ byn-butyl bromide and methylcobalt(iii) bisdimethylglyoximate in acetonitrile was studied using cyclic voltammetry. The reaction with the methylcobalt complex occursvia anodic activation (oxidation of the methylcobalt complex), while the reaction withn-butyl bromide is initiated by cathodic activation (reduction of GeI₂). Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2157–2160, December, 1997.     

Iron(III)–salen–H<Subscript>2</Subscript>O<Subscript>2</Subscript> as a peroxidase model: electron transfer reactions with anilines

Abstract  

Iron(III)–salen complexes catalyze the H₂O₂ oxidation of various ring-substituted anilines in MeCN have been studied, and [O=Fe^IV(salen)]^+· is proposed as the active species. Study of the kinetics of the reaction by spectrophotometry shows the emergence of a new peak at 445 nm in the spectrum which corresponds to azobenzene. Further oxidation of azobenzene by H₂O₂ leads to the formation of azoxybenzene. ESI–MS studies also support the formation of these products. The rate constants for the oxidation of meta- and para-substituted anilines were determined from the rate of decay of oxidant as well as the rate of formation of azobenzene, and the reaction follows Michaelis–Menten kinetics. The rate data show a linear relationship with the Hammett σ constants and yield a ρ value of −1.1 to −2.4 for substituent variation in the anilines. A reaction mechanism involving electron transfer from aniline to [O=Fe(salen)]^+· is proposed. The presence of axial ligands modulates the activity of the complex.     

On the protonation of dibenzo-18-crown-6

Abstract  

The stability constant of the dibenzo-18-crown-6·H₃O⁺ cationic complex species dissolved in nitrobenzene saturated with water has been determined from extraction experiments in the two-phase water–nitrobenzene system and from γ-activity measurements. Various structures of protonated dibenzo-18-crown-6 are discussed.     

Study of <Emphasis Type="Italic">cis</Emphasis>–<Emphasis Type="Italic">trans</Emphasis> isomerization mechanism of 3,3′-azobenzene disulphonate in the lowest singlet and triplet electronic states by density functional theory

Abstract  

The cis–trans isomerization pathways of 3,3′-azobenzene disulphonate in the S₀ and T₁ states are studied by DFT method at the B3LYP/6-31G(d,p) level. In the S₀ state, the cis–trans isomerization concerns the complex pathway that is characterized by the inversion of one NNC angle combined with rotation around the NC bond, and the three sequential transition states are also found on the potential energy profile. Therefore, the cis–trans isomerization of 3,3′-azobenzene disulphonate can be understood in terms of a pathway involving successive rotation, inversion, and rotation processes. The energy barrier of the S₀ state is 22.79 kcal mol⁻¹. In the T₁ state, the isomerization mainly concerns the rotational pathway around the NN double bond, and the two isomers are connected through only one transition state. The isomerization of the T₁ state is related to a lower energy barrier, 5.02 kcal mol⁻¹, but requires a change in spin-multiplicity.     

The first step of oxidation of alkylbenzenes by permanganates in acidic aqueous solutions

Data on the kinetics, kinetic isotope effects, substrate selectivety, and activation parameters for the first step of oxidation of alkylbenzenes by permanganante in acidic aqueous solutions are surveyed. The MnO₄ ⁻, HMnO₄, and MnO₃ ⁺ species serve as oxidants at different acidities. The increase in the positive charge in this series enhances the electrophilicity of the reagent, which manifests itself as an increase in the reaction rate and a change in the site of attack on the alkylbenzene molecule (either the aromatic ring or C−H bond in the alkyl group). The oxidation of the alkyl C−H bonds in alkylbenzenes and in alkanes follows similar mechanisms, while the attack on the aromatic ring proceedsvia the electrophilic aromatic substitution mechanism with a transition state intermediate between the charge transfer complex and σ-complex. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 765–780, May, 2000.     

Extraction and DFT study on the complexation of H3O+ ion with tetrakis(2-ethoxyethoxy)-tetra-p-tert-butylcalix[4]arene

Abstract  

Extraction experiments in the two-phase water/nitrobenzene system and γ-activity measurements were used to determine the stability constant of protonated tetrakis(2-ethoxyethoxy)-tetra-p-tert-butylcalix[4]arene in nitrobenzene saturated with water. Density functional theory (DFT) calculations were applied to derive the most probable structure of the tetrakis(2-ethoxyethoxy)-tetra-p-tert-butylcalix[4]arene·H₃O⁺ complex species.     

