Absolute rate constants for the addition of hydroxymethyl radicals to alkenes in methanol solution

Absolute rate constants and their temperature dependencies were determined for the addition of hydroxymethyl radicals (CH₂OH) to 20 mono- or 1,1-disubstituted alkenes (CH₂ = CXY) in methanol by time-resolved electron spin resonance spectroscopy. With the alkene substituents the rate constants at 298 K (k₂₉₈) vary from 180 M^?1s^?1 (ethyl vinylether) to 2.1 middot; 10⁶ M^?1s^?1 (acrolein). The frequency factors obey log A/M^?1s^?1 = 8.1 ± 0.1, whereas the activation energies (E_a) range from 11.6 kJ/mol (methacrylonitrile) to 35.7 kJ/mol (ethyl vinylether). As shown by good correlations with the alkene electron affinities (EA), log k₂₉₈/M^?1s^?1 = 5.57 + 1.53 · EA/eV (R² = 0.820) and E_a = 15.86 ? 7.38 · EA/eV (R² = 0.773), hydroxymethyl is a nucleophilic radical, and its addition rates are strongly influenced by polar effects. No apparent correlation was found between E_a or log k₂₉₈ with the overall reaction enthalpy. © 1995 John Wiley & Sons, Inc.     

The kinetics of aquation and racemization of some cis- chloro-, hydroxo-, and aquabis(ethylenediamine)(amine)cobalt(III) complexes

Aquation rates forcis-CoCl(en)₂(A)²⁺ (A = 3,5-lutidine, imidazole, N-methylimidazole, benzimidazole) have been determined by halide release titration in 1.0 M HNO₃ at 50–80°C. Kinetic parameters are (in the above order of A) 10⁷k₂₉₈ (sec^?1), 7.4, 5.7, 1.3, 9.7; E_a (kJ/mole), 103, 101, 130, 112; log PZ (sec^?1), 11.89, 11.53, 16.04, 13.58; ΔS₂₉₈^? (J/°K· mole),?26, ?32, +54,+7. Rates of racemization for active cis-Co(en)₂(A)-(OH₂)³⁺ were measured spectropolarimetrically in 0.1 M HClO₄, 0.02 M Hg²⁺ for A = ammonia, cyclohexylamine, 3,5-lutidine, N-methylimidazole. Kinetic parameters are (units as above) 10⁹k₂₉₈ = 1.8, 38.0, 7.7, 1.7; E_a = 150, 135, 152, 157; log PZ = 17.49, 16.18, 18.56, 18.87; ΔS₂₉₈^? = +82, +57, +102,+108. Rates of racemization of the active hydroxo complexes cis-Co(en)₂(A)(OH)²⁺ (A = NH₃, CH₃NH₂, 3,5-lutidine, imidazole) were measured similarly at pH = 8 (π = 2.0, NaClO₄). The racemization of the hydroxo is ca. (3–4) × 10³ faster than for the corresponding aqua complex. Kinetic parameters are 10⁶k₂₉₈ = 2.55, 7.2, 30.1, 7.0; E_a = 138, 123, 122, 128; log PZ = 18.58, 16.39, 16.85, 17.18; ΔS₂₉₈^? = +102,+60, +69,+76. Racemization rates for aquahydroxo mixtures (A = CH₃NH₂, 3,5-lutidine, imidazole) were also determined in the pH range of 4–8 (θ = 2.0, NaClO₄) at 50.6°C, and pK_a data calculated from the pH versus k plot are 5.50, 5.65, and 6.40, respectively, for A.     

