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.     

Acetaldehyde oxidation in the negative temperature coefficient regime: Experimental and modeling results

Acetaldehyde oxidation has been studied in experiments at temperatures of 553 and 713 K carried out in a low pressure, static reactor and in numerical modeling calculations using a detailed chemical kinetic reaction mechanism. The results of the experimental study were used to construct and validate the reaction mechanism, which was then used to examine acetaldehydeoxidation in the negative temperature coefficient regime between 550 and 900 K. This mechanism was also tested against independent measurements of acetaldehyde oxidation carried out by Baldwin, Matchan, and Walker. The overall rate of reaction and the properties of the negative temperature coefficient regime were found to be sensitive to the competition between radical decomposition reactions and the addition of molecular oxygen to acetyl and methyl radicals, including particularly During these experiments, an upper limit to the rate of decomposition ofCH₃O₂H was measured at 553 K. Implications of the results for future kinetic modeling of engine knock are discussed.     

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.     

Mutual interactions of the methyl and methylperoxy radicals studied by flash photolysis and kinetic spectroscopy

The decadic extinction coefficient of the methyl radical at 216.4 nm and the rate constant for mutual combination were redetermined as: . The application of the Beer–Lambert law to these measurements was justified experimentally. The absorption spectrum of the methylperoxy radical was characterized as a weak, broad, structureless band, having a maximum at 240 nm with ?(240) = 1.55 × 10³ l./mol cm. The mutual interaction of methylperoxy radicals leads to the generation of methoxy and hydroperoxy radicals as a consequence of the nonterminating interaction . Each derivative radical may consume a significant fraction of the methylperoxy radicals, and either of these cross interactions may be made predominant by a suitable choice of oxygen pressure. The mutual interaction was studied under both conditions. The overall mechanism was analyzed by a precise computational method, and the rate constant of the total mutual interaction was estimated as .     

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.     

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.     

Effect of olefins on the thermal decomposition of propane part I. The model reaction

Study of the thermal decomposition of propane at very low conversions in the temperature range 760–830 K led to refinement of the mechanism of the reaction. The quotient V/V characterizing the two decomposition routes connected with the 1- and 2-propyl radicals proved to depend linearly on the initial propane concentration. This suggested the occurrence of intermolecular radical isomerization: in competition with decomposition of the 2-propyl radical: The linearity led to the conclusion that the selectivity of H-abstraction from the methyl and methylene groups by the methyl radical is practically the same as that by the H atom. The temperature-dependence of this selectivity ( μ = kCH₃/kCH₂) was given by Further evaluation of the dependence gave the Arrhenius representation for the ratio of the rate coefficients of the above isomerization and decomposition reactions. Steady-state treatment resulted in the rate equation of the process, comparison of which with measurements gave further Arrhenius dependences.     

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.     

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.     

Gas-phase thermolysis of sulfur compounds. Part VIII. Chloromethyl allyl,cyanomethyl allyl, 1-cyanoethyl allyl and neopentyl allyl sulfides

The pyrolyses of four alkyl allyl sulfides with substituents on the α? C atom of the alkyl moiety have been studied in a stirred-flow system over the temperature range 340-400°C and pressures between 2 and 12 torr. The only products formed are propene and thioaldehydes. The reactions showed first-order kinetics with the rate coefficients following the Arrhenius equations: Chloromethyl allyl sulfide: Cyanomethyl allyl sulfide: 1-cyanoethyl allyl sulfide: Neopentyl allyl sulfide: The effects of these and other substituents on the reactivity is discussed in relation with the stabilization of a polar six-centered transition state. The results support a non-concerted mechanism where the 1–5 α? H atom shift is assisted by its acidic character.     

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.     

Kinetics of formation,dissociation, and redox reactions of the benzyl radical-Cu(II) ion complex in a 10% acetonitrile-90% water mixture

The rate constant of formation, dissociation, and redox reaction of the benzyl radical-copper(II) ion complex have been determined at temperatures from 274 K to 333 K by laser flash photolysis of 1,3-diphenyl-2-propanone (DBK) in a 10% acetonitrile-90% water mixture containing copper(II)perchlorate. Rate constants were calculated from time resolved absorbances of both benzyl radicals (Bz) at 315 nm and benzyl-copper complexes (BzCu²⁺) at 375 nm. Values of the rate constants of the formation of BzCu²⁺ complex were fitted to the equation and the rate constant of the reverse reaction of BzCu²⁺ complex to the equation The rate constant of the redox reaction of BzCu²⁺ complex to benzyl alcohol and copper(I) ion can be described by the equation The first order rate constant of dissociation of phenylacetyl radical formed initially in the flash photolysis of DBK is given by the equation      

