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.     

Kinetics of the gas-phase reaction CH3F + I2 ⇆ CH2FI + HI: The C?H bond dissociation energy in methyl and methylene fluorides

The kinetics of the gas-phase reaction of CH₃F with I₂ have been studied spectrophotometrically from 629 to 710 K, and were determined to be consistent with the following mechanism: (1) A least-squares analysis of the kinetic data taken in the initial stages of reaction resulted in where θ = 4.575T/1000 kcal/mol. The errors represent one standard deviation. The experimental activation energy E₄ = 30.8 ± 0.2 kcal/mol was combined with the assumption E₃ = 1 ± 1 kcal/mol and estimated heat capacities to obtain The enthalpy change at 298 K was combined with selected thermochemical data to derive The kinetic studies of ?HF₂ and CH₂F₂ have been reevaluated to yield These results are combined with literature data to yield the C? H, C? F, and C? Cl bond dissociation energies in their respective fluoromethanes, and the effect of α-fluorine substitution is discussed.     

The gas-phase kinetics of the reaction of 1,1,1-trifluoroethyl iodine with HI: The C?I bond dissocation energy in 2,2,2-trifluoroethyl iodide

The kinetics of the gas-phase reaction of 2,2,2-trifluoroethyl iodide with hydrogen iodide has been studied over the temperature range of 525°K to 602°K and a tenfold variation in the ratio of CF₃CH₂I/HI. The experimental results are in good agreement with the expected free radical-mechanism: An analysis of the kinetic data yield: where θ =2.303RT in kcal/mol. If these results are combined with the assumption that E₂ = 0 ± 1 kcal/mol, then one obtains DH (CF₃CH₂? I) = 56.3 kcal/mol. This result may be compared with DH(CH₃CH₂? I) = 52.9 kcal/mol and suggests that substitution of three fluorines for hydrogen in the beta position strengthens the C? I bond slightly.     

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.     

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.     

Free radical decomposition of bromoacetonitrile in liquid cyclohexane

The kinetics of the gamma radiation induced free radical chain decomposition of BrCH₂CN in liquid cyclohexane (RH) was investigated over the temperature range of 60–170°C. In addition competitive experiments in the presence of CCl₄ were carried out between 80 and 180°C. For the reactions the following Arrhenius expressions were derived: where θ = 2.303RT in kcal/mol. The effect of CN substitution on the activation energies of reactions (2) and (3) was evaluated based on the present and previously published results. The CN group effect on halogen atom abstraction [reaction (2)] is discussed in terms of inductive and enthalpic factors. The differences E₃ ? E(CH₃ + RH) and E(CCl₂CN + RH) ? E(CCl₃ + RH), which yield a value of about 5.5 kcal/mol, are considered to reflect the cyano stabilization effect at the radical center confirming D(CH₂(CN)–H) ～ 93 kcal/mol.     

The reactions of methyl radicals with chloromethanes

The reaction of methyl radicals with CCl₄ and CCl₃Br have been reinvestigated in the gas phase over a wide range of temperatures and pressures using both the photolysis of acetone and the pyrolysis of di-tertiary butyl peroxide (dtBP) as the methyl radical sources. The results are in essential agreement with previous work; however, these new studies provide evidence that at higher pressures the major source of HCl in the reactions is due to methyl radical attack on CH₃CCl₃, formed via the combination of methyl and trichloromethyl radicals. From these investigations Arrhenius parameters for the reactions have been determined: Pyrolysis of dtBP in the presence of relatively high-pressure mixtures of CCl₄ and CCl₃Br resulted in no enhanced methane formation, since, under these conditions, the only termination product is C₂Cl₆, and the HCl precursor CH₃CCl₃ is not formed. A competitive technique has been used in which dtBP was pyrolysed in the gas phase in the presence of high-pressure mixtures of CCl₃Br and a chloromethane. Arrhenius parameters were obtained for the reactions and the results were used to provide information on the importance of polar effects for hydrogen abstraction from halogenated methanes.     

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.     

The addition of methyl radicals to hexafluoroacetone

The reaction of methyl radicals (Me) with hexafluoroacetone (HFA), generated from ditertiary butyl peroxide (dtBP), was studied over the temperature range of 402–433 K and the pressure range of 38–111 torr. The reaction resulted in the following displacement process taking place: where TFA refers to trifluoroacetone. The trifluoromethyl radicals that were generated abstract a hydrogen atom from the peroxide: such that k_6a is given by: where θ = 2.303RT kcal/mol. The interaction of methyl and trifluoromethyl radicals results in the following steps: Product analysis shows that k₁₇/kk = 2.0 ± 0.2 such that k₁₇ = 10^10.4±0.5M^?1 · s^?1. The rate constant k₅ is given by: It is concluded that the preexponential factor for the addition of methyl radicals to ketones is lower than that for the addition of methyl radicals to olefins.     

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      

Kinetic study of gas-phase reactions with CF2Cl2 and CFCl3. Analysis using the transition state theory (TST) of the chlorine atom transfer reactions by CF3 and CH3 radicals from halomethanes

The following gas-phase reactions: were studied by the competitive method with CF₃I as the source of radicals. The kinetic parameters obtained in the temperature range 533–613 K and 503–613 K respectively for chlorine atom transfer reactions are given by: where θ = 2.303 RT (cal mol^?1). The Arrhenius A values were calculated for seven chlorine atom transfer reactions (CF₂Cl₂, CFCl₃, CCl₄ with CF₃ radicals; CF₃Cl, CF₂Cl₂, CFCl₃ and CCl₄ with CH₃ radicals) by using the thermochemical kinetic version of the Transition State Theory (TST).     