Rate constants for the elementary reactions between CN radicals and CH4, C2H6, C2H4, C3H6, and C2H2 in the range: 295 ⩽ T/K ⩽ 700

Pulsed laser photolysis, time-resolved laser-induced fluorescence experiments have been carried out on the reactions of CN radicals with CH₄, C₂H₆, C₂H₄, C₃H₆, and C₂H₂. They have yielded rate constants for these five reactions at temperatures between 295 and 700 K. The data for the reactions with methane and ethane have been combined with other recent results and fitted to modified Arrhenius expressions, k(T) = A′(298) (T/298)ⁿ exp(?θ/T), yielding: for CH₄, A′(298) = 7.0 × 10^?13 cm³ molecule^?1 s^?1, n = 2.3, and θ = ?16 K; and for C₂H₆, A′(298) = 5.6 × 10^?12 cm³ molecule^?1 s^?1, n = 1.8, and θ = ?500 K. The rate constants for the reactions with C₂H₄, C₃H₆, and C₂H₂ all decrease monotonically with temperature and have been fitted to expressions of the form, k(T) = k(298) (T/298)ⁿ with k(298) = 2.5 × 10^?10 cm³ molecule^?1 s^?1, n = ?0.24 for CN + C₂H₄; k(298) = 3.4 × 10^?10 cm³ molecule^?1 s^?1, n = ?0.19 for CN + C₃H₆; and k(298) = 2.9 × 10^?10 cm³ molecule^?1 s^?1, n = ?0.53 for CN + C₂H₂. These reactions almost certainly proceed via addition-elimination yielding an unsaturated cyanide and an H-atom. Our kinetic results for reactions of CN are compared with those for reactions of the same hydrocarbons with other simple free radical species. © John Wiley & Sons, Inc.     

Arrhenius parameters for the reactions CH3. + C4H10 → CH4 + C4H9. and C2H5. + C4H10 → C2H6 + C4H9.

Study of n-butane pyrolysis at high temperature in a flow system allows measurement of the sum of the rate constants of the initiation reactions and of the Arrhenius parameters of the reactions Established data for k₁/k₂ allow estimation of k₁ for 951°K and this, with recent thermochemical data, yields the result log k_?1 (l.mole s^?1) = 8.5, in remarkable agreement with a recent measurement [20] but over si×ty times smaller than conventional assumption. The product k₃k₄ (l.²mole^?2s^?2) is found to be associated with the Arrhenius parameters log (A₃A₄) = 21.90 ± 0.6 and (E₃ + E₄) = 38.3 ± 2.7 kcal/mole. These values are much higher than would be e×pected on the basis of low temperature estimates. Independent evaluation gives log A₄ = 10.5 ± 0.4 (l.mole^?1s^?1) and E₄ = 20.1 ± 1.7 kcal/mole, hence log A₃ = 11.4 ± 0.8 (l.mole^?1s^?1) and E₃ = 18.2 ± 3.2 kcal/mole. These values are shown to be entirely consistent with a wide range of results from pyrolytic studies, and it is argued that they further confirm the view that Arrhenius plots for alkyl radical–alkane metathetical reactions are strongly curved, in part due to tunneling and, appreciably, to other as yet unidentified effects. Since there is published evidence that metathetical reactions involving hydrogen atoms show even greater curvature, it is suggested that this may be a characteristic of many metathetical reactions.     

Rate constant and activation energy for the gaseous reaction between hydroxyl and formaldehyde

The rate constant k₄ has been measured at 268°, 298°, and 334° K for the reaction CH₂O + 2OH → CO + 2H₂O relative to that for OH + OH (k₂) by competition experiments in a discharge flow tube using mass-spectrometric analysis. Based on k₂ = 2.24 × 10^?12cm³/molec·sec at 298°K and E₂ = 4 kJ/mol, k₄ = (6.5 ± 1.5) × 10^?12cm³/molec·sec at 298°K and E₄ = (6 ± 2)kJ/mol.     

