Kinetics and mechanism of oxidation of neutral α ‐amino acids by sodium N‐chloro‐p‐toluenesulfonamide in acid medium

Kinetics of oxidation of α ‐amino acids, glycine, valine, alanine, and phenylalanine, by sodium N‐chloro‐p‐toluenesulfonamide or chloramine‐T (CAT) has been investigated in HClO₄ medium at 30°C. The rate shows first‐order dependence on both CAT and amino acid concentrations and an inverse first‐order on [H⁺]. The variation of ionic strength and the addition of p‐toluenesulfonamide and Cl^? ion had no effect on the reaction rate. Decrease of dielectric constant of the medium by increasing the MeOH content decreased the rate. Rate studies in D₂O medium showed the inverse solvent‐isotope effect of k_D2O/k_H2O=0.50. Proton‐inventory studies were carried out using H₂O–D₂O mixtures. The activation parameters have been computed. The proposed mechanism and the derived rate law are consistent with the observed kinetic data. An isokinetic relationship is observed with β=323 K, indicating enthalpy as a controlling factor. The rate of oxidation increases in the following order: Gly < Val < Phe < Ala. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 34: 49–55, 2002     

Über Chalkogenolate. 89. Untersuchungen über N-Dicyandithiocarbamidsäure Darstellung und Eigenschaften der freien Säure

On Chalcogenolates. 89. Studies on N-Dicyandithiocarbamic Acid. Preparation and Properties of the Free Acid Colorless N-dicyandithiocarbamic acid (melting point: 112°C) has been prepared by reaction between a suspension of K[S₂C? N(CN)₂] in diethyl ether and a solution of HCl in (C₂H₅)₂O at 0°C; the ether was distilled off at 0°C in vacuo. The compound has been characterized by means of infared spectra, electron absorption spectra, ¹H-NMR spectra, and mass spectra. The dissociation constant of N-dicyandithiocarbamic acid in water is K_a = (1.69 ± 0.1) X 10^?1 20°C. The thermodynamic data of the dissociation were calculated.     

Über Chalkogenolate. 93. Untersuchungen über Trithioallophansäure 2. Darstellung und Eigenschaften der freien Säure [1]

On Chalcogenolates. 93. Studies on Trithioallophanic Acid 2. Preparation and Properties of the Free Acid Yellow trithioallophanic acid H₂N? CS? NH? CS(SH) has been prepared by reaction between a suspension of K[S₂C? NH? CS? NH₂] in diethyl ether and a solution of HCl in (C₂H₅)₂O at ?15°C; the ether was distilled off at ?15°C in vacuo. The compound has been characterized by means of infrared spectra, electron absorption, ¹H-NMR spectra, and mass spectra. The dissociation constant of trithioallophanic acid in water is K_a = (1,41 ± 0,08) · 10^?2 at 20°C.     

Kinetics and mechanism of the oxidation of primary alcohols by sodium N-chloroethylcarbamate

The oxidation of primary alcohols by sodium N-chloroethylcarbamate in acid solution, results in the formation of corresponding aldehydes. The reaction is first order with respect to the oxidant and alcohol. The rate increases with an increase in acidity. The oxidation of α,α-dideuterioethanol exhibited a primary kinetic isotope, k_H/k_D = 2.11 at 298 K. The value of solvent isotope effect k(H₂O)/k(D₂O) = 2.23 at 298 K. Addition of ethyl carbamate does not affect the rate. (EtOC(OH)NHCl)⁺ has been postulated as the reactive species. Plots of (log k₂ + Ho) against (Ho + log[H⁺]) are linear with the slope, ?, having values from 1.78–1.87. This suggested a proton abstraction by water in the rate-determining step. The rates of oxidation of alcohols bearing both electron-withdrawing and electron-donating groups are more than that of methanol. A concerted mechanism involving transfer of a hydride ion from the C? H bond of the alcohol tothe oxidant and removal of a proton from the O? H group by a water molecule has been proposed.     

