Gas-phase pyrolysis mechanism and kinetics of 3-t-butoxyquadricyclane

The gas-phase pyrolysis of 3-t-butoxyquadricyclane [1] was investigated over the temperature range 511–542 K at one atm in helium. The initial pyrolysis step is the isomerization of 3-t-butoxyquadricyclane to 7-t-butoxynorbornadiene (Ea = 38.49 ± 0.85 kcal/mole, log A = 15.44 ± 0.35). 7-t-butoxynorbornadiene exhibits a single unimolecular reaction pathway which produces a mixture of t-butoxycycloheptatrienes (Ea = 38.44 ± 0.63 kcal/mole, log A = 15.05 ± 0.26). This two-step mechanism affords fewer reactions than unsubstituted quadricyclane in the gas phase and could be useful for its reduced sooting potential. © 1996 John Wiley & Sons, Inc.     

Chemistry of trifluorostyrenes and their dimmers: 3. Solvent effect on the kinetic parameters of the thermal cyclodimerization of α, β, β-trifluorostyrene1

Rate constants for the thermal cyclodimerization of α, β, β-trifluorostyrene (TFS) were determined in six solvents at 393°K. The products of this reaction were mixtures of roughly equal amounts of cis-trans isomers. The rate constants in 3 solvents, were calculated according to Arrhenius equation. In n-hexane, log A = 6.02±0.18, E_a= 19.5±0.3 kcal.mol^?1; in glyme, logA = 5.31 ± 0.19, E_a= 18.0±0.3 kcal.mol^?1; in methanol, IogA=4.93±0.13, E_a=17.1±0.3 kcal mol^?1. All data are consistent with a stepwise radical mechanism, and our reaction in this solvent series obeys an isokinetic relationship, with β = 478°K.     

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     

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.     

Reactivity ratios of styrene and methyl methacrylate at 90°C

From the conversion–composition data of Gruber and Elias, the reactivity ratios of styrene (M₁) and methyl methacrylate (M₂) were calculated to be r₁ = 0.55 ± 0.02 and r₂ = 0.58 ± 0.06 at 90°C. The least-squares method was then used on these and literature values at other temperatures to obtain the Arrhenius expressions: In r₁ = 0.04736 – (235.45/T), and ln r₂ = 0.1183 – (285.36/T). Using literature values for the homopolymerization steps, A₁₁ = 2.2 × 10⁷l./mole-sec., E₁₁ = 7.8 kcal./mole, and A₂₂ = 0.51 × 10⁷ l./mole-sec.^?1, E₂₂ = 6.3 kcal./mole, activation energies and frequency factors were then calculated for the cross-polymerization steps: A₁₂ = 2.1 × 10⁷ l./mole-sec., E₁₂ = 7.3 kcal./mole, and A₂₁ = 0.45 × 10⁷ l./mole-sec., E₂₁ = 5.7 kcal./mole.     

Applications of carbon-13 resonance spectroscopy—XV : The degenerate cope-rearrangement of bullvalene

The temperature dependence of the ¹³C-NMR spectrum of bullvalene has been studied from ?67 to +128°C using fourier transform spectroscopy and ¹H broadband decoupling. Lineshape analysis based on the Anderson-Kubo-Sack theory yielded E_a=13·9±0·1 kcal/mole, log A= 14·0±0·1, ΔH3 = 13·3±0·1 kcal/mole, and ΔS3 = 3·4±0·4 e.u. The pertinent features of dynamic ¹³C-NMR spectroscopy are discussed.     

The carbon–hydrogen bond strength in dimethyl ether and some properties of iodomethyl methyl ether

The kinetics and mechanism of the reaction between iodine and dimethyl ether (DME) have been studied spectrophotometrically from 515–630°K over the pressure ranges, I₂ 3.8–18.9 torr and DME 39.6–592 torr in a static system. The rate-determining step is, where k₁ is given by log (k₁/M^?1 sec^?1) = 11.5 ± 0.3 – 23.2 ± 0.7/θ, with θ = 2.303RT in kcal/mole. The ratio k₂/k_?1, is given by log (k₂/k_?1) = ?0.05 ± 0.19 + (0.9 ± 0.45)/θ, whence the carbon-hydrogen bond dissociation energy, DH° (H? CH₂OCH₃) = 93.3 ± 1 kcal/mole. From this, ΔH°_f(CH₂OCH₃) = ?2.8 kcal and DH°(CH₃? OCH₂) = 9.1 kcal/mole. Some nmr and uv spectral features of iodomethyl ether are reported.     