Kinetic study of the gas‐phase reaction of hydroxyl radical with CF3CH2OCH2CF3 using the laser photolysis–laser induced fluorescence method

The laser photolysis‐laser‐induced fluorescence method was used for measuring the kinetic parameters of the reaction of OH radicals with CF₃CH₂OCH₂CF₃ (2,2,2‐trifluoroethyl ether), in the temperature range of 298–365 K. The bimolecular rate coefficient at 298 K, k_II(298), was measured to be (1.47 ± 0.03) × 10^?13 cm³ molecule^?1 s^?1, and the temperature dependence of k_II was determined to be (4.5 ± 0.8) × 10^?12exp [?(1030 ± 60)/T] cm³ molecule^?1 s^?1. The error quoted is 1σ of the linear regression of the respective plots. The rate coefficient at room temperature is very close to the average of the three previous measurements, whereas the values of E_a/R and the A‐factor are higher than the two previously reported values. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 519–525, 2010     

Solvent effects on the rate of reaction of Cl2˙− and SO4˙− radicals with unsaturated alcohols

Rate constants have been measured in several aqueous/organic solvent mixtures for the addition reaction of Cl₂^˙? radicals with 2-propen-1-o1 and 2-buten-1-o1 as a function of temperature and with 2, 3-dimethyl-2-butene at room temperature. The rate constants were in the range of 10⁶–10⁹ L mol^?1 s^?1, the activation energies were relatively low (1–10 kJ mol^?1), and the pre-exponential factors varied over the range log A = 7.9 to 9.4. The rate constants (k) decreased (by up to a factor of 30) upon increasing the fraction of organic solvent and log k correlated linearly with the dielectric constant for a given water/organic solvent system, but the lines for the different solvent systems had different slopes. A better correlation of log k was found with a combination of the solvatochromic factor, E_T(30), and the hydrogen-bond donor acidity factor, α. This suggests that the rate of reaction is influenced by the solvent polarity and also by specific solvation of the ionic reactant and product. Solvent effect on the reaction of SO₄^˙? with 2-propen-1-o1 was studied for comparison. © 1993 John Wiley & Sons, Inc.     

Kinetics and mechanism of oxidation of D-fructose and D-glucose by sodium salts of N-(chloro)-mono/di-substituted benzenesulfonamides in aqueous alkaline medium

In an effort to introduce N-chloroarylsulfonamides of different oxidizing strengths, nine sodium salts of mono- and di-substituted N-chloroarylsulfonamides are employed as oxidants for studying the kinetics of oxidation of D -fructose and D -glucose in aqueous alkaline medium. The results are analyzed along with those by the sodium salts of N-chlorobenzenesulfonamide and N-chloro-4-methylbenzenesulfonamide. The reactions show first-order kinetics each in [oxidant], [Fru/Glu], and [OH⁻]. The rates slightly increase with increase in ionic strength of the medium. Further, the rate of oxidation of fructose is higher by 4 to 5 times than that of the glucose oxidation, by the same oxidant. Similarly, E_a values for glucose oxidations are higher by about 1.5 times the E_a values for fructose oxidations. The results have been explained by a plausible mechanism, and the related rate law deduced. The significant changes in the kinetics and thermodynamic data are observed with change of substituent in the benzene ring. It is because Cl⁺ is the effective oxidizing species in the reactions of N-chloroarylsulfonamides. The oxidative strengths of the latter therefore depend on the ease with which Cl⁺ is released from them. The ease with which Cl⁺ is released from N-chloroarylsulfonamides depends on the electron density of the nitrogen atom of the sulfonamide group, which in turn depends on the nature of the substituent in the benzene ring. The following Hammett equations are valid for the oxidation of fructose and glucose, log k_obs = −3.13 + 0.54 σp and log k_obs = −3.81 + 0.28 σp, respectively. The enthalpies and entropies of activations for oxidations by all the N-chloroarylsulfonamides correlate well with isokinetic temperatures of 301 K and 299 K, for fructose and glucose oxidations, respectively. The effect of substitution in the oxidants on the E_a and log A for the oxidations is also considered. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 572–582, 2005     

A laser flash photolysis kinetic study of reactions of the Cl2− radical anion with oxygenated hydrocarbons in aqueous solution

