Kinetics and thermochemistry of the gas phase reaction of methyl ethyl ketone with iodine. I. The resonance energy of the methylacetonyl radical

The gas phase reaction of iodine (2.8–43.3 torr) with methyl ethyl ketone (MEK) (7.4–303.4 torr) has been studied over the temperature range 280–355°C in a static system. The initial rate of disappearance of I₂ is first order in MEK and half order in I₂. The rate-determining step is the abstraction of a secondary hydrogen atom by an iodine atom: where k₁ is given by and θ = 2.303RT in kcal/mole. This activation energy is equivalent to a secondary C? H bond strength of 92.3 ± 1.4 kcal/mole and ΔH of the methylacetonyl radical = -16.8 ± 1.7 kcal/mole. By comparison with 95 kcal/mole for the secondary C? H bond strength, when delocalization of the unpaired electron with a pi bond is not possible, the resonance stabilization of the methylacetonyl radical is calculated to be 2.7 ± 1.7 kcal/mole. This value is 10 kcal/mole less than the stabilization energy of the isoelectronic methylallyl radical. The difference in pi bond energies in the canonical forms of the methylacetonyl radical is shown to account for the variation in stabilization energies.     

Kinetics of the gas phase reaction of methyl benzoate with bromine and iodine: The methoxyl C?H bond strength of methyl esters

The gas phase reactions of PhCOOCH₃ with I₂ and Br₂ were studied spectrophotometrically in a static system over the temperature ranges 344–359° and 246–303°, respectively. For each system the initial rate was first order in PhCOOCH₃ and half order in halogen as the concentration of PhCOOCH₃ was varied from 1.4 to 15.2 torr, that of I₂ from 6.2 to 26.4 torr, and that of Br₂ from 3.0 to 13.6 torr. The rate-determining step is the extraction of a methoxyl hydrogen atom: Empirical assignment of A-factors for k₁ lead to for the I₂ system, and to for the Br₂ system, where ? = 2.303RT in kcal/mole. Combined with the assumption that E_–1 = 1 ± 1 kcal/mole and 2 ± 1 kcal/mole for HI and HBr, respectively, DH (PhCOOCH₂? H) calculated from the two systems shows excellent agreement at 100.2 ± 1.3 kcal/mole and 100.1 ± 1.3 kcal/mole. Using a value of δH (PhCOOMe) = –65.6 ± 1.5 kcal/mole obtained from group additivity estimates, δH_f,298⁰ (PhCOOCH₂) is calculated to be –16.7 ± 2.0 kcal/mole. Unimolecular decomposition of the Ph(CO)O°CH₂ radical was also observed: with a rate constant equal to The abnormally high methoxyl C? H bond strength is discussed in relation to the bonding in ethers, alkanes, and esters.     

The kinetics of the gas-phase reaction between iodine and phenylsilane and the bond dissociation energy D(C6H5SiH2?H)

The title reaction has been investigated in the temperature range of 490-573 K. Initial reactant pressures were varied in the range of 0.2-5.2 torr (I₂) and 2-20 torr (C₆H₅SiH₃). The rate of iodine consumption, monitored spectrophotometrically, was found to obey both by initial rate and integrated equation fitting procedures. The effect of added initial HI conformed to this expression. The data are consistent with a conventional I-atom propagated chain reaction, and for the step the rate constant is given by From this is derived the bond dissociation energy value C₆H₅SiH₂? H = 374 kJ/mol(88 kcal/mol). A comparison with other Si? H dissociation energy values indicates that the “silabenzyl” stabilization energy is small, ≈7 kJ/mol.     