1‐Methyl‐4‐nitraminopyridinium nitrate and 4‐nitraminopyridinium methanesulfonate

In the title compounds, C₆H₈N₃O₂⁺·NO₃^? and C₅­H₆­N₃­O₂⁺·­CH₃SO₃^?, respectively, the cations are almost planar; the twist of the nitr­amino group about the C—N and N—N bonds does not exceed 10°. The deviations from coplanarity are accounted for by intermolecular N—H?O interactions. The coplanarity of the NHNO₂ group and the phenyl ring leads to the deformation of the nitr­amino group. The C—N—N angle and one C—C—N angle at the junction of the phenyl ring and the nitr­amino group are increased from 120° by ca 6°, whereas the other junction C—C—N angle is decreased by ca 5°. Within the nitro group, the O—N—O angle is increased by ca 5° and one O—N—N angle is decreased by ca 5°, whereas the other O—N—N angle remains almost unchanged. The cations are connected to the anions by relatively strong N—H?O hydrogen bonds [shortest H?O separations 1.77 (2)–1.81 (3) Å] and much weaker C—H?O hydrogen bonds [H?O separations 2.30 (2)–2.63 (3) Å].     

The reaction of O(1D) with CH4

The room-temperature photolysis of N₂O (10–100 torr) at 2139 Å to produce O(¹D) has been studied in the presence of CH₄ (10–891 torr). The reactions of O(¹D) with CH₄ were found to be The method of chemical difference was used to measure the rate constant ratio k₄/(k₂ + k₃), where reactions (2) and (3) are The CH₃ radicals produced in reaction (4) react with the O₂ and NO produced in reactions (2) and (3). Thus, near the endpoint of the internal titration, ?{C₂H₆} gives an accurate measure of k₄/(k₂ + k₃). For the translationally energetic O(¹D) atoms produced in the photolysis, k₄/(k₂ + k₃) = 2.28 ± 0.20. However, if He is added to remove the excess translational energy, then k₄/(k₂ + k₃) drops to 1.35 ± 0.3.     

Relative rates of nucleophilic reactions of benzyl carbocation formed in photolysis of benzyl chloride and benzyl acetate in aqueous solution

Benzyl chloride and benzyl acetate were photolyzed in 30% methanol–water mixtures (V/V) at 0°C. The photolysis produces benzyl carbocations that react with nucleophiles. The reaction products were analyzed by gas chromatography or liquid chromatography. From the amounts of products the relative values of rate constants of reactions of benzyl carbocation with nucleophiles N and water k(N)/k(H₂O) were calculated. Benzyl carbocation reacts with I^?, Br^?, Cl^?, and Ac^? ions with approximately diffusion-controlled rate. A value of 2.4 × 10⁷ dm³ mol^?1 s^?1 for the rate constant k(H₂O) and a lifetime of 0.7 ns were estimated for benzyl carbocation in the aqueous solution.     

Über Pterinchemie. 69 Mitteilung [1]. Konformationsanalyse von 5,6,7,8-Tetrahydropteroinsäure und 5,6,7,8-Tetrahydro-L-folsäure

Conformational analysis of 5,6,7,8-tetrahydropteroic acid and 5,6,7,8-tetrahydro-L -folic acid In the 360-MHz-¹H-NMR.-spectrum of (6R, S)-9,9-dideuterio-5, 6, 7, 8-tetrahydropteroic acid (racemic) (XIII) (AMX-System, Fig. 4) and (6R, S)-9,9-dideuterio-5, 6, 7, 8-tetrahydro-L -folic acid (diastereomeric) (XVI) the H_a–C(6) and H_a–C(7) show a vicinal coupling constant of 6,7 Hz and the H_a–C(6) and H_e–C(7) one of 3,2 Hz. The first coupling constant provides evidence for an approximate trans-diaxal arrangement of H_a–C(6) and H_a–C(7), and the second for a gauche conformation of H_a–C(6) and H_e–C(7). The tetrahydropyrazine ring in the racemic 5, 6, 7, 8-tetrahydropteroic acid (III) and in the diastereomeric 5, 6, 7, 8-tetrahydro-L -folic acid (XVII) exists therefore in a half-chair conformation with a pseudoequatorial position of the side chain at C(6) (Fig.5).     