A laser photolysis–long path laser absorption (LP‐LPLA) experiment has been used to determine the rate constants for H‐atom abstraction reactions of the dichloride radical anion (Cl₂⁻) in aqueous solution. From direct measurements of the decay of Cl₂⁻ in the presence of different reactants at pH = 4 and I = 0.1 M the following rate constants at T = 298 K were derived: methanol, (5.1 ± 0.3)·10⁴ M⁻¹ s⁻¹; ethanol, (1.2 ± 0.2)·10⁵ M⁻¹ s⁻¹; 1‐propanol, (1.01 ± 0.07)·10⁵ M⁻¹ s⁻¹; 2‐propanol, (1.9 ± 0.3)·10⁵ M⁻¹ s⁻¹; tert.‐butanol, (2.6 ± 0.5)·10⁴ M⁻¹ s⁻¹; formaldehyde, (3.6 ± 0.5)·10⁴ M⁻¹ s⁻¹; diethylether, (4.0 ± 0.2)·10⁵ M⁻¹ s⁻¹; methyl‐tert.‐butylether, (7 ± 1)·10⁴ M⁻¹ s⁻¹; tetrahydrofuran, (4.8 ± 0.6)·10⁵ M⁻¹ s⁻¹; acetone, (1.41 ± 0.09)·10³ M⁻¹ s⁻¹. For the reactions of Cl₂⁻ with formic acid and acetic acid rate constants of (8.0 ± 1.4)·10⁴ M⁻¹ s⁻¹ (pH = 0, I = 1.1 M and T = 298 K) and (1.5 ± 0.8) · 10³ M⁻¹ s⁻¹ (pH = 0.42, I = 0.48 M and T = 298 K), respectively, were derived. A correlation between the rate constants at T = 298 K for all oxygenated hydrocarbons and the bond dissociation energy (BDE) of the weakest C‐H‐bond of log k_2nd = (32.9 ± 8.9) − (0.073 ± 0.022)·BDE/kJ mol⁻¹ is derived. From temperature‐dependent measurements the following Arrhenius expressions were derived: k (Cl₂⁻ + HCOOH) = (2.00 ± 0.05)·10¹⁰·exp(−(4500 ± 200) K/T) M⁻¹ s⁻¹, E_a = (37 ± 2) kJ mol⁻¹ k (Cl₂⁻ + CH₃COOH) = (2.7 ± 0.5)·10¹⁰·exp(−(4900 ± 1300) K/T) M⁻¹ s⁻¹, E_a = (41 ± 11) kJ mol⁻¹ k (Cl₂⁻ + CH₃OH) = (5.1 ± 0.9)·10¹²·exp(−(5500 ± 1500) K/T) M⁻¹ s⁻¹, E_a = (46 ± 13) kJ mol⁻¹ k (Cl₂⁻ + CH₂(OH)₂) = (7.9 ± 0.7)·10¹⁰·exp(−(4400 ± 700) K/T) M⁻¹ s⁻¹, E_a = (36 ± 5) kJ mol⁻¹ Finally, in measurements at different ionic strengths (I) a decrease of the rate constant with increasing I has been observed in the reactions of Cl₂⁻ with methanol and hydrated formaldehyde. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 169–181, 1999     

Kinetics of the reactions of Cl atoms with CH3Br and CH2Br2

The rate coefficients for the reactions of Cl atoms with CH₃Br, (k₁) and CH₂Br₂, (k₂) were measured as functions of temperature by generating Cl atoms via 308 nm laser photolysis of Cl₂ and measuring their temporal profiles via resonance fluorescence detection. The measured rate coefficients were: k₁ = (1.55 ± 0.18) × 10^?11 exp{(?1070 ± 50)/T} and k₂ = (6.37 ± 0.55) × 10^?12 exp{(?810 ± 50)/T} cm³ molecule^?1 s^?1. The possible interference of the reaction of CH₂Br product with Cl₂ in the measurement of k₁ was assessed from the temporal profiles of Cl at high concentrations of Cl₂ at 298 K. The rate coefficient at 298 K for the CH₂Br + Cl₂ reaction was derived to be (5.36 ± 0.56) × 10^?13 cm³ molecule^?1 s^?1. Based on the values of k₁ and k₂, it is deduced that global atmospheric lifetimes for CH₃Br and CH₂Br₂ are unlikely to be affected by loss via reaction with Cl atoms. In the marine boundary layer, the loss via reaction (1) may be significant if the Cl concentrations are high. If found to be true, the contribution from oceans to the overall CH₃Br budget may be less than what is currently assumed. © 1994 John Wiley & Sons, Inc.     