Synthesis, characterization, and electron transfer reaction of some surfactant–cobalt(III) complex ions

Abstract  

The surfactant complex ion cis-[Co(tmd)₂(C₁₂H₂₅NH₂)₂]³⁺ (tmd = 1,3-propanediamine, C₁₂H₂₅NH₂ = dodecylamine) has been synthesized and characterized by elemental analysis and spectral data. In addition we have determined the critical micelle concentration of the surfactant–cobalt(III) complex and studied the kinetics and mechanism of the complex with ferrocyanide anion. The reaction is found to be second order, and the second-order rate constant increases with increasing initial concentration of the surfactant–cobalt(III) complex due to the presence of self-micelles formed by the complex itself. The thermodynamic parameters were determined. The results have been analyzed.     

Computational study on the reaction mechanism of the gas-phase atom-negative ion of S?+?NO2 ?: comparative study of mechanism with S?+?O3 reaction as isoelectronic and isostructure systems

The reaction mechanism of sulfur vapor (S) with nitrite ion (NO₂ ⁻) has been investigated theoretically on the triplet and singlet potential energy surfaces (PESs). All stationary points for the title reaction have been optimized at the B3LYP/6-311+G(3df) level. The energetic data have been obtained at the CCSD(T)//B3LYP level employing the 6-311+G(3df) basis set. Five stable collision complexes, ³IN1 (S–ONO⁻), ³IN2 (cyclic SONO⁻), ¹IN1 (cis S–ONO⁻), ¹IN2 (S–NO₂ ⁻), and ¹IN3 (trans S–ONO⁻), have been considered on the triplet and singlet PESs through barrier-less and exothermic processes. By starting from these complexes, a simple mechanism has been obtained on the triplet PES while a complex mechanism has been considered on the singlet PES. The calculated results show that there are no favorable paths for the reaction of S with NO₂ ⁻ on the singlet PES. Therefore, the S + NO₂ ⁻ reaction proceeds only on the triplet PES to produce ³SO + ³NO⁻ as main products. The results from the comparative study of S + NO₂ ⁻ reaction mechanism with S + O₃ (as isoelectronic and isostructure reactions) on the singlet PES show similarities in the overall trend of reaction mechanism and atom connectivity and differences in the stability of intermediates and the energy barriers of transition states.     

Oxovanadium(IV)‐Salen Ion Catalyzed H2O2 Oxidation of Tertiary Amines to N‐Oxides– Critical Role of Acetate Ion as External Axial Ligand

The oxovanadium(IV)‐salen ion catalyzed H₂O₂ oxidation of N,N‐dimethylaniline forms N‐oxide as the product of the reaction. The reaction follows Michaelis–Menten kinetics and the rate of the reaction is accelerated by electron donating groups present in the substrate as well as in the salen ligand. This peculiar substituent effect is accounted for in terms of rate determining bond formation between peroxo bond of the oxidant and the N‐atom of the substrate in the transition state. Trichloroacetic acid (TCA) shifts the λ_max value of the oxidant to the red region and catalyzes reaction enormously. The cleavage of N? O bond by vanadium complex leads to moderate yield of the product. But the percentage yield of the product becomes excellent in the presence of TCA.     

Manganese(II), cobalts(II) and nickel(II) complexes of tris(2-pyridyl)phosphine and their catalytic activity toward oxidation of tetralin

Abstract  

Monomeric Mn²⁺, Co²⁺ and Ni²⁺ complexes of tris(2-pyridyl)phosphine (P(2-py)₃ were synthesized through the reaction of the hydrated metal(II) chlorides with P(2-py)₃ in near-quantitative yields. The solid-state structure of the Mn complex was determined by single-crystal X-ray diffraction. All three complexes were tested as homogeneous catalysts for the oxidation of tetralin to α-tetralone with tert-butyl hydroperoxide (TBHP) as oxidant. The influences of temperature, solvent, catalyst molar ratio and time of the reaction on the catalyzed reactions were investigated.     

