Thermal stability of alcohols

3,3-Dimethylbutanol-2 (3,3-DMB-ol-2) and 2,3-dimethylbutanol-2 (2,3-DMB-ol-2) have been decomposed in comparative-rate single-pulse shock-tube experiments. The mechanisms of the decompositions are The rate expressions are They lead to D(iC₃H₇? H) – D((CH₃)₂(OH) C? H) = 8.3 kJ and D(C₂H₅? H) – D(CH₃(OH) CH? H) = 24.2 kJ. These data, in conjunction with reasonable assumptions, give and The rate expressions for the decomposition of 2,3-DMB-1 and 3,3-DMB-1 are and      

Kinetics,mechanism, and endo selectivity of Diels–Alder reactions of alkylmonosubstituted ethenes with cyclohexa-1,3-diene in the gas phase

The reactions where Y = CH₃ (M), C₂H₅ (E), i? C₃H₇ (I), and t? C₄H₉ (T) have been studied between 488 and 606 K. The pressures of CHD ranged from 16 to 124 torr and those of YE from 57 to 625 torr. These reactions are homogeneous and first order with respect to each reagent. The rate constants (in L/mol·s) are given by The Arrhenius parameters are used as a test for a biradical mechanism and to discuss the endo selectivity of the reactions.     

Time-resolved vibrational chemiluminescence: Rate constants for the reactions of F atoms with H2O and HCN,and for the relaxation of HF (v = 1) by H2O and HCN

Rate constants have been determined at (298 ± 4) K for the reactions: and the relaxation processes: Time-resolved HF(1,0) emission was observed following the photolysis of F₂ with pulses from an excimer laser operating on XeCl (λ = 308 nm). Analysis of the emission traces gave first-order constants for reaction and relaxation, and their dependence on [H₂O] and [HCN] yielded:      

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.     

Unexpectedly rapid vapor-phase reaction between bromine and methyl iodide

The overall reaction (1) occurs readily in the gas phase, even at room temperature in the dark. The reaction is much faster than the corresponding process and does not involve the normal bromination mechanism for gas phase reactions. Reaction (1) is probably heterogeneous although other mechanisms cannot be excluded. The overall reactions (1) (2) proceed, for all practical purposes, completely to the right-hand side in the vapor phase. The expected mechanism is (3) (4) (5) (6) (7) where reaction (3) is initiated thermally or photochemically. Reaction (4) is of interest because little kinetic data are available on reactions involving abstraction of halogen by halogen and also because an accurate determination of the activation energy E₄ would prmit us to calculate an acccurate value of the bond dissociation energy D(CH₃? I).     

A shock tube study of H + HNCO → NH2 + CO

The reaction of atomic hydrogen with isocyanic acid (HNCO) to produce the amidogen radical (NH₂) and carbon monoxide, has been studied in shock-heated mixtures of HNCO dilute in argon. Time-histories of the ground-state NH₂ radical were measured behind reflected shock waves using cw, narrowlinewidth laser absorption at 597 nm, and HNCO time-histories were measured using infrared emission from the fundamental v₂-band of HNCO near 5 μm. The second-order rate coefficient of reaction (2(a)) was determined to be: cm³ mol^?1 s^?1, where f and F define the lower and upper uncertainty limits, respectively. An upper limit on the rate coefficient of was determined to be:      

Effects of olefins on the thermal decomposition of propane part V. Effect of butene-2-Cis

The thermal decomposition of butene-2-cis at low conversion and its effect on the pyrolysis of propane have been studied in the temperature range 779-812 K. It was established that 2-butene decomposes in a long-chain process, with the chain cycle (Besides the radical path, the molecular reaction can also play a role in the formation of the products.) The thermal decomposition of propane is considerably inhibited by 2-butene, which can be explained by the fact that the less reactive radicals formed in the reactions between the olefin and the chain-carrying radicals regenerate the chain cycle more slowly than the original radicals in the above chain cycle or in the reactions The reactions of the 2-propyl radical are further initiation steps. The ratios of the rate coefficients of the elementary steps of the decomposition (Table III) have been determined via the ratios of the products. Estimation of the radical concentrations indicated that only the methyl, 2-propyl and methylallyl radicals are of importance in the chain termination. On the basis of the inhibition-influenced curves, the role of the bimolecular initiation steps. could be clarified in the presence of 2-butene.     