Kinetics of the reactions of C2H5, n‐C3H7, and n‐C4H9 radicals with Cl2 at the temperature range 190–360 K

The kinetics of the C₂H₅ + Cl₂, n‐C₃H₇ + Cl₂, and n‐C₄H₉ + Cl₂ reactions has been studied at temperatures between 190 and 360 K using laser photolysis/photoionization mass spectrometry. Decays of radical concentrations have been monitored in time‐resolved measurements to obtain reaction rate coefficients under pseudo‐first‐order conditions. The bimolecular rate coefficients of all three reactions are independent of the helium bath gas pressure within the experimental range (0.5–5 Torr) and are found to depend on the temperature as follows (ranges are given in parenthesis): k(C₂H₅ + Cl₂) = (1.45 ± 0.04) × 10^?11 (T/300 K)^{?1.73 ± 0.09} cm³ molecule^?1 s^?1 (190–359 K), k(n‐C₃H₇ + Cl₂) = (1.88 ± 0.06) × 10^?11 (T/300 K)^{?1.57 ± 0.14} cm³ molecule^?1 s^?1 (204–363 K), and k(n‐C₄H₉ + Cl₂) = (2.21 ± 0.07) × 10^?11 (T/300 K)^{?2.38 ± 0.14} cm³ molecule^?1 s^?1 (202–359 K), with the uncertainties given as one‐standard deviations. Estimated overall uncertainties in the measured bimolecular reaction rate coefficients are ±20%. Current results are generally in good agreement with previous experiments. However, one former measurement for the bimolecular rate coefficient of C₂H₅ + Cl₂ reaction, derived at 298 K using the very low pressure reactor method, is significantly lower than obtained in this work and in previous determinations. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 614–619, 2007     

KINETICS OF FORMATION AND STABILITY CONSTANTS OF MONO AND BIS l-TYROSINE COMPLEXES OF Cu(II), Co(II), AND Ni(II)

Abstract

The kinetics and stability constants of l-tyrosine complexation with copper(II), cobalt(II) and nickel(II) have been studied in aqueous solution at 25° and ionic strength 0.1 M. The reactions are of the type M(HL)^(3-n)+ _n-1 + HL- ? M(HL)^(2-n)+_n(k_n, forward rate constant; k_-n, reverse rate constant); where M=Cu, Co or Ni, HL^? refers to the anionic form of the ligand in which the hydroxyl group is protonated, and n=1 or 2. The stability constants (K_n=k_n/k_-n) of the mono and bis complexes of Cu²⁺, Co²⁺ and Ni²⁺ with l-tyrosine, determined by potentiometric pH titration are: Cu²⁺, log K₁=7.90 ± 0.02, log K₂=7.27 ± 0.03; Co²⁺, log K₁=4.05 ± 0.02, log K₂=3.78 ± 0.04; Ni²⁺, log K₁=5.14 ± 0.02, log K₂=4.41 ± 0.01. Kinetic measurements were made using the temperature-jump relaxation technique. The rate constants are: Cu²⁺, k₁=(1.1 ± 0.1) × 10⁹ M ^?1 sec^?1, k_-1=(14 ± 3) sec^?1, k₂=(3.1 ± 0.6) × 10⁸ M ^?1 sec^?1, k^?2=(16 ± 4) sec^?1; Co²⁺, k₁=(1.3 ± 0.2) × 10⁶ M ^?1 sec^?1, k_-1=(1.1 ± 0.2) × 10² sec^?1, k₂=(1.5 ± 0.2) × 10⁶ M ^?1 sec^?1, k_-2=(2.5 ± 0.6) × 10² sec^?1; Ni²⁺, k₁=(1.4 ± 0.2) × 10⁴ M ^?1 sec^?1, k_-1=(0.10 ± 0.02) sec^?1, k₂=(2.4 ± 0.3) × 10⁴ M ^?1 sec^?1, k_-2=(0.94 ± 0.17) sec^?1. It is concluded that l-tyrosine substitution reactions are normal. The presence of the phenyl hydroxyl group in l-tyrosine has no primary detectable influence on the forward rate constant, while its influence on the reverse rate constant is partially attributed to substituent effects on the basicity of the amine terminus.     