The gas-phase kinetics of the reaction of 1,1-difluoroiodoethane with hydrogen iodide: The C–I bond dissociation energy in 1,1-difluoroiodoethane

The kinetics of gas-phase reaction of CH₃CF₂I with HI were studied from 496 to 549K and have been shown to be consistent with the following mechanism: A least squares treatment of the data gave where θ = 2.303 RT kcal/mole. The observed activation energy E₁ was combined with E₂ = 0 ± 1 kcal/mole to yield The result, combined with data for several C? I bond dissociation energies, leads us to conclude that the C(sp³)? I bond is relatively insensitive to F for H substitution and that the C(sp²)–I bond has considerable double-bond character.     

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 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.     

Pyrolysis of 4,4-bisperfluoromethylbutene-1,4 01 in the gas phase

Pyrolysis of (CF₃)₂C(OH)CH₂CH=CH₂, the reverse of the reaction between perfluoroacetone and propene, has been studied in the gas phase between 475° and 598°K. Even at 573°K, the unimolecular reaction rate constant appears to be in its pressure-independent region at 20.0 torr pressure. In a quartz vessel, the decomposition is homogeneous. The specific unimolecular rate constant is where the limits are for one standard deviation. Combining these results with the previously reported results on the reverse reaction, the equilibrium constant for the reaction is It is noteworthy that in the temperature range of the study of the forward reaction (448° to 573°K), the percentage of back reaction in the times of the experiments varies from less than 0.1 to 1.5. Using group additivities and the above ΔH⁰, ΔH of (CF₃)₂CO is calculated to be ?325.2 kcal/mole at 600°K and the average C? C bond is 42.0 kcal/mole.     

Kinetics of the gas-phase reaction of cyclopropylcarbinyl iodide and hydrogen iodide. Heat of formation and stabilization energy of the cyclopropylcarbinyl radical

The rate of the gas phase reaction has been measured spectrophotometrically over the range 480°–550°K. The rate constant fits the equation where θ = 2.303RT in kcal/mole. This result, together with the assumption that the activation energy for the back reaction is 0 ± 1 kcal/mole, allows calculation of DH (Δ? CH₂? H) = 97.4 ± 1.6 kcal/mole and ΔH (Δ? CH₂·) = 51.1 ± 1.6 kcal/mole. These values correspond to a stabilization energy of 0.4 ± 1.6 kcal/mole in the cyclopropylcarbinyl radical.     

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.     

The kinetics of secondary reactions in the iodination of monogermane. The effect of iodine substitution on the Ge?H bond dissociation energy

A study has been made both of secondary reactions occurring during the reaction of I₂ with GeH₄, and of the direct reaction between I₂ and GeH₃I. Both these studies show that the abstraction reaction occurs about 30 times faster than the reaction in the temperature range of 425–446 K. This information is used to show that iodine substitution weakens Ge–H bonds by 14.4 ± 2.5 kJ/mol and that D(H₂IGe? H) = 332 ± 10 kJ/mol (79.3 kcal/mol). Possible reasons for the effects of halogen substituents on Ge? H and Si? H bond strengths are discussed.     

Thermal stability of primary amines

Tertiary-amyl amine has been decomposed in single-pulse shock-tube experiments. Rate expressions for several of the important primary steps are This leads to D(CH₃? H) – D(NH₂? H) = ?10.5 kJ and D[(CH₃)₃C? H] – D[(CH₃)₂NH₂C? H] = + 6 kJ. The present and earlier comparative rate single-pulse shock-tube data when combined with high-pressure hydrazine decomposition results-(after correcting for fall off effects through RRKM calculations) gives where k_r(…) is the recombination rate involving the appropriate radicals. This suggests that in this context amino radical behavior is analogous to that of alkyl radicals. If this agreement is exact, then Rate expressions for the primary step in the decomposition of a variety of primary amines have been computed. In the case of benzyl amine where data exist the agreement is satisfactory. The following differences in bond energies have been estimated:      

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      

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.     

A kinetic study of the thermal elimination of hydrogen fluoride from 1,2-difluoroethane. Determination of the bond dissociation energies D(CH2F?CH2F) and D(CH2F?H).

