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1.
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.  相似文献   

2.
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.  相似文献   

3.
The abstraction of hydrogen and deuterium from 1,2-dichloroethane, 1,1,2-trichloroethane, and two of their deuterated analogs by photochemically generated ground state chlorine atoms has been investigatedin the temperature range 0–95°C using methane as a competitor. Rate constants and their temperature coefficients are reported for the following reactions Over the temperature range of this investigation an Arrhenius law temperature dependence was observed in all cases. Based on the adopted rate coefficient for the chlorination of methane [L.F. Keyser, J. Chem. Phys., 69 , 214 (1978)] which is commensurate with the present temperature range, the following rate constant values (cm3 s?1) are obtained: The observed pure primary, and mixed primary plus α- and β3-secondary kinetic isotope effects at 298 K are k3/k6 = 2.73 ± 0.08, and k1/k2 = 4.26 ± 0.12, respectively. Both show a normal temperature dependence decreasing to k3/k6 = 2.39 ± 0.06 and k1/k2 = 3.56 ± 0.09 at 370 K. Contrary to some simple theoretical expectations, the kinetic isotope effect for H/D abstraction decreases with increasing number of chlorine substituents in the geminal group in a parallel manner to the trend established previously for C1-substitution in the adjacent group. The occurrence of a β-secondary isotope effect, k4/k5, is established; this effect suggests a slight inverse temperature dependence.  相似文献   

4.
Aqueous iodination of trans-2-butenoic acid proceeds via hydrolysis of I2 to form HOI and I?, then rapid addition of HOI across the double bond to form the iodohydrin product. In the presence of iodate to keep iodide concentration low, the reaction proceeds at a conveniently measurable rate. The rate for the addition reaction is ?d[C4H6O2]/dt = 5900 [H+][C4H6O2][HOI]M/s at 25.0°C when [IO] = 0.025M and ionic strength = 0.3. The overall rate law in the presence of iodate is where [H+] and [IO] are total concentrations used to prepare the solution.  相似文献   

5.
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 k6a 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 = 1010.4±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.  相似文献   

6.
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.  相似文献   

7.
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   相似文献   

8.
The reaction of CF3 radicals with H2O (D2O) has been studied over the range of 533–723 K using the photolysis and the pyrolysis of CF3I as the free radical source. Arrhenius parameters for the reactions where X = H or D, relative to CF3 radical recombination are given by where k/k is in cm3/2/mol1/2·s1/2 and θ = 2.303RT/cal/mol. The activation energy and the primary kinetic isotope effect have been compared with those derived from the BEBO method.  相似文献   

9.
Metastable N2(A3Σu+), υ = 0, υ = 1, molecules are produced by a pulsed Tesla-type discharge of a dilute N2/Ar gas mixture. Rate coefficients for quenching these metastable levels by O2, O, N, and H were obtained by time-resolved emission measurements of the (0, 6) and (1, 5) Vegard–Kaplan bands. In units of cm3/mole · sec at 300°K and with an experimental uncertainty of ±20%, these rate coefficients for N2(A3Σu+) are Within the limits of error these coefficients apply to quenching N2(A3Σu+) υ′ = 1 as well.  相似文献   

10.
H2S increases the thermal isomerization of butene-2 cis (Bc) to butene-1 (B1) and butene-2 trans (Bt) 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 Bt formation results from the addition–elimination processes: . It is shown that the initiation step of pure Bc 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 k1', 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.  相似文献   

11.
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, δHf,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.  相似文献   

12.
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.  相似文献   

13.
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 k2c/k2a 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.  相似文献   

14.
4-Methylhexyne-1, 5-methylhexyne-1, hexyne-1, and 6-methylheptyne-2 have been decomposed in comparative-rate single-pulse shock-tube experiments. Rate expressions for the initial decomposition reactions at 1100°K and from 2 to 6 atm pressure are In combination with previous results, rate expressions for propargyl C? C bond cleavage are related to that for the alkanes by the expression These results yield a propargyl resonance energy of D(nC3H7-H) – D(C3H3-H) = 36 ± 2 kJ, in excellent agreement with a previous shock-tube study. They also lead to D(CH3C≡CCH2-H) – D(C3H3-H) = 0.6 ± 3 kJ, D(sC4H9-H) – D(iC3H7-H) = 0 ± 3 kJ, D(iC4H9-H) – D(nC3H7-H) = 2 ± 3 kJ, and D(nC3H7-H) – D(iC3H7-H) = 13.9 ± 3 kJ (all values are for 300°K). The systematics of the molecular decomposition process are explored.  相似文献   

15.
A kinetic study has been made of the gas phase, I2-catalyzed decomposition of (CH3)2S at 630–650 K. Some I2 is consumed initially, reaching a steady-state concentration. The initial major products are CH4 and CH2S together with small amounts of CH3SCH2I, CH3I, HI, and CS2. The initial reaction corresponds to a pseudo-equilibrium: accompanied by: and which brings (I2) into steady state and a final complex reaction: From the initial rate of I2 loss it is possible to obtain Arrhenius parameters for the iodination: We measure k1, (644 K) = 150 L/mol s and from both the Arrhenius plot and independent estimates A1 (644 K) = 1011.2 ± 0.3 L/mol s. Thus, E1 = 26.7 ± 1 kcal/mol. From the steady-state I2 concentration, an assumed mechanism and the known rate parameters for the CH3I/HI system. It is possible to deduce KA (644) = 3.8 × 10?2 with an uncertainty of a factor of 2. Using an estimated ΔS (644) = 4.2 ± 1.0 e.u. we find ΔHA (644) = 7.0 ± 1.1 kcal. With 〈ΔCPA〉644 = 1.2 this becomes: ΔHA(298) = 6.6 ± 1.1 kcal/mol. Then ΔH (CH3SCH2I) = 6.3 ± 1 kcal/mol. Making the assumption that E?1 = 1.0 ± 0.5 kcal/mol we find ΔH (644) = 25.7 ± 0.7 kcal/mol and with 〈ΔCPI〉 = 1.2; ΔH = 25.3 ± 0.8 kcal/mol. This gives ΔH (CH3S?H2) = 35.6 ± 1.0 kcal/mol and DH (CH3SCH2? H) = 96.6 ± 1.0 kcal/mol. This then yields Eπ(CH2S) = 52 ± 3 kcal. From the observed rate of pressure increase in the system and the preceding data k3, is calculated for the step CH3SCH2 → CH3 + CH2S. From an estimated A-factor E3 is deduced and from the overall thermochemistry values for k?3 and E?3. A detailed mechanism is proposed for the I-atom catalyzed conversion of CH2S to CS2 + CH4.  相似文献   

16.
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.  相似文献   

17.
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:   相似文献   

18.
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 ΔHf(trimethylene) ≤ 280 kJ/mol at 300°K.  相似文献   

19.
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   相似文献   

20.
The equilibrium constant for the reaction has been determined between 331 and 480°K using a variable-temperature flowing afterglow. These data give ΔH°(1) = -1.03 ± 0.21 kcal/mol and ΔS°(1) = —4.6 ± 1.0 cal/mol°K. When combined with the known thermochemical values for HBr, Br?, and HNO3, this yields ΔH(NO3?) = -74.81 ± 0.54 kcal/mol and S(NO3?) = 59.4 cal/mol·°K. In addition ΔHn-1,n and ΔSn-1,nfor the gas-phase reactions were determined for n = 2 and 3. The implications of these measurements to gas-phase negative ion chemistry are discussed.  相似文献   

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