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 of CH radical reactions with N2O,SO2, OCS,CS2, and SF6

Pulsed laser photolysis/laser-induced fluorescence (LIF) is utilized to measure absolute rate constants of CH radical reactions as a function of temperature and pressure. Multiphoton dissociation of CHBr₃ at 266 nm is employed for the generation of CH (X²Π) radicals. The CH radical relative concentration is monitored by exciting fluorescence on the R₁(2) line of the (A²Δ – X²Π) transition at 429.8 nm. A resistively heated cell allows temperature studies to be performed from room temperature to ≈?670 K. The following Arrhenius equations are derived: With the exception of SF₆, the reactions of sulfur containing species proceed at rates that are near the theoretical gas kinetic collision frequency. Additionally, these reactions all have activation energies that are near zero or slightly negative. These observations are consistent with an insertion-decomposition mechanism being dominant under these conditions.     

Kinetics of the pyrolyses of endo- and exo-5-methylbicyclo (2.2.2) oct-2-ene in the gas phase

The pyrolyses of endo- and exo-5-methylbicyclo (2.2.2) oct-2-ene (endo- and exo-MBO) have been studied between 608 and 679°K at pressures between 7 and 37 torr. These reactions correspond to parallel first-order eliminations of propene and ethylene: The rate constants (in sec^?1) for endo-MBO are given by and those for exo-MBO by Reaction mechanisms involving diradicals are shown to be compatible with the experimental results. The heats of formation and the entropies of endo and exo-MBO are estimated.     

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).     

On the gas-phase free radical displacement reaction CH3 + CD3COCD3 → CD3 + CH3COCD3

The gas-phase free radical displacement reaction has been studied in the temperature range of 240–290°C and at 140°C with the thermal decomposition of azomethane (AM) and di-tert-butylperoxide (DTBP), respectively, as methyl radical sources. The reaction products of the CD₃ radicals were analyzed by mass spectrometry. Assuming negligible isotope effects, Arrhenius parameters for the elementary radical addition reaction were derived: The data are discussed with respect to the back reaction and general features of elementary addition reactions.     

Kinetics of UV-laser induced dehydrochlorination of 1-chloro-1,1-difluoroethane

The decomposition of 1-chloro-1,1-difluoroethane by a radical chain reaction has been studied in a flow reactor in the temperature range from 503 to 773 K. For the initiation of the chain small amounts of added chlorine were photolyzed with a XeCl laser (λ = 308 nm). The formation of the dehydrochlorination and chlorination products, vinylidene fluoride, and 1,2-dichloro-1,1-difluoroethane respectively, is described by a kinetic model. Arrhenius parameters for the two abstraction reactions and were determined by a competition method: Experimental and modeling results are discussed with respect to former studies on the thermal reaction of 1-chloro-1,1-difluoroethane.     

Effects of alkenes on the thermal decomposition of propane part II. Influence of ethylene

Experiments with propane-ethylene mixtures in the temperature range 760–830 K resulted in refinement of the role of ethylene inhibition in the decomposition of propane. The source of the rate-reducing effect of ethylene is the reaction This replaces the decomposition chains more slowly by means of the reactions than H-atoms do by direct H-abstraction from propane. Analysis of the ratios of the product formation rates showed that the selectivity of the ethyl radical for the abstraction of hydrogen of different bond strengths from propane was practically the same as that of the H-atom. The ratio of the rate constants of hydrogen addition to ethylene and methyl-hydrogen abstraction from propane by the H-atom (3) was determined as was that of the decomposition and the similar H-abstraction of the ethyl radical Interpretation of the influence of ethylene required the completion of the mechanism with further initiation of the reaction besides termination via ethyl radicals.     

Pyrolysis of tetrafluorohydrazine

The pyrolysis of tetrafluorohydrazine has been studied from 578 to 791°K. The stoichiometry has been established as Reaction rates have been measured and the effects of surface area, inert gas pressure, and nitric oxide have been examined. The rate-determining step proposed is and the general rate expression obtained for this is