Kinetics of some reactions of vinyl and isopropyl radicals in the gas phase

Vinyl and isopropyl radicals were generated by the pyrolysis of azoisopropane in the presence of acrolein at 473–563 K. Reaction products were analyzed by gas chromatography. Rate constant ratios k₂/k₁ = 0.02 ± 0.01 and k₄/k₃ = 0.01 ± 0.005 are suggested for the following reactions: The rate constant ratio of reactions (7) and (c) obeys the Arrhenius equation The Arrhenius equation was derived for (k₈ + k₉).     

Hydrolysis of 2-aminoethanethiolsulfate ions

The kinetics of the acid catalyzed hydrolysis of 2-aminoethanethiolsulfate (AETS) ions were investigated. The dependence of the hydrolysis rate constant on acidity and temperature was determined. The hydrolysis rate equation can be expressed as where H_o is the Hammett acidity function. The rate constant, k, can be expressed as The pK_a's for the compound were measured and literature value of pK_a was found to be in error. The values determined in this study are pK_a1 < ?0.5 and pK_a2 = 9.1 ± 0.1. General acid catalysis of the hydrolysis reaction was found not to proceed to a significant degree. © 1994 John Wiley & Sons, Inc.

1 This article is a US Government work and, as such, is in the public domain in the United States of America.

     

Incorporating glycidyl methacrylate into block copolymers using poly(methacrylate‐ran‐styrene) macroinitiators synthesized by nitroxide‐mediated polymerization

Methyl methacrylate/styrene (MMA/S), ethyl methacrylate/styrene (EMA/S) and butyl methacrylate/styrene (BMA/S) feeds (>90 mol % methacrylate) were copolymerized in 50 wt % p‐xylene at 90 °C with 10 mol % of additional SG1‐free nitroxide mediator relative to unimolecular initiator (BlocBuilder^®) to yield methacrylate rich copolymers with polydispersities _w/ _n = 1.23–1.46. k_pK values (k_p = propagation rate constant, K = equilibrium constant) for MMA/S copolymerizations were comparable with previous literature, whereas EMA/S and BMA/S copolymerizations were characterized by slightly higher k_pK's. Chain extensions with styrene at 110 °C initiated by the methacrylate‐rich macroinitiators (number average molecular weight _n = 12.9–33.5 kg mol^?1) resulted in slightly broader molecular weight distributions with _w/ _n = 1.24–1.86 and were often bimodal. Chain extensions with glycidyl methacrylate/styrene/methacrylate (GMA/S/XMA where XMA = MMA, EMA or BMA) mixtures at 90 °C using the same macroinitiators resulted frequently in bimodal molecular weight distributions with many inactive macroinitiators and higher _w/ _n = 2.01–2.48. P(XMA/S) macroinitiators ( _n = 4.9–8.9 kg mol^?1), polymerized to low conversion and purified to remove “dead” chains, initiated chain extensions with GMA/MMA/S and GMA/EMA/S giving products with _w/ _n ～ 1.5 and much fewer unreacted macroinitiators (<5%), whereas the GMA/BMA/S chain extension was characterized by slightly more unreacted macroinitiators (～20%). © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2574–2588, 2009     

Studies on anionic polymerization of lactams. Part III. Copolymerization of pyrrolidone and caprolactam

On reacting acetylcaprolactam (AcCL) and pyrrolidonate MgBr (Py^?) in tetrahydrofuran solution, transacetylation takes place, giving acetylpyrrolidone (AcPy) and caprolactamate MgBr (CL^?). The rate constants for the transacetylation reactions were measured at 25°C. Their values in units of liters/mole-second were: The rate constants for the addition reactions measured were: As the transacetylation is much faster than the addition reaction the copolymer composition should be given by the equation: where K_trans, the transacetylation equilibrium constant, equals 0.3 while K_acidity reflects the relative acidities of the monomers and its value (from the literature) is about 0.4. Pyrrolidone is, therefore, more reactive than caprolactam in anionic copolymerization by a factor of about 8.     

The very-low-pressure study of the kinetics and equilibrium: Cl + CH4 ⇄ CH3 + HCl at 298 K. The heat of formation of the CH3 radical

The bimolecular rate constant for the direct reaction of chlorine atoms with methane was measured at 25°C by using the very-low-pressure-pyrolysis technique. The rate constant was found to be In addition, the ratio k₁/k_?1 was observed with about 25% accuracy: K₁₍₂₉₈₎ = 1.3 ± 0.3. This gives a heat of formation of the methyl radical ΔH° _{f 298}(CH₃) = 35.1 ± 0.15 kcal/mol. A bond dissociation energy BDE (CH₃ ? H) = 105.1 ± 0.15 kcal/mol in good agreement with literature values was obtained.     