The kinetics of the thermal elimination of HF from 1,2-difluoroethane have been studied in a static system over the temperature range 734–820°K. The reaction was shown to be first order and homogeneous, with a rate constant of where θ = 2.303RT in kcal/mole. The A-factor falls within the normal range for such reactions and is in line with transition state theory; the activation energy is similarly consistent with an estimate based on data for the analogous reactions of ethyl fluoride and other alkyl halides. The above activation energy has been compared with values of the critical energy calculated from data on the decomposition of chemically activated 1,2-difluoroethane by the RRKM theory and the bond dissociation energy, D(CH₂F? CH₂F) = 88 ± 2 kcal/mole, derived. It follows from thermochemistry that ΔH_f⁰(CH₂F) = -7.8 and D(CH₂F? H) = 101 ± 2 kcal/mole. Bond dissociation energies in fluoromethanes and fluoroethanes are discussed.     

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.     

Kinetics of the gas-phase reaction of allyl alcohol with iodine. The heat of formation and stabilization energy of the hydroxyallyl radical

The reaction of iodine with allyl alcohol has been studied in a static system, following the absorption of visible light by iodine, in the temperature range 150-190°C and in the pressure range 10-200 torr. The rate-determining step has been found to be and k₃ is consistent with the equation From the activation energy and the assumption E_-3 = 1 ± 1 kcal mol^?1, it has been calculated that kcal mol^?1. The stabilization energy of the hydroxyallyl radical has been found to be 11.4 ± 2.2 kcal mol^?1.     

Kinetics of the gas-phase reaction of acetone with iodine: Heat of formation and stabilization energy of the acetonyl radical

The kinetics of the gas-phase reaction CH₃COCH₃ + I₂ ? CH₃COCH₂I + HI have been measured spectrophotometrically in a static system over the temperature range 340–430°. The pressure of CH₃COCH₃ was varied from 15 to 330 torr and of I₂ from 4 to 48 torr, and the initial rate of the reaction was found to be consistent with \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH}_3 {\rm COCH}_3 + {\rm I}^{\rm .} \stackrel{1}{\rightarrow}{\rm CH}_{\rm 3} {\rm COCH} + {\rm HI} $\end{document} as the rate-determining step. An Arrhenius plot of the variation of k₁ with temperature showed considerable scatter of the points, depending on the conditioning of the reaction vessel. After allowance for surface catalysis, the best line drawn by inspection yielded the Arrhenius equation, log [k₁/(M^?1 sec^?1)] = (11.2 ± 0.8) – (27.7 θ 2.3)/θ, where θ = 2.303 R T in kcal/mole. This activation energy yields an acetone C? H bond strength of 98 kcal/mole and δH (CH₃CO?H₂) radical = ?5.7 ± 2.6 kcal/mole. As the acetone bond strength is the same as the primary C? H bond strength in isopropyl alcohol, there is no resonance stabilization of the acetonyl radical due to delocalization of the radical site. By contrast, the isoelectronic allyl resonance energy is 10 kcal/mole, and reasons for the difference are discussed in terms of the π-bond energies of acetone and propene.     

Kinetic studies of the reactions CH2I2 + HI⇄CH3I + I2 and 2 CH3I⇄CH4 + CH2I2 the heat of formation of the iodomethyl radical

The rate of the reaction CH₂I₂ + HI ? CH₃I + I₂ has been followed spectrophotometrically from 201.0 to 311.2°. The rate constant for the reaction fits the equation, log (k₁/M^?1 sec^?1) = 11.45 ± 0.18 - (15.11 ± 0.44)/θ. This value, combined with the assumption that E₂ = 0 ± 1 kcal/mole, leads to ΔH (CH₂I, g) = 55.0 ± 1.6 kcal/mole and DH (H? CH₂I) = 103.8 ± 1.6 kcal/mole. The kinetics of the disproportionation, 2 CH₃I ? CH₄ + CH₂I₂ were studied at 331° and are compatible with the above values.     

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.     

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.