Kinetics of methoxy-NNO-Azoxymethane hydrolysis in strong acids

The kinetics of methoxy-NNO-azoxymethane (I) hydrolysis in concentrated solutions of strong acids (HBr, HCl, HClO₄, and H₂SO₄) has been investigated by a manometric method. The gas evolution rate is described by the equation corresponding to two consecutive first-order reactions, with the rate constant of the second reaction considerably exceeding the rate constant of the first reaction, i.e., k ₂ {ie17-1} k ₁. The temperature dependences of k ₁ (s⁻¹) in 47.59% HBr in the temperature range from 60 to 90°C and in 64.16% H₂SO₄ between 80 and 130°C are described by Arrhenius equations with IogA= 12.7 ± 1.5 and 13.6 ± 1.4 and E _a = 115 ± 10 and 137 ± 10 kJ/mol, respectively. The parameters of the Arrhenius equation for the rate constant k ₂ for the reaction in 64.16% H₂SO₄ between 80 and 130°C are IogA= 9.1 ± 2.5 and E _a = 91 ± 18 kJ/mol. An analysis of the UV spectra of compound I in concentrated H₂SO₄ shows that I is a weak base $ (pK_{BH^ + } \approx - 6) $ (pK_{BH^ + } \approx - 6) . The rate-determining step of the hydrolysis of I is the attack of the nucleophile on the carbon atom of the MeO group of the protonated molecule of I. The resulting methyldiazene dioxide decomposes via a complicated mechanism to evolve N₂, NO, and N₂O. The pseudo-first-order rate constant k ₁ of the reaction at 80°C depends strongly on the acid concentration and on the type of nucleophile (Br⁻, Cl⁻, or H₂O). The relationship between k ₁ and the rate constant k of the bimolecular nucleophilic substitution reaction (S_N2) is given by the linear equation log$ [k_1 /(C_H + C_{Nu} )] = m^ \ne m*X_0 + \log (k/K_{BH^ + } ) $ [k_1 /(C_H + C_{Nu} )] = m^ \ne m*X_0 + \log (k/K_{BH^ + } ) , where $ C_{H^ + } $ C_{H^ + } and C _Nu are the concentrations of H+ and nucleophile, respectively; X ₀ is the excess acidity; and m ^≠ and m* are coefficients. The Swain-Scott equation log$ (k_{Nu} /k_{H_2 O} ) = ns $ (k_{Nu} /k_{H_2 O} ) = ns , where n is the nucleophilicity factor and s is the substrate constant (s = 0.72), is applicable to the rate constants k of the S_N2 reactions of the protonated molecule of I with Br⁻, Cl⁻, and H₂O.     

Temperature dependence of the rate constants for reactions of the carbonate radical with organic and inorganic reductants

Rate constants have been measured by pulse radiolysis for the reactions of the carbonate radical, CO₃^·?, with a number of organic and inorganic reactants as a function of temperature, generally over the range 5 to 80°C. The reactants include the substitution-inert cyano complexes of Fe^II, Mo^IV, and W^IV, the simple inorganic anions SO₃^2?, ClO₂^?, NO₂^?, I^?, and SCN^?, several phenolates, ascorbate, tryptophan, cysteine, cystine, methionine, triethylamine, and allyl alcohol. The measured rate constants ranged from less than 10⁵ to 3 × 10⁹ M^?1 s^?1, the activation energies ranged from ?11.4 to 18.8 kJ mol^?1, and the pre-exponential factors ranged from log A = 6.4 to 10.7. The activation energies for the metal complexes and inorganic anions generally decrease with increasing driving force for the reaction, as expected for an outer sphere electron transfer. For highly exothermic reactions, however, the activation energy appears to increase, probably reflecting the temperature dependence of diffusion. For many of the organic reactants, the activation energies were low and independent of driving force, suggesting that the oxidation is via an inner sphere mechanism.     