Kinetics of the molybdate catalyzed oxidation of iodide by hydrogen peroxide

The kinetics of the oxidation of iodide by hydrogen peroxide catalyzed by acidic molybdate have been studied by a spectrophotometric stopped-flow method. The results are interpreted in terms of the mechanism and the implied rate law where [mol] is total analytical concentration of molybdate. The values obtained for the rate and equilibrium constants are k₄ = (3.3 ± 1) × 10² 1./mole · s, K₁ = (1.2 ± 0.6) × 10⁴ 1./mole, K₂ = (1.3 ± 0.7) × 10³ 1./mole, and K₃ = (4 ± 3) × 10² 1./mole at 298°K.     

Linear free energy relationships between reaction rate constants and equilibrium constants of complex compounds: III. Kinetics and mechanisms of ternary complex formation between (5-X-1, 10-phenanthroline)copper(II) and threonine

The kinetics of ternary complex formation involving Cu(5-X-1, 10-phen) and threonine (CuAL, A=5-X-1, 10-phen; L=threonine or represented by O-N; X=NO₂, Cl, H, CH₃) has been studied by temperature-jump and stopped-flow methods. The formation rate constants, k_f(M^?1·s^?1), for the complexation reaction, CuA + L CuAL, are as follows; X=NO₂, 8.68×10⁸; X = Cl, 7.13×10⁸; X=H, 6.12×10⁸; X=CH₃, 5.42×10⁸. The rate constants for zwitterion attack are nil within experimental error. It has been found that a linear free energy relationship exists between the stability (logK^CuA_CuAL) of the complexes CuAL and log k_f as follows: logK^CuA_CuAL = 0.13+0.83 logk_f, r = 0.99. It suggested that the formation rate governed the stability of the ternary complexes. The rates of formation of the ternary complexes increased with decreasing electron-donating property of the substituents. A linear relationship was found to exist as expressed by the following equation: log(k^R_f/K^O_f = 0.097σ, r = 0.96. A mechanism involves a rapid equilibrium between CuA and L followed by a slow ring closure of L.     

Radiation-induced dehalogenation of 1,1,1,2 tetrachloroethane and 1,1,2 trichloro-1 bromoethane reactions of 1,1,2 trichloroethyl radicals

The kinetics of the radiation-induced free radical chain reactions in solutions of CCl₃CClH₂ and CCl₂BrCH₂Cl in cyclohexane (RH) were studied in the temperature range of 90–225°C. 1,1,2 trichloroethyl and 1,1,1,2 tetrachloroethyl radicals were produced by the reaction of radiolytically generated cyclohexyl (R) radicals with solutes. The reactions studied were The following rate expressins were obtained: where θ = 2.303RT in kcal/mole. From the activation parameters of the k₄/k₅ rate constant ratio and the assumption that E₄ = E, E₅ was calculated to be 20.2 ± 0.2 kcal/mole. The Arrhenius parameters for the Cl atom elimination reaction from chloroethyl radicals derived from liquid and gas-phase studies are compared.     

Hydrogen/deuterium abstraction by chlorine atoms from gaseous ethyl chlorides. Secondary kinetic isotope effects in the system CH3CH2Cl,CH3CHDCl,CH3CD2Cl

The abstraction of hydrogen/deuterium from CH₃CH₂Cl, CH₃CHDCl, and CH₃CD₂Cl by photochemically generated ground-state chlorine atoms has been investigated over the temperature range of 8–94°C using methane as a competitor. Rate constant data for the following reactions have been obtained: The temperature dependence of the relative rate constants k_i/k_j was found to conform to the Arrhenius rate law, where the stated error limits are one standard deviation: and k_r is the rate constant for the reference reaction (CH₄ + Cl → CH₃ + HCl). The β secondary kinetic isotope effects (k₂/k₃/k₄) are close to unity and show a slight inverse temperature dependence. Both preexponential factors and activation energies decrease as a result of deuterium substitution in the adjacent chloromethyl group. The trends are well outside the limits of experimental error.     

Reaction between nitrous acid and hydrogen peroxide in perchloric acid medium

A kinetic investigation on the reaction has been carried out in HClO₄ medium under different conditions. A spectrophotometric method of estimation of nitrous acid at various time intervals has been employed. The results are interpreted on the basis ofthe following mechanism: The absolute rate constant value of 39.7 M^?1 plusmn; s^?1 for k₄ and the equilibrium constant K_eq = 116M^?1 for reaction (2) have been evaluated. The activation energy of the overall reaction has also been determined as E_a = 13.2 kcal/mol.     