Kinetics and mechanism of the oxidation of 2,3,6-trimethylphenol with hydrogen peroxide in the presence of Ti-monosubstituted polyoxometalates

The product composition and reaction kinetics are reported for 2,3,6-trimethylphenol (TMP) oxidation with hydrogen peroxide in acetonitrile catalyzed by a Ti-monosubstituted polyoxometalate (Ti-POM) with a Keggin structure ([Bu₄N]₄[PTi(OMe)W₁₁O₃₉]) and for the stoichiometric reaction between TMP and the peroxo complex [Bu₄N]₄[HPTi(O)₂W₁₁O₃₉] (I). The main products of the stoichiometric reaction are 2,3,5-trimethyl-1,4-benzoquinone (TMBQ) and 2,2′,3,3′,6,6′-hexamethyl-4,4′-biphenol (BP). The TMBQ yield increases as the TMP/I molar ratio is decreased. The catalytic reaction is first-order with respect to H₂O₂ and the catalyst and has a variable order (1-0) with respect to TMP. The rate of the reaction increases as the water concentration in the reaction mixture is raised. The stoichiometric reaction is first-order with respect to peroxo complex I and has a variable order (1-0) with respect to TMP. There is no kinetic isotope effect for this reaction (k _ArOH/k _ArOD = 1). A TMP oxidation mechanism is suggested, which includes the coordination of a TMP molecule and peroxide on a Ti site of the catalyst with the formation of a reactive intermediate. The one-electron oxidation of TMP in this intermediate yields a phenoxyl radical. The subsequent conversions of these ArO^° radicals yield the reaction products.     

Mechanistic investigations of ruthenium(III) catalyzed oxidation of pentoxifylline by copper(III) periodate complex in aqueous alkaline medium

Abstract  

The kinetics of the oxidation of ruthenium(III)-catalyzed oxidation of pentoxifylline (PTX) by diperiodatocuprate(III) (DPC) in aqueous alkaline medium at a constant ionic strength of 0.30 mol dm⁻³ was studied spectrophotometrically. The reaction between PTX and DPC in alkaline medium in the presence of Ru(III) exhibits 1:2 stoichiometry (PTX:DPC). The reaction was of first order in DPC, less than the unit order in [PTX] and [OH⁻] and negative fractional order in [IO₄ ⁻]. The order in [Ru(III)] was unity. Intervention of free radicals was observed in the reaction. The main products were identified by TLC and spectral studies including LC-MS. The oxidation reaction in alkaline medium has been shown to proceed via a Ru(III)-PTX complex, which reacts with monoperiodatocuprate(III) to decompose in a rate determining step followed by a fast step to give the products. The reaction constants involved in different steps of the mechanism were calculated. The activation parameters with respect to the slow step of the mechanism were computed and discussed, and thermodynamic quantities were also determined. The active species of catalyst and oxidant have been identified.     

New synthetic route to modafinil drug including desulfobenzhydrylation of sodium carbamoylmethyl thiosulfate: experimental and quantum chemical studies

A new synthetic route to 2-benzhydrylsulfinylacetamide (1), a nootropic drug modafinil, is described. The synthesis includes the alkylation of sodium thiosulfate with chloroacetamide to sodium carbamoylmethyl thiosulfate, the desulfobenzhydrylation of the latter by benzhydrol in formic acid to form benzhydrylthioacetamide (3a), and the further oxidation of this thioamide with hydrogen peroxide. According to B3LYP/6-31G** DFT calculations, the key step of the synthesis, namely, desulfobenzhydrylation of salt 6a, occurs only insignificantly due to the energetically unfavorable direct attack of this salt by benzhydryl formate; the reaction mainly involves the attack by the benzhydrilium carbocation Ph₂CH⁺. The oxidation of sulfide 3a to sulfinylacetamide 1 is efficiently catalyzed by side proton-donor molecules (constituents of the transition states of the reaction). The oxidant can be the anionic form of the reactant (HO₂ ^- ion), which reacts with sulfide 3a via the unusual noncatalytic mechanism. At the step of transition state formation, this mechanism resembles the SN ₂ substitution.     

Inner-sphere oxidation of 2-aminomethylpyridinecobalt(II) complex by N-bromosuccinimide

The kinetics of the oxidation of the 2-aminomethylpyridineCo^II complex by N-bromosuccinimide (NBS), have been studied in aqueous solutions under various conditions, and obey the following rate law:Rate = [NBS][Co(L)(H₂O)₂]²⁺[k₂₊k₃/[H⁺]]An inner-sphere mechanism is proposed for the oxidation pathway for both protonated and deprotonated complex species, with the formation of an intermediate, which is slowly converted into the final oxidation products. The reaction rate is increased by increasing the pH, T, [complex], and decreased by increasing ionic strength over the range studied.