Thermolysis reaction of 2-acetyl-1-oxo-five-, six-, and seven-membered ring

The rates of gas-phase thermolysis reactions of 2-acetylcyclopentanone 1,2-acetylcyclohexanone 2, N-acetylcaprolactam 3,2-acetylbutyrolactone 4,2-acetyl-2-methylbutyrolactone 5, and 3-acetyl-2-oxazolidinone 6 have been measured over a temperature range of 50 K. They undergo unimolecular first-order elimination reactions for which log A = 11.7, 11.7, 11.2, 11.4, 11.5, and 11.1 s^?1 and E_a = 193.4, 189.5, 153.2, 201.0, 206.8, and 176.1 kJ mol^?1, respectively. The effect of the ring size together with the effect of a heteroatom in the ring on the rate of thermolysis reactions for compound 1–6 is the subject of this work. © 1995 John Wiley & Sons, Inc.     

Interaction Between Pd(RaaiR′)Cl<Subscript>2</Subscript> and Adenine: Reaction Dynamics and Mechanism [RaaiR′ = 1-alkyl-2-(arylazo)imidazole]

Nucleophilic substitution of Pd(RaaiR′)Cl₂ [RaaiR′=1-alkyl-2-(arylazo)imidazole, p-R—C₆H₄— N=N—C₃H₂NN-1-R′; where R= H(a)/Me(b)/Cl(c) and R′ = Et(1)/Bz(2)] with adenine (A) in MeCN–water (1:1) at 298 K, to form [Pd(A)₂]Cl₂, has been studied spectrophotometrically under pseudo-first-order conditions and the analyses support a nucleophilic association path. The reaction follows the rate law, rate = {a+k [A] ₀²[Pd(RaaiR′)Cl₂]: first-order in Pd(RaaiR′)Cl₂ and second-order in A. The rate increases as follows: Pd(RaaiEt)Cl₂(1) < Pd(RaaiBz)Cl₂(2) and Pd(MeaaiR′)Cl₂(b) < Pd(HaaiR′)Cl₂(a) < Pd(ClaaiR′)Cl₂(c). External addition of Cl⁻ (LiCl) suppresses the rate (rate 1/[Cl⁻]). The activation parameters, H⁰ and S⁰ of the reactions were calculated from the Eyring plot and support the proposed mechanism.     

Nucleophilic Substitution Reactions at Planar Tetra-coordinate Bis-cationic-platinum(II) Complexes. Kinetics of Displacement of Pyridine from {Pt[2,6-bis(methylthiomethyl)pyridine](py)}2+, {Pt[bis(2-pyridylmethyl)amine] (py)}2+ and {Pt[bis(2-(pyridylmethyl)sulphide](py)}2+ Cations

The kinetics of nucleophilic substitution of pyridine in bis-cationic [Pt(L)(py)]²⁺ complexes (L=SNS, NNN, NSN) [SNS=bis(methylthiomethyl)pyridine, NNN=bis(2-pyridylmethyl)amine, NSN=bis(2-pyridylmethyl)sulphide] by a series of nucleophiles (Cl⁻, Br⁻, I⁻, N₃⁻, (C₂H₅)₂S, NH₃, thiourea (tu), NO₂⁻, C₅H¹0NH, SeCN⁻, SCN⁻, CN⁻ when L=SNS; Cl⁻, Br⁻, I⁻, N₃⁻, (C₂H₅)₂S, SCN⁻, NH₃, NO₂⁻ when L=NNN; Br⁻, N₃⁻, NO₂⁻, NH₃, C₅H₁₀NH when L=NSN) have been measured in MeOH at 25 °C, μ =0.1 mol dm⁻³ (LiClO₄ or LiCF₃SO₃). The logarithms of the second-order rate constants calculated at μ=0, log k° ₂, do not follow the dependence upon the n° _Pt scale. In particular, the reactivity of the biphilic reagents tu, SeCN⁻, SCN⁻ and, to a lesser extent, NO⁻ ₂, towards these doubly charged substrates is largely lower than expected on the basis of the n° _Ptscale. There are good linear relationships between logk° ₂ for the bis-cationic substrate [Pt(SNS)(py)]²⁺, chosen as the standard, and log k° ₂ for the same reactions with [Pt(NNN)(py)]²⁺, [Pt(NSN)(py)]²⁺ and other double charged complexes previously studied. A new wide nucleophilicity scale based on [Pt(SNS)(py)]²⁺, that is appropriate to all the bis-cationic substrates, is here proposed     