The rate constant for t-C4H9 → H + i-C4H8 and the thermodynamic parameters of t-C4H9

The rate of the reverse reaction of the system has been measured in the range of 584–604 K from a study of the azomethane sensitized pyrolysis of isobutane. Assuming the published value for the rate constant of recombination of t-butyl we obtain Combination with our published data for k₁ permits the evaluation We have modified a previously published structural model of t-butyl by the inclusion of a barrier to free rotation of the methyl groups in order to calculate values of the entropy and enthalpy of t-butyl as a function of temperature. Using standard data for H and for i-C₄H₈ we obtain We have obtained other, independent values of this quantity by a reworking of published data using our new calculations of the entropy and enthalpy of t-butyl. There is substantial agreement between the different values with one exception, namely, that derived from published data on the equilibrium which is significantly lower than the other values. We conclude that the value obtained from the present work and a reworking of published data which involves the use of experimental data on t-butyl recombination is incompatible with the result based on iodination data.     

Chlorine-photosensitized reactions in the Cl2 + O2 + 1,1,1,2-C2H2Cl4 system

The gas-phase photochlorination (λ = 436 nm) of the 1,1,1,2-C₂H₂Cl₄ has been studied in the absence and the presence of oxygen at temperatures between 360 and 420°K. Activation energies have been estimated for the following reaction steps: The dissociation energy D(CCl₃CHCl? O₂) ± (24.8 ± 1.5) kcal/mole has also been estimated from the difference in activation energy of the direct and reverse reactions The mechanism is discussed and the rate parameters are compared to those obtained for a series of other chlorinated ethanes.     

Thermal decomposition of cyclopentane and related compounds

Cyclopentane has been decomposed in comparative-rate single-pulse shock-tube experiments. The pyrolytic mechanism involves isomerization to 1-pentene and also a minor pathway leading to cyclopropane and ethylene. This is followed by the decomposition of 1-pentene and cyclopropane. The rate expressions over the temperature range of 1000°–1200° K are Details of the cyclopentane decomposition processes are considered, and it appears that if the trimethylene radical is an intermediate, then ΔH_f(trimethylene) ≤ 280 kJ/mol at 300°K.     

The kinetics of thermal dimerization of hexatriene

The thermal isomerization of cis-hexatriene (cHT) to cyclohexadiene (CHD) and the dimerization of CHD and trans-hexatriene (tHT) in the liquid phase in the temperature range 380 K-473 K are reported. The rate coefficients are: for the cHT to CHD isomerization for tHT dimerizationlog and for CHD dimerization; endo form exo form © 1993 John Wiley & Sons, Inc.     

Kinetics and the mechanism of the thermal reaction between trifluoromethylhypofluorite and hexafluoropropene

The kinetics of the thermal reaction between CF₃OF and C₃F₆ have been investigated between 20 and 75°C. It is a homogeneous chain reaction of moderate length where the main product is a mixture of the two isomers 1-C₃F₇OCF₃ (68%) and 2-C₃F₇OCF₃ (32%). Equimolecular amounts of CF₃OOF₃ and C₆F₁₄ are formed in much smaller quantities. Inert gases and the reaction products have no influence on the reaction, whereas only small amounts of oxygen change the course of reaction and larger amounts produce explosions. The rate of reaction can be represented by eq. (I): The following mechanism explains the experimental results: Reaction (5) can be replaced by reactions (5a) and (5b), without changing the result: Reaction (4) is possibly a two-step reaction: For ∣CF₃ = ∣C₃F₆∣, ν_20°C = 36.8, ν_50°C = 24.0, and ν_70°C = 14.2.     

The Competitive dehydrohalogenation of 1,1,1-trifluoro-2-chloroethane in reflected shock waves

The thermal decomposition of 1,1,1-trifluoro-2-chloroethane has been investigated in the single-pulse shock tube between 1120° and 1300deg;K at total reflected shock pressures from ～2610 to 3350 torr. Under these conditions, the major reaction is the α,α-elimination of hydrogen chloride, with The decomposition also involves the slower α,β-elimination of hydrogen fluoride, with the first-order rate constant given by At temperatures above 1270°K, two additional minor products were observed. These were identified as CF₂CFCl and CF₃CHCl₂ and suggest C? Cl rupture as a third reaction channel leading to complicated kinetics.     

Rate constants for abstraction of hydrogen from ethylene by methyl and ethyl radicals relative to abstraction from propane and isobutane

A method is described for the measurement of relative rate constants for abstraction of hydrogen from ethylene at temperatures in the region of 750 K. The method is based on the effect of the addition of small quantities of propane and isobutane on the rates of formation of products in the thermal chain reactions of ethylene. On the assumption that methane and ethane are formed by the following reactions, (1) measurements of the ratio of the rates of formation of methane and ethane in the presence and absence of the additive gave the following results: Values for k₂ and k₃ obtained from these ratios are compared with previous measurements.     