Thermal stability of primary amines

Tertiary-amyl amine has been decomposed in single-pulse shock-tube experiments. Rate expressions for several of the important primary steps are This leads to D(CH₃? H) – D(NH₂? H) = ?10.5 kJ and D[(CH₃)₃C? H] – D[(CH₃)₂NH₂C? H] = + 6 kJ. The present and earlier comparative rate single-pulse shock-tube data when combined with high-pressure hydrazine decomposition results-(after correcting for fall off effects through RRKM calculations) gives where k_r(…) is the recombination rate involving the appropriate radicals. This suggests that in this context amino radical behavior is analogous to that of alkyl radicals. If this agreement is exact, then Rate expressions for the primary step in the decomposition of a variety of primary amines have been computed. In the case of benzyl amine where data exist the agreement is satisfactory. The following differences in bond energies have been estimated:      

The kinetics of free radical chain reactions in solutions of trichlorofluorethylene in cyclohexane

The kinetics of the gamma-radiation-induced free radical chain reaction in solutions of C₂Cl₃F in cyclohexane (RH) was investigated over a temperature range of 87.5–200°C. The following rate constants and rate constant ratios were determined for the reactions: In competitive experiments in ternary solutions of C₂Cl₄ and C₂Cl₃F in cyclohexane the rate constant ratio k_2c/k_2a was determined By comparing with previous data for the addition of cyclohexyl radicals to other chloroethylenes it is shown that in certain cases the trends in activation energies for cyclohexyl radical addition can be correlated with the C? Cl bond dissociation energies in the adduct radicals.     

The reactions of C2F5 radicals with benzene in the vapor phase

C₂F₅ radicals were generated in the presence of benzene vapor by the reaction The radicals react with the benzene by addition and pseudo H abstraction The rate constant k_add for the addition reaction (7) is given by where θ = 2.303RT cal/mole and k_c is the rate constant for combination of C₂F₅ radicals. The addition becomes reversible above 110°C. The reactions of CF₃ and C₂F₅ radicals with benzene vapor are compared.     

Electron density and its derivatives at the nucleus for spherically confined hydrogen atom

It is shown that the energy of a hydrogen‐like atom confined inside a spherical cavity of radius, R, and potential barrier, V₀, is quantitatively defined by the ratio . Here, the conventional spherical density (r) is scaled as η_l(r) = and the ratio of the second derivative η(r) to η_l(r) is evaluated at the nucleus. Numerical results of the ratios are presented for 1s, 2s, 2p, and 3d states at several values of V₀. For such states, the characteristic radii of confinement leading to the well‐defined values of energy are identified. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Lewis acid‐base properties of cellulose acetate butyrate by inverse gas chromatography

The dispersive component of the surface‐free energy, , of cellulose acetate butyrate (CAB) has been determined using the net retention volume, V_N, of n‐alkanes (C₅? C₈) probes in the temperature range 323.15–393.15 K. The values decrease nonlinearly with increase in temperature, and the temperature coefficients of are ? 0.32 (mJ/m²K) and ? 0.10 (mJ/m²K) in the range 323.15–353.15 K and 353.15–393.15 K, respectively. This variation in has been attributed to the structural changes that take place on the surface of CAB at ～353.15 K. The specific components of the enthalpy of adsorption, , and entropy of adsorption, , calculated using V_N of polar solutes are negative. The values are used to evaluate Lewis acidity constant, K_a, and Lewis basicity constant, K_b, for the CAB surface. The K_a and K_b values are found to be 0.126 and 1.109, respectively, which suggest that the surface is predominantly basic. The K_a and K_b results indicate for the necessary surface modifications of CAB which act as biodegradable adsorbent material. Copyright © 2010 John Wiley & Sons, Ltd.     

The gas phase reaction of CF3 radicals with C6F5H in the temperature range 373–658 K

CF₃ radicals were generated by the photolysis of perfluoroacetic anhydride. In the presence of pentafluorobenzene, the CF₃ radicals react according to the following mechanism: It was found that the addition reaction (3) becomes reversible above ca. 453 K. The addition rate parameters have been revised and they satisfactorily agree with those reported previously. At temperatures higher than 593 K, only true H-abstraction occurs. The rate constant k_H for reaction (5) is given by: where θ = 2.303 RT kJmol^?1 and k_c is the rate constant for combination of CF₃ radicals. The reactions of CF₃ with benzene and pentafluorobenzene are compared.     