Substituent effects on enol nitrosation of 1,3‐diketones

The keto–enol tautomerism of 3‐chloro‐pentane‐2,4‐dione (ClPD) was studied in aqueous micellar solutions of cationic, anionic, and nonionic surfactants. The enol of ClPD tautomerizes rapidly in water to the equilibrium proportions of the keto form, K_E=0.55; whereas the keto–enol conversion of 3‐ethyl‐pentane‐2,4‐dione (EPD) is a much slower reaction than the enol nitrosation. Kinetics of enol –nitrosation of both ClPD and EPD in aqueous acid medium using nitrous acid shows first‐order dependence upon [ketone] and linear or curve relationships of the observed rate constant, k_o, as a function of [nitrite] or [H⁺]; the observed behavior depends on the molecular structure of diketone and varies with the experimental conditions. The reaction is strongly catalyzed by Cl^?, Br^?, or SCN^?, and the observed rate constant shows a curve dependence on [Br^?] or [SCN^?], which is more pronounced at high acidity. The results are consistent with a reaction mechanism in which the nitrosation occurs initially on the enol–oxygen and releasing a proton to form a chelate–nitrosyl complex intermediate in steady state. Fine differences on the mechanistic spectrum of enols nitrosation are considered on the basis of the molecular structure of the diketone. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 668–679, 2012     

Kinetics of the thermal isomerization of 1,1,2,2‐tetramethylcyclopropane

Reaction rates for the structural isomerization of 1,1,2,2‐tetramethylcyclopropane to 2,4‐dimethyl‐2‐pentene have been measured over a wide temperature range, 672–750 K in a static reactor and 1000–1120 K in a single‐pulse shock tube. The combined data from the two temperature regions give Arrhenius parameters E_a=64.7 (±0.5) kcal/mol and log₁₀(A, s^?1) = 15.47 (±0.13). These values lie at the upper end of the ranges of E_a and log A values (62.2–64.7 kcal/mol and 14.82–15.55, respectively) obtained from three previous experimental studies, each of which covered a narrower temperature range. The previously noted trend toward lower E_a values for structural isomerization of methylcyclopropanes as methyl substitution increases extends only through the dimethylcyclopropanes (1,1‐ and 1,2‐); E_a then appears to increase with further methyl substitution. In contrast, the pre‐exponential factors for isomerization of cyclopropane and all of the methylcyclopropanes through tetramethylcyclopropane lie within ±0.3 of log₁₀(A, s^?1) = 15.2 and show no particular trend with increasing substitution. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 483–488, 2006     

Kinetics of OH reactions with aliphatic thiols

The kinetics of OH reactions with 1–4 carbon aliphatic thiols have been investigated over the temperature range 252–430 K. OH radicals were produced by flash photolysis of water vapor at λ > 165 nm and detected by time-resolved resonance fluorescence spectroscopy. All thiols investigated react with OH at nearly the same rate; k(298 K) = 3.2–4.6 × 10^?11 cm³ molecule^?1 s^?1, -E_act = 0.6–1.0 kcal/mol, A = 0.6–1.2 × 10^?11 cm³ molecule^?1 s^?1. CH₃SH and CH₃SD react with OH at identical rates over the entire temperature range investigated. We conclude that the dominant reaction pathway is addition to the sulfur atom.