Kinetics of the reactions Br2 + HCN,BrCN + I−, S(CN)2 + I− in aqueous acid solution

Spectrophotometric methods have been used to obtain rate laws and rate parameters for the following reactions: with k_a, k_b, E_a, E_b having the values 85±5 l./mole · s, 5.7±0.2 s^?1 (both at 298.2°K), and 56±4 and 66±2 kJ/mole, respectively. with k_c=0.106±0.004 l./mole ·s at 298.2°K and E_c=67±2 kJ/mole. with k_d=(3.06 ±; 0.15) × 10^?3 l./mole ·s at 298.2°K and E_d=66±2 kJ/mole. Mechanisms for these reactions are discussed and compared with previous work.     

Mechanisms and kinetics of the thermal decompositions of trichlorosilane,dichlorosilane, and monochlorosilane

Decomposition studies of trichlorosilane, dichlorosilane, and monochlorosilane at 921 K, 872 K, and 806 K, respectively, are reported. The studies were made at fixed reactant pressures over a range of total pressures in a wall conditioned, quartz reactor connected to a quadrupole mass-spectrometer. Products were monitored sequentially and continuously in time. The dichlorosilane decomposition was also studied by the comparative-rate single-pulse shock-tube method at temperatures around 1250 K. Two mechanisms of decomposition are considered: a silylene based mechanism initiated by molecular elimination reactions (Scheme I), and a free radical based mechanism initiated by bond fission reactions (Scheme V). Modeling tests of these mechanisms show that only the former is consistent with the experimental data. The decompositions are shown to be essentially nonchain processes initiated by the following pressure dependent reactions: HSiCl₃(SINGLEBOND)4→ SiCl₂+HCl, H₂SiCl₂(SINGLEBOND)1→ SiCl₂+H₂ and H₃SiCl(SINGLEBOND)5→ HSiCl+H₂. High pressure Arrhenius parameters recommended for these reactions are A_4,∞=A_1,∞=A_5,∞=10^14.5±0.5 s⁻¹, E_4,∞=71.9±2.1 kcal/mol, E_1,∞=69.2±2.0 kcal/mol, and E_5,∞=60.6±1.8 kcal/mol. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet: 30: 69–88, 1998.     

Very low-pressure pyrolysis (VLPP) of t-butylmethyl ether

The thermal decomposition of t-butylmethyl ether has been studied using the VLPP technique. The recommended Arrhenius parameters for the molecular elimination, reaction (1), are A(800°K) = 10^{1 3, 9} sec^?1 and E_a (800°K) = 59.0 ± 1.0 kcal/mole. No radical reactions occur under the conditions used. These parameters are in good agreement with earlier experimental work and with theoretical estimates of both A and E.     

The dissociation of 3-chloro-1-butene and the resonance energy of the chloro-allyl radical

Methane is a primary product of pyrolysis of 3-chloro-l-butene at temperatures in the range 776–835°K, and from its rate of formation values have been obtained for the limiting high-pressure rate constant of the reaction These may be represented by the expression log [(k₁)_∞/sec^?1] = (16.7 ± 0.3) ? (71.5 ± 1.5)/θ, where θ = 2.303RT kcal/mole. Assuming a zero activation energy for the reverse reaction and that over the experimental temperature range the rates at which a methyl radical adds on to chlorobutene are comparable to those at which it abstracts hydrogen, the activation energy for the dissociation reaction leads to a value of 83.2 ± 1.9 ckal/mole for D(H? CHClCH:CH₂) at 298°K. Taking D(H? CHClCH₂CH ₃) = 95.2 ± 1.0 kcal/mole a value of 12.0 ± 2.1 kcal/mole is obtained for the resonance energy of the chloroallyl radical. This value in conjunction with resonance energies obtained in earlier work indicates that substitution of a hydrogen atom on the carbon atom adjacent to the double bond in the allyl radical leads to no significant variation in the allylic resonance energy.     

Very low-pressure pyrolysis of cyclobutyl cyanide. The cyano stabilization energy

A very low-pressure pyrolysis (VLPP) apparatus has been constructed and shown to yield kinetic data consistent with other VLPP systems. The technique has been applied to the pyrolysis of cyclobutyl cyanide over the temperature range of 833–1203°K. The reaction was found to proceed via a single pathway to yield ethylene and vinyl cyanide. If A_∞ is based on previous high-pressure data for this reaction and for cyclobutane pyrolysis, then RRKM theory calculations show that the experimental unimolecular rate constants are consistent with the high-pressure Arrhenius parameters given by where θ=2.303 RT in kcal/mole. If A_∞ is adjusted relative to the more recent parameters for cyclobutane pyrolysis suggested by VLPP studies, then the Arrhenius expression becomes The cyano group reduces the activation energy for cyclobutane pyrolysis by 6±1 kcal/mole, and on the basis of a biradical mechanism this value may be attributed to the cyano stabilization energy.     