Kinetics of the simultaneous oxidation of nickel(II) and sulfur(IV) by oxygen in alkaline medium in Ni(II)–sulfur(IV)–O2 system

In the Ni(II)–S(IV)–O₂ system in the region of pH > 8.4, both Ni(II) and S(IV) are simultaneously autoxidized, and when sulfur is consumed fully NiOOH precipitates. At pH > 8.4, ethanol has no effect on the rate, whereas ammonia strongly inhibits the reaction when pH > 7.0. The kinetics of the reaction, in both the presence and the absence of ethanol, is defined by the rate law where k is the rate constant, K_O is the equilibrium constant for the adsorption of O₂ on ? Ni(OH)₂ particle surface. In ammonia buffer, the factor F is defined by where K, K_OH, K₁, K₂, K₃, and K₄ are the stability constants of NiSO₃, NiOH⁺, Ni(NH₃)²⁺, Ni(NH₃), Ni(NH₃), and Ni(NH₃), respectively. In unbuffered medium, the factor F reduces to The values of k and K^sp were found to be (1.3 ± 0.08) × 10^?1 s^?1 and (4.2 ± 3.5) × 10^?16, respectively, at 30°C. A nonradical mechanism that assumes the adsorption of both SO₃^2? and O₂ on the ? Ni(OH)₂ particle surface has been proposed. At pH ≤ 8.2, Ni(II) displays no catalytic activity for sulfur(IV)‐autoxidation and it is also not oxidized to NiOOH. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 464–478, 2010     

Thermal decomposition of 3,4-dimethylpentene-1, 2,3,3-trimethylpentane, 3,3-dimethylpentane,and isobutylbenzene in a single pulse shock tube

Several hydrocarbons have been pyrolyzed in a single pulse shock tube. Rate parameters for the main bond breaking step have been found to be In combination with similar studies carried out earlier and through application of the well-established experimental rule (k(AB)/k_r(AA)k_r(BB))^1/2 ～ 2 where A and B are radicals and the rate constants are for the combination of these radicals, rate parameters for the thermal decomposition of all the hydrocarbons formed from any pair of the following radicals: methyl, ethyl, isopropyl, t-butyl, t-amyl, allyl, methylallyl, and benzyl have been calculated. The available calculated and experimental values of the decomposition rate constants are in excellent agreement. It appears that, with the possible exception of reactions involving the ejection of methyl radicals, the frequency factors per bond are nearly constant, depending only upon the type of carbon–carbon bond that is being broken. These values are all lower than those expected from the radical recombination rates. Heats of formation of ethyl, t-amyl, benzyl, methylallyl, n-propyl, s-butyl, isobutyl, neopentyl, and 3-pentyl radicals have been derived. Rate parameters for the decomposition of some simple ketones and ethers have also been estimated.     

A kinetic study of the oxidation of L-ascorbic acid by chromium(VI)

The reaction between chromium(VI) and L-ascorbic acid has been studied by spectrophotometry in the presence of aqueous citrate buffers in the pH range 5.69–7.21. The reaction is slowed down by an increase of the ionic strength. At constant ionic strength, manganese(II) ion does not exert any appreciable inhibition effect on the reaction rate. The rate law found is where K_p is the equilibrium constant for protonation of chromate ion and k_r is the rate constant for the redox reaction between the active forms of the oxidant (hydrogenchromate ion) and the reductant (L-hydrogenascorbate ion). The activation parameters associated with rate constant k_r are E_a = 20.4 ± 0.9 kJ mol^?1, ΔH^≠ = 17.9 ± 0.9 kJ mol^?1, and ΔS^≠=?152 ± 3 J K^?1 mol^?1. The reaction thermodynamic magnitudes associated with equilibrium constant K_p are ΔH⁰ = 16.5 ± 1.1 kJ mol^?1 and ΔS⁰ = 167 ± 4 J K^?1 mol^?1. A mechanism in accordance with the experimental data is proposed for the reaction. © 1993 John Wiley & Sons, Inc.     

2‐(2,4‐Dinitrobenzyl)pyridine (DNBP): A Potential Light‐Activated Proton Shuttle

The well‐known photochromic tautomerism of 2‐(2,4‐dinitrobenzyl)pyridine ( 1 ; CH; Scheme 1) was re‐investigated by flash photolysis in aqueous solution in view of its potential application as a light‐activated proton pump. Irradiation of 1 yields the enamine tautomer NH (λ_max=520 nm) that rapidly equilibrates with its conjugate base CNO^? (λ_max=420 nm). The pH–rate profile for the first‐order decay of NH and CNO^? provides a direct determination of the acidity constant of NH, pK =5.94±0.12 (I=0.1M ) and serves to clarify the mechanisms of proton transfer prevailing in aqueous solutions. The acidity constant of protonated 1 (CHNH⁺), pK =4.18±0.02, was determined by spectrophotometric titration.