Weitere Evidenz für die mikroskopische Gleichzeitigkeit des Bruches zweier Bindungen bei einer E2-Reaktion: H-Isotopeneffekte und Substituentenwirkungen bei der Reaktion von 2,2-Diphenyläthyl-benzolsulfonat mit Methoxid-Ion

Second order rate constants are determined for the E2 reactions of 2,2-diphenyl-ethyl benzenesulfonates Ph₂C^xHCH₂O · O₂S–C₆H₄–X with CH₃ONa in methyl cellosolve solution (^xH = H or D, X = p-CH₃O, p-CH₃, H or p-NO₂). The HAMMETT ? values are of the same order of magnitude as those found in the first order solvolyses of methyl, ethyl or isopropyl benzenesulfonates. The primary deuterium isotope effects k_H/k_D are 5.27, 5.42 and 6.70 for X = p-CH₃O, H and p-NO₂. The ? values as well as the increase of k_H/k_D with introduction of p-NO₂ supply evidence for simultaneous C_αO and C_βH bond cleavages also in the E2 reactions of these compounds.     

Autoxidation of poly-4-methylpentene-1. I. The role of diffusion in autoxidation kinetics

An apparatus is described for the measurement of oxygen uptake into a polymer sample at constant oxygen pressures in the range 20–1000 mm Hg. Measurements of the rate of oxygen uptake into poly-4-methylpentene-1 show that the rate is accurately first-order in oxygen pressure over the range 50–800 mm pressure for temperatures ranging from 122 to 154°C and film thickness in the range 0.001–0.025 cm. A theoretical treatment of the kinetics of a reaction in which oxygen diffuses into both faces of a thin film, in which it is consumed by a first-order reaction shows that the oxidation rate ρ per unit area of film surface is given by ρ = ρ_∞ tanh ßL/2 where ρ_∞ is the limiting oxidation rate for a thick film, L is the film thickness, and ß = (k/D)^1/2, k being the oxidation rate constant and D the diffusion constant. Values of D and the activation energy for diffusion calculated from autoxidation data are in good agreement with values determined directly.     

Rate constants for aqueous‐phase reactions of SO4− with C2F5C(O)O− and C3F7C(O)O− at 298 K

Perfluorocarboxylic acids and their anions (PFCAs), such as perfluorooctanate (C₇F₁₅C(O)O^?), have been generally recognized to be global pollutants and are believed to persist in the environment. Kinetic data for reactions of sulfate anion radicals (SO₄^?) with PFCAs are needed to evaluate the residence times of PFCAs in the environment, but no kinetic data have been reported, except for the rate constant for the reaction of SO₄^? with trifluoroacetate (CF₃C(O)O^?) (k₁). In this study, using the fact that PFCAs react with SO₄^? to form shorter chain PFCAs, we determined rates relative to k₁ of the reactions of photolytically generated SO₄^? with two short‐chain PFCAs, pentafluoropropionate (C₂F₅C(O)O^?; k₂) and heptafluorobutyrate (C₃F₇C(O)O^?; k₃), along with conversion ratios for conversion of C₂F₅C(O)O^? into CF₃C(O)O^? (α) and conversion of C₃F₇C(O)O^? into C₂F₅C(O)O^? (β) and CF₃C(O)O^? (γ) at 298 K. Values of k₁, k₂, or k₃ might change over the course of reaction with increasing ionic strength. Nevertheless, if the values of k₁/k₂, k₂/k₃, α, β, and γ remain almost constant during the reaction, a simple equation involving relative rates, such as k₁/k₂, can be used to relate the concentrations of C₃F₇C(O)O^?, C₂F₅C(O)O^?, and CF₃C(O)O^?. We compared the relative rates, such as k₁/k₂, and the conversion ratios determined from various experimental runs with different initial conditions to check whether relative rates and conversion ratios remained almost constant during each experimental run. The values of k₁/k₂, k₂/k₃, α, β, and γ seemed to remain almost constant, which facilitated determination of k₂/k₁ = 0.89 ± 0.07, k₃/k₁ = 0.84 ± 0.08, α = 0.88 ± 0.05, β = 0.75 ± 0.05, and γ = 0.17 ± 0.02. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 276–288, 2007     