Kinetics of chemically activated ethane

Chemically activated ethane, with an excitation energy of 114.9 ± 2 kcal/mole, was formed by reaction with methane of excited singlet methylene radicals produced by the 4358 Å photolysis of diazomethane. A decomposition rate constant of (4.6 ± 1.2) × 10⁹ sec^?1 was measured for the chemically activated ethane. This result agrees, via RRKM theory, with most other chemically activated ethane data, and the result predicts, via RRKM and absolute rate theory for E₀ = 85.8 kcal/mole, E* = 114.9 kcal/mole, and k_E = 4.6 × 101 sec^?1, a thermal A-factor at 600°K of 10^16.6±0.2 sec^?1, in approximate agreement with the more recent experimental values. Combining 2 kcal/mole uncertainties in E₀ and E* with the uncertainty in our rate constant yields an A-factor range of 10^16.6±0.7 sec^?1. It is emphasized that this large uncertainty in the A-factor results from an improbable combination of uncertainty limits for the various parameters. These decomposition results predict, via absolute rate theory (with E₀(recombination) = 0) and statistical thermodynamic equilibrium constants, methyl radical recombination rates at 25°C of between 4.4 × 10⁸ to 3.1 × 10⁹ l.-mole^?1-sec^?1, which are 60 to 8 times lower, respectively, than the apparently quite reliable experimental value. A value of E₀(recombination) greater than zero offers no improvement, and a value less than zero would be quite unusual. Activated complexes consistent with the experimental recombination rate and E₀(recombination) = 0 greatly overestimate the experimental chemical activation and high pressure thermal decomposition rate data. Absolute rate theory as it is applied here in a straightforward way has failed in this case, or a significant amount of internally consistent data are in serious error. Some corrections to our previous calculations for higher alkanes are discussed in Appendix II.     

Catalytic action of metallic salts in autoxidation and polymerization. IV. Polymerization of methyl methacrylate by cobalt(II) or (III) acetylacetonate–tert-butyl hydroperoxide or dioxane hydroperoxide

Polymerization of methyl methacrylate was carried out by four initiating systems, namely, cobalt(II) or (III) acetylacetonate–tert-butyl hydroperoxide (t-Bu HPO) or dioxane hydroperoxide (DOX HPO). Dioxane hydroperoxide systems were much more effective for the polymerization of methyl methacrylate than tert-butyl hydroperoxide systems, and cobaltous acetylacetonate was more effective than cobaltic acetylacetonate in both hydroperoxides. The initiating activity order and activation energy for the polymerization were as follows: Co(acac)₂–DOX HPO (E_a-9.3 kcal/mole) > Co (acac)₃–DOX HPO (E_a = 12.4 kcal/mole) > Co(acac)₂–t-Bu HPO (E_a = 15.1 kcal/mole) > Co(acac)₃–t-Bu HPO (E_a-18.5 kcal/mole). The effects of conversion and hydroperoxide concentration on the degree of polymerization were also examined. The kinetic data on the decomposition of hydroperoxides catalyzed by cobalt salts gave a little information for the interpretation of polymerization process.     

Thermal rearrangements of polybutadienes with different vinyl contents

A study was made of the loss of double bonds in equibinary (1,4-1,2) polybutadiene (EB) and in polybutadienes with 30% 1,2, 70% 1,4 (FI), and 10% 1,2, 90% 1,4 (DI) double-bond content, when heated in vacuum under nonpyrolytic conditions (temperature range 220–280°C). These polymers were found to undergo second-order loss of 1,2 unsaturation with similar activation energies (E_a = 34.0 ± 3 kcal/mole), by analogy to the previously reported thermally induced loss of double bonds in 1,2-polybutadiene (VB) (E_a = 33.6 ± 3 kcal/mole). Moreover, EB and FI exhibited also second-order loss of 1,4 unsaturation, with E_a ca. 36 and 40 kcal/mole, respectively, while DI showed negligible loss of 1,4 unsaturation below 260°C, in common with cis-1,4-polybutadiene (CB) (with 2% 1,2 double bonds) examined earlier. The loss of 1,2 double bonds in the various polybutadienes with different vinyl contents is accompanied by substantial methyl production, ranging from about one methyl group formed for every 4–5 vinyl units lost in VB, to one methyl for every two vinyls lost in EB, and to almost one methyl for each vinyl lost in DI or CB. Mechanisms are proposed for the thermally induced loss of 1,2 and 1,4 unsaturation in various polybutadienes and for the accompanying methyl production.     