The kinetics of the reactions of i-C3F7 radicals with BR2 and HBr

The reactions have been studied competitively in the vapor phase over the range of 52–204°C. The i-C₃F₇ radicals were generated by means of the reaction It was found that where θ = 2.303RT J/mol. Absolute Arrhenius parameters are derived for the reactions where R = CF₃, C₂F₅, and i-C₃F₇.     

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 kinetics and the mechanism of the thermal gas-phase reaction between NO2 and 1,1-dichlorodifluoroethylene,CF2CCl2

The kinetics of the gas phase reaction between NO₂ and CF₂CCl₂ has been investigated in the temperature range from 50 to 80°C. The reaction is homogeneous. Three products are formed: O₂NCF₂CCl₂NO₂ and equimolecular amounts of CINO and of O₂NCF₂C(O)Cl. The rate of consumption of the reactants is independent of the total pressure, the reaction products, and added inert gases and can be represented by a second-order reaction: However, the distribution of the products is influenced by the pressure of the present gases, which favor the formation of the dinitro-compound in a specific way. The effect of CF₂CCl₂ is the greatest. In the absence of added gases, the ratio of O₂NCF₂CCl₂NO₂ to that of O₂NCF₂C(O)Cl is proportional to (CF₂CCl₂ + γ_P products). The experimental results can be explaned by the following mechanism: P and X represent the products and the added gases:      

Influence of butadiene-1,3 on ethanal pyrolysis at 495°C

At 495°C and a low extent of reaction, ethanal pyrolysis is slightly inhibited by the addition of small quantities of butadiene-1,3, whereas it is accelerated by more important quantities. The inhibiting effect is interpreted in terms of a free-radical chain mechanism in which the main chain carriers of ethanal pyrolysis (CH₃.free radicals) reversibly add to butadiene-1,3 and yield penten-2-yl (R.) free radicals. These free radicals either react in a metathetical step: or in terminating steps. But butadiene-1,3 also gives rise to new initiation steps: which account for the accelerating effect. Process (i₃) seems to be more important than process (i₂) in the experimental conditions, but its nature could not be identified. The results are consistent with literature data and the following value of k₆: (4.57T in cal/mol).     

Kinetics and equilibria of the reaction between bromine and ethyl ether. The C?H bond dissociation energy in ethyl ether

The kinetics and equilibria of the reaction: have been studied in the temperature range 298–333 K by using the very low pressure reactor (VLPR) technique. Combining the estimated entropy change of reaction (1), ΔS = 8.1 ± 1.0 eu, with the measured ΔG, we find ΔH = 4.2 ± 0.4 kcal/mol; ΔH(CH₃CHOC₂H₅) = ?20.2 kcal/mol, and DH° [Et OCH(Me)-H] = 91.7 ± 0.4 kcal/mol. We find: where θ = 2.3 RT in kcal/mol. It has been shown that the reaction proceeds via a loose transition state and the “contact TS” model calculation gives a very good agreement with the observed value.     

Mechanism of pyrolysis of C2Cl6

Using published data on the kinetics of pyrolysis of C₂Cl₆ and estimated rate parameters for all the involved radical reactions, a mechanism is proposed which accounts quantitatively for all the observations: The steady-state rate law valid for after about 0.1% reaction is and the reaction is verified to proceed through the two parallel stages suggested earlier whose net reaction is A reported induction period obtained from pressure measurements used to follow the rate is shown to be compatible with the endothermicity of reaction A, giving rise to a self-cooling of the gaseous mixture and thus an overall pressure decrease. From the analysis, the bond dissociation energy DH⁰(C₂Cl₅? Cl) is found to be 70.3 ± 1 kcal/mol and ΔH_f300⁰(·C₂Cl₅) = 7.7 ± 1 kcal/mol. The resulting π? bond energy in C₂Cl₄ is 52.5 ± 1 kcal/mol.     

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: .     

The kinetics of the Diels–Alder addition of cyclopentadiene to acetylene and the decomposition of norbornadiene

The title reactions have been investigated in a static system. The addition of acetylene to cyclopentadiene (CPD) results in formation of norbornadiene (BCH), cycloheptatriene (CHT), and toluene (T), while BCH decomposition produces CPD, C₂H₂, CHT, and T. Kinetic studies, comprising both product–time evolution and initial pressure variation, support a mechanism These reactions are almost certainly homogeneous and molecular in nature. Least mean square analysis of the data yield for temperatures of 525–656°K, log k_?1 (1./mole·s)=7.51±0.05? (24.19±0.15 kcal/mole)/RT ln 10, and for temperatures of 584–630°K,      

Radiation-induced dechlorination of carbon tetrachloride in cyclohexane and n-hexane mixtures. III. The kinetics of liquid-phase reactions of trichloromethyl radicals

The kinetics of gamma-radiation-induced free-radical chain reactions in solutions of carbon tetrachloride in mixtures of varying composition of cyclohexane and n-hexane was investigated in the temperature range of 296°–413°K. Trichloromethyl radicals were produced by the reaction of radiolytically generated alkyl radicals with the solute. The kinetics of the following reactions were studied: The following rate expression was obtained: The error limits are the standard deviation from the least mean-square Arrhenius plots. The present results, combined with previously measured activation parameters for hydrogen-atom abstraction from c-C₆H₁₂ and n-C₆H₁₄ by CCl₃ radicals relative to CCl₃ combination, afford experimental evidence that the decay of trichloromethyl species in alkane solutions is a diffusion-limited process. The thesis that activation energies of reactions (4) and (5) in the liquid phase are equal to their respective values in the gas phase is confirmed.