Radical‐initiated polymerization of β‐methyl‐α‐methylene‐γ‐butyrolactone

β‐Methyl‐α‐methylene‐γ‐butyrolactone (MMBL) was synthesized and then was polymerized in an N,N‐dimethylformamide (DMF) solution with 2,2‐azobisisobutyronitrile (AIBN) initiation. The homopolymer of MMBL was soluble in DMF and acetonitrile. MMBL was homopolymerized without competing depolymerization from 50 to 70 °C. The rate of polymerization (R_p) for MMBL followed the kinetic expression R_p = [AIBN]^0.54[MMBL]^1.04. The overall activation energy was calculated to be 86.9 kJ/mol, k_p/k_t^1/2 was equal to 0.050 (where k_p is the rate constant for propagation and k_t is the rate constant for termination), and the rate of initiation was 2.17 × 10^?8 mol L^?1 s^?1. The free energy of activation, the activation enthalpy, and the activation entropy were 106.0, 84.1, and 0.0658 kJ mol^?1, respectively, for homopolymerization. The initiation efficiency was approximately 1. Styrene and MMBL were copolymerized in DMF solutions at 60 °C with AIBN as the initiator. The reactivity ratios (r₁ = 0.22 and r₂ = 0.73) for this copolymerization were calculated with the Kelen–Tudos method. The general reactivity parameter Q and the polarity parameter e for MMBL were calculated to be 1.54 and 0.55, respectively. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1759–1777, 2003     

Experimental determination of the rate constants of the reactions of HO2 + DO2 and DO2 + DO2

The rate constants of the reactions of DO₂ + HO₂ (R1) and DO₂ + DO₂ (R2) have been determined by the simultaneous, selective, and quantitative measurement of HO₂ and DO₂ by continuous wave cavity ring-down spectroscopy (cw-CRDS) in the near infrared, coupled to a radical generation by laser photolysis. HO₂ was generated by photolyzing Cl₂ in the presence of CH₃OH and O₂. Low concentrations of DO₂ were generated simultaneously by adding low concentrations of D₂O to the reaction mixture, leading through isotopic exchange on tubing and reactor walls to formation of low concentrations of CH₃OD and thus formation of DO₂. Excess DO₂ was generated by photolyzing Cl₂ in the presence of CD₃OD and O₂, small concentrations of HO₂ were always generated simultaneously by isotopic exchange between CD₃OD and residual H₂O. The rate constant k₁ at 295 K was found to be pressure independent in the range 25–200 Torr helium, but increased with increasing D₂O concentration k₁ = (1.67 ± 0.03) × 10⁻¹² × (1 + (8.2 ± 1.6) × 10⁻¹⁸ cm³ × [D₂O] cm⁻³) cm³ s⁻¹. The rate constant for the DO₂ self-reaction k₂ has been measured under excess DO₂ concentration, and the DO₂ concentration has been determined by fitting the HO₂ decays, now governed by their reaction with DO₂, to the rate constant k₁. A rate constant with insignificant pressure dependence was found: k₂ = (4.1 ± 0.6) × 10⁻¹³ (1 + (2 ± 2) × 10⁻²⁰ cm³ × [He] cm⁻³) cm³ s⁻¹ as well as an increase of k₂ with increasing D₂O concentration was observed: k₂ = (4.14 ± 0.02) × 10⁻¹³ × (1 + (6.5 ± 1.3) × 10⁻¹⁸ cm³ × [D₂O] cm⁻³) cm³ s⁻¹. The result for k₂ is in excellent agreement with literature values, whereas this is the first determination of k₁.     