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

The Arrhenius parameters for the gas phase, unimolecular structural isomerizations of 1,1,2‐trimethylcyclopropane to three isomeric methylpentenes and two dimethylbutenes have been determined over a wide range of temperatures, 688–1124 K, using both static and shock tube reactors. For the overall loss of reactant, E_a = 63.7 (± 0.5) kcal/mol and log₁₀ A = 15.28 (± 0.12). These values are higher by 2.6 kcal/mol and 0.7–0.8 than previously reported from experimental work or predicted from thermochemical calculations. E_a for the formation of trans‐4‐methyl‐2‐pentene is 1.5 kcal/mol higher than E_a for the formation of the cis isomer, which is identical to the E_a difference previously reported for the formation of trans‐ and cis‐2‐butene from methylcyclopropane. Substitution of methyl groups for hydrogen atoms on the cyclopropane ring is expected to weaken the C? C ring bonds, and it has been reported previously that activation energies for structural isomerizations of methylcyclopropanes do decrease substantially over the series cyclopropane > methylcyclopropane > 1,1‐ or 1,2‐dimethylcyclopropane. However, the present study shows that the trend does not continue beyond dimethylcyclopropane isomerization. Besides reductions in C? C bond energy, steric interactions may be increasingly important in determining the energy surface and conformational restrictions near the transition state in isomerizations of the more highly substituted methylcyclopropanes. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 475–482, 2006     

The rate of the gas phase iodination of cyclobutane. The heat of formation of the cyclobutyl radical

The gas phase iodination of cyclobutane was studied spectrophotometrically in a static system over the temperature range 589° to 662°K. The early stage of the reaction was found to correspond to the general mechanism where the Arrenius parameters describing k₁ are given by log k₁/M^?1 sec^?1 = 11.66 ± 0.11 – 26.83 ± .31/θ, θ = 2.303RT in kcal/mole. The measured value of E₁, together with the fact that E_?1 = 1 ± 1 kcal/mole, provides ΔH(c-C₄H_7.) = 51.14 ± 1.0 kcal/mole, and the corresponding bond dissociation energy, D(c-C₄H₇? H) = 96.8 ± 1.0 kcal/mole. A bond dissociation energy of 1.8 kcal/mole higher than that for a normal secondary C? H bond corresponds to one half of the extra strain energy in cyclobutene compared to cyclobutane and is in excellent agreement with the recent value of Whittle, determined in a completely different system. Estimates of ΔH and entropy of cyclobutyl iodide are in very good agreement with the equilibrium constant K₁₂ deduced from the kinetic data. Also in good agreement with estimates of Arrhenius parameters is the rate of HI elimination from cyclobutyl iodide.     

Abstraction of chlorine atoms from chloromethanes by the cyclohexyl radical

The kinetics of chlorine atom abstraction from the chloromethanes (CM)CCl₄, CHCl₃, and CH₂Cl₂ by radiolytically generated cyclohexyl radicals has been studied in the liquid phase by a competitive method. The halogen abstraction data have been put on an absolute basis by comparing the rates of the metathetical reactions with the known rate of addition of cyclohexyl radicals to C₂Cl₄. The following Arrhenius parameters were obtained:

The pyrolysis of 2-nitrosoisobutane and the bond dissociation energies of nitroso compounds

Study of the reaction by very-low-pressure pyrolysis (VLPP) in the temperature range of 550–850°K yields for the high-pressure Arrhenius parameters where θ = 2.303RT in kcal/mole. These in turn yield for the high-pressure second-order recombination of tBu + NO, k_?1 = (3.5 ± 1.7) × 10⁹ 1./mole·sec at 600°K. For the competing reaction l./mole·sec and E₄ ≥ 4.2 kcal/mole. The bond dissociation energy DH^o (tBu-NO) was determined to be (39.5 ± 1.5) kcal/mole, both from the equilibrium constant and from the activation energy of reaction (1), obtained from RRKM calculations. A ‘free-volume’ model for the transition state for dissociation is consistent with the data. A limited study of the system at 8–200 torr showed an extremely rapid inhibition by products and a very complex set of products.