Pyrolysis of 2,4-dimethylhexene-l and the stability of isobutenyl radicals

2,4-Dimethylhexene-l has been decomposed in single-pulse shock tube experiments. Rate expressions for the initial reactions are and sec^?1 at 1.5–5 atm and 1050°K. This leads to ΔH^°_f300 (CH₂ = C(CH₃)CH₂) = 124 kJ/mol, or an allylic resonance energy of 50 kJ/mol. Rate expressions for the decomposition of the appropriate olefins which yield isobutenyl radicals and methyl, ethyl, isopropyl, n-propyl, t-butyl, and t-amyl radicals, respectively, are presented. The rate expression for the decomposition of isobutenyl radical is (at the beginning of the fall-off region). For the combination of isobutenyl and methyl radicals, the rate constant at 1020°K is Combination of this number and the calculated rate expression for 2-methylbutene-1 decomposition gives S. (1100) = 470 J/mol °K. This yields It is demonstrated that an upper limit for the rate of hydrogen abstraction by isobutenyl from toluene is      

Flash photolysis of biacetyl in gas phase. Rate constants of reactions between methyl and acetyl radicals

The flash photolysis of biacetyl produces CO, C₂H₆, and CH₃COCH₃ as main products, and in small amounts CO₂, C₂H₄, and CH₃CHO. The rate constants of reactions (2) and (3) of thermally equilibrated radicals were calculated from the amounts of products: .     

H2S-promoted thermal isomerization of butene-2 CIS to butene-1 or butene-2 trans around 500°C

H₂S increases the thermal isomerization of butene-2 cis (B_c) to butene-1 (B₁) and butene-2 trans (B_t) around 500°C. This effect is interpreted on the basis of a free radical mechanism in which buten-2-yl and thiyl free radicals are the main chain carriers. B₁ formation is essentially explainedby the metathetical steps: whereas the free radical part of B_t formation results from the addition–elimination processes: . It is shown that the initiation step of pure B_c thermal reaction is essentially unimolecular: and that a new initiation step occurs in the presence of H₂S: . The rate constant ratio has been evaluated: and the best values of k₁ and k_1', consistent with this work and with thermochemical data, are . From thermochemical data of the literature and an “intrinsic value” of E_?3 ? 2 kcal/mol given by Benson, further values of rate constants may be proposed: is shown to be E₄ ? 3.5 ± 2 kcal/mol, of the same order as the activation energy of the corresponding metathetical step.     

Photolysis of azomethane–propane mixtures. Arrhenius parameters for the hydrogen abstraction reactions of methyl with azomethane and with propane

Mixtures of up to 14% azomethane in propane have been photolyzed using mainly 366 nm radiation in the ranges of 323–453 K and 25–200 torr. Detailed measurements were made of the yields of nitrogen, methane, and ethane. Other products observed were isobutane, n-butane, ethene, and propene. A detailed mechanism is proposed and shown to account for the observed variation of product yields with experimental conditions. The quantum yield of the molecular process is found to be given by the temperature-independent equation The values of rate constants obtained are where the reactions are and it is assumed that the rate constant for the reaction is given by      

The kinetics of the thermal bromination of CF3I. Determination of the bond dissociation energy D(CF3−I)

The kinetics of the thermal bromination reaction have been studied in the range of 173–321°C. For the step we obtain where θ=2.303RT cal/mole. From the activation energy for reaction (11), we calculate that This is compared with previously published values of D(CF₃?I). The relevance of the result to published work on k_c for a combination of CF₃ radicals is discussed.     

Effects of Olefins on the Thermal Decomposition of Propane Part IV. Influence of Propylene

On the basis of the thermal decomposition of mixtures of propylene and propane with molar ratios of 0.0–0.33 in the temperature range 779–812K, the influencing functions describing the inhibition by propylene of the decomposition of propane were determined. The rate-reducing effect is explained mainly by the reactions (in which ^.R = ^.H, ^.CH₃ and 2-?₃H₇) and also by the addition reactions It was established that the bulk of the allyl radicals formed participate in the chain step, but, due to their lower reactivity, they restore the decomposition chain more slowly than the original radicals do. From the characteristic change in the ratio υ/υ, the rate ratios of hydrogenabstraction reaction by radicals from propylene and propane could be determined. In these reactions there was no significant difference between the selectivities of the radicals. For an interpretation of the changes, the decomposition mechanism must be completed with the reaction Evaluation of the influencing curves revealed that the initiation reactions must be taken into account. By parameter estimation we have determined the rate ratios characterizing the above initiation reactions, the unimolecular decomposition of propane, hydrogen abstraction by radicals from propane and propylene, intermolecular isomerization of the 2-propyl radical via propane and propylene, and abstraction of propane hydrogens by the ethyl and methyl radicals; these are given in Tables II.