The reaction of O(1D) with H2O and the reaction of OH with C3H6

N₂O was photolyzed at 2139 Å to produce O(¹D) atoms in the presence of H₂O and CO. The O(¹D) atoms react with H₂O to produce HO radicals, as measured by CO₂ production from the reaction of OH with CO. The relative importance of the various possible O(¹D )–H₂O reactions is The relative rate constant for O(¹D) removal by H₂O compared to that by N₂O is 2.1, in good agreement with that found earlier in our laboratory. In the presence Of C₃H₆, the OH can be removed by reaction with either CO or C₃H₆: From the CO₂ yield, k₃/k₂ = 75,0 at 100°C and 55.0 at 200°C to within ± 10%. When these values are combined with the value of k₂ = 7.0 × 10^?13exp (–1100/RT) cm³/sec, k₃ = 1.36 × 10^?11 exp (–100/RT) cm³/sec. At 25°C, k₃ extrapolates to 1.1 × 10^?11 cm³/sec.     

Zur Diazotierung aromatischer Amine in Schwefelsäure

The diazotization of 2-chloro-4,6-dinitroaniline, 2,6-dichloro-4-nitroaniline, and 4-nitroaniline in concentrated sulfuric acid is very strongly catalysed by water. At a given water concentration the reaction rates of these amines are in the ratio 50/20/1. The relation between the bimolecular rate constants k and the acidity function H₀ is very simple, the plots of log k versus H₀ being linear with a slope of 2.     

Encapsulation of AGE‐Breaker Alagebrium by Cucurbit[7]uril Improved the Stability of Both Its Carbonyl α‐Hydrogen and Thiazolium C2‐Hydrogen

As determined by both ¹H NMR and UV/Vis spectroscopic titration, ESI‐MS, isothermal titration calorimetry, and DFT molecular modeling, advanced glycation end products (AGE) breaker alagebrium (ALA) formed 1:1 guest–host inclusion complexes with cucurbit[7]uril (CB[7]), with a binding affinity, K_a, in the order of magnitude of 10⁵ m ^?1, thermodynamically driven by both enthalpy (ΔH=?6.79 kcal mol^?1) and entropy (TΔS=1.21 kcal mol^?1). For the first time, a dramatic inhibition of keto–enol tautomerism of the carbonyl α‐hydrogen of ALA has been observed, as evidenced by over an order of magnitude decrease of both the first step rate constant, k₁, and the second step rate constant, k₂, during hydrogen/deuterium exchange in D₂O. Meanwhile, as expected, the reactivity of C2‐hydrogen was also inhibited significantly, with an upshift of 2.09 pK_a units. This discovery will not only provide an emerging host molecule to modulate keto–enol tautomerism, but also potentially lead to a novel supramolecular formulation of AGE‐breaker ALA for improved stability and therapeutic efficacy.     

Synthesis,kinetics and mechanism of the reaction of a quasi‐aromatic copper(II) complex, [Cu(PnAO)‐6H]0, with aromatic aldehydes

The quasi‐aromatic metal complex (1,1,2,8,9,9‐hexamethyl‐4,6‐dioxa‐5‐hydro‐3,7,10,14‐tetraazacyclotetradecane‐2,7,10,12‐tetraene)copper(II), [Cu(PnAO)‐6H]⁰ (AH), was synthesized. Reactions of AH were studied spectrophotometrically in acidic media (pH = 1 ∼ 2, EtOH:H₂O = 1:4 v/v) with para‐substituted benzaldehydes (ald). The Cu,2N,3C quasi‐aromatic heterocyclic ring in AH is highly reactive at the central‐aromatic‐carbon atom, C12, to most aldehydes. A novel parallel, competitive and consecutive second‐order reaction mechanism is proposed. To obtain the rate constants following this mechanism, the Gauss‐Newton‐Marquardt and Runge‐Kutta methods were employed. Consistent results were obtained. Effects of acidity, solvent, temperature and substituent R (RH, CH₃, OCH₃, Cl) of the aromatic aldehydes on the reaction rate constants were studied. The results support the proposed SN2 mechanism. A linear free energy relationship between the rate constant k₁ and the Hammett parameters for the substituted benzaldehydes as well the activation parameters are presented. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 33: 1–8, 2001     

Reactivity of dpph• in the oxidation of catechol and catechin

A kinetic study of the reduction of pyrocatechol and catechin by dpph^? radical has been carried out in various ratios of CH₃OH/H₂O mixed solvent at pH 5.5–7.5, μ = 0.10 M [(n‐Bu)₄N]ClO₄, and T = 25°C. The rate constants of oxidation in aqueous solvent, k, were obtained from the extrapolation of the linear plots of the specific rate constants k vs. % H₂O plots at each pH value. A linear relationship between k and 1/[H⁺] was observed for both flavonoids with k = k₁K_a1/[H⁺], where K_a1 was the first acid dissociation constant on the catechol ring and k₁ is the rate constant of the oxidation of the mononegative species HX^?. The values of k₁ obtained from the slopes of the plots are (8.2 ± 0.2) × 10⁵ and (6.1 ± 0.1) × 10⁵ M^?1 s^?1 for pyrocatechol and catechin, respectively. The analysis of the reaction on the basis of Marcus theory for an outer‐sphere electron transfer reaction yielded a value of 3.7 × 10³ M^?1 s^?1 for the self‐exchange rate constant of dpph^?/dpphH couple. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 43: 147–153, 2011     

Nickelphosphankomplexe der Mercaptoessigsäure

Nickel Complexes of Mercaptoacetic Acid The reaction of [Cp°₂Zr(OOCCH₂SH‐κ¹O)(OOCCH₂SH‐κ²O, O′)] (Cp° = C₅EtMe₄) with [NiCl₂(PMe₂Ph)₂] or [NiCl₂(dppe)] (dppe = PPh₂CH₂CH₂PPh₂) in the presence of NEt₃ yields the tetranuclear Zr^IV/Ni^II complex [{Cp°₂Zr(κ¹O‐OOCCH₂S‐κ²O′, S)(κ²O, O′‐OOCCH₂S‐κ¹S)Ni(PMe₂Ph)}₂] ( 1 ) and the chelate complexes [Ni(OOCCH₂S‐κ²O, S)L₂] [L = PMe₂Ph ( 2 ), L₂ = dppe ( 3 )]. 2 and 3 are also accessible from [NiCl₂(PMe₂Ph)₂] or [NiCl₂(dppe)] and mercaptoacetic acid in the presence of NEt₃ in quantitative yield. The structure of 2 is dynamic in solution, whereby a complex with three‐coordinate nickel atom is formed. 2 and 3 were characterized spectroscopically (¹H, ¹³C, ³¹P NMR and IR) and by crystal structure determination.     

Reaktivität von Koordinationsverbindungen,XXIII [1]. Bildung und Zerfall des Sauerstoffadduktes mit N,N′-Bis-(4-(5)-imidazolylmethyl)-äthylendiamin-kobalt (II)-Ionen

In aqueous solution N, N′-bis-(4-(5)-imidazolylmethyl)-ethylenediamine-cobalt (II) (CoIMEN²⁺) takes up molecular oxygen giving μ-dioxygen-μ-hydroxo-bis-[N, N′-bis-(4-(5)-imidazolylmethyl)-ethylenediamine]-dicobalt (II). (Co IMEN)₂ O₂ (OH)³⁺ is exceptionally stable against irreversible autoxydation to Co^III species. Its absorption spectrum is very similar to that of the known analogous complex (CoTRIEN)₂ O₂ (OH)³⁺. The kinetics of formation and dissociation of (CoIMEN)₂O₂(OH)³⁺ are studied by spectrophotometry and with an oxygen specific electrode. The rate of the forward reaction is described by v_f = [CoIMEN²⁺]² · [O₂] · (k₁ + k₂ · [OH^?]) with k₁ = 9 · 10⁴ M^?2 s^?1 and k₂ = 1 · 10¹²M^?3 S^?1, at 25° and I = 0,2. A mechanism including hydroxylated as well as nonhydroxylated intermediates is proposed. Dissociation is preceeded by protonation of the oxygen adduct. At pH 1–2 the rate of dissociation is independent of [H⁺] and follows first order kinetics: v_D = k₃ · [(CoIMEN)₂O₂(OH)³⁺] with k₃ = 2.15 · 10^?2 S^?1.