- Homolytic dissociation of 1-substituted cyclohexa-2,5-diene-1-carboxylic acids: An EPR spectroscopic study of chain propagation
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Hydrogen abstraction from 1-substituted cyclohexa-2,5-diene-1-carboxylic acids containing linear, branched and cyclic alkyl substituents, as well as allyl, propargyl (prop-2-ynyl), cyanomethyl and benzyl substituents, has been studied by EPR spectroscopy. For each carboxylic acid, EPR spectra of the corresponding cyclohexadienyl radicals were observed at lower temperatures, followed by spectra due to ejected carbon-centred radicals at higher temperatures. Rate constants, for release of the carbon-centred radicals from the cyclohexadienyl radicals, were determined from radical concentration measurements for the above range of substituents. The rate of cyclohexadienyl radical dissociation increased with branching in the 1-alkyl substituent and with electron delocalisation in the ejected carbon-centred radical; 3,5-and 2,6-dimethyl-substitution of the cyclohexadienyl ring led to reductions in the dissociation rate constants. Rate data for abstraction of bisallylic hydrogens from the cyclohexadienyl acids were also obtained for ethyl, n-propyl and isopropyl radicals. These results indicated a sharp drop in the rate of hydrogen abstraction as the degree of branching in the attacking radical increased. Small decreases in the hydrogen abstraction rate constants were observed for cyclohexadienes containing CO2R substituents.
- Jackson, Leon V.,Walton, John C.
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p. 1758 - 1764
(2007/10/03)
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- Exploitation of aldoxime esters as radical precursors in preparative and EPR spectroscopic roles
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Photolyses of aldoxime esters, containing a considerable range of alkyl groups, lead to cleavage of their N-O bonds and formation of aryliminyl and alkyl radicals. The process was found to be favoured by 4-methoxyacetophenone as a photosensitiser and by methoxy substituents in the aryl rings. 4-Nitro- and pentafluoro-substitutions of the aryl rings were, on the other hand, deleterious. The intermediate iminyl radicals, together with primary, secondary and tertiary alkyl radicals were characterised by 9 GHz EPR spectroscopy. Cyclopropyl, CF3, and CCl3 radicals were probably also formed, but were too reactive for direct EPR spectroscopic detection. Photosensitised reaction of benzophenone oxime O-nonanoyl ester produced the diphenylmethaniminoxyl, as well as the expected n-octyl and iminyl radicals. This indicated that O-C bond scission accompanied O-N scission for this ketoxime ester. At higher temperatures the C-centred radicals added to the starting oxime esters to produce alkoxyaminyl radicals that were also spectroscopically detected in some cases. No evidence for abstraction of the iminyl hydrogen by tertbutoxyl radicals was obtained. Instead, the t-BuO radicals added to the C=N double bonds of the oxime esters. Similarly, chlorine abstraction from alkylbenzohydroximoyl chlorides by trimethyltin radicals did not take place. Preparative scale experiments with oxime esters containing suitably unsaturated alkyl groups showed that good yields of cyclised products could be obtained in the presence of the photosensitiser. This process constitutes a general method by which carboxylic acids or acid chlorides can be converted into alkyl radicals and hence to cyclised derivatives.
- McCarroll, Andrew J.,Walton, John C.
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p. 2399 - 2409
(2007/10/03)
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- The efficiency of alkyl radical generation and hydrogen transfer from 1-alkylcyclohexa-2,5-diene-1-carboxylic acids
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A novel EPR spectroscopic technique has been used to determine kinetic data for alkyl radical generation and hydrogen transfer from 1-alkylcyclohexa-2,5-diene-1-carboxylic acids; the implications of these data for preparative chain reactions of these reagents are inferred.
- Jackson, Leon,Walton, John C.
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p. 7019 - 7021
(2007/10/03)
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- The reactivity of ketyl and alkyl radicals in reactions with carbonyl compounds
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A parabolic model of bimolecular radical reactions was used for analysis of the hydrogen transfer reactions of ketyl radicals: >C+OH + R1COR2 → >C=O + R1R2C+OH. The parameters describing the reactivity of the reagents were calculated from the experimental data. The parameters that characterize the reactions of ketyl and alkyl radicals as hydrogen donors with olefins and with carbonyl compounds were obtained: >C+OH + R1CH=CH2 → >C=O + R1C+ HCH3; >R1CH=CH2 + R2C+HCH2R3 → R2C+HCH3 + R2CH=CHR3. These parameters were used to calculate the activation energies of these transformations. The kinetic parameters of reactions of hydrogen abstraction by free radicals and molecules (aldehydes, ketones, and quinones) from the C-H and O-H bonds were compared.
- Denisov
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p. 2110 - 2116
(2007/10/03)
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- Kinetics and mechanisms of the reactions of chlorine atoms with ethane, propane, and n-butane
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Absolute (flash photolysis) and relative (FTIR-smog and GC) rate techniques were used to study the gas-phase reactions of Cl atoms with ethane (k1), propane (k3), and n-butane (k2). Experiments performed at 298-540 K give k2÷k1=(2.0±0.1)exp((183±20)÷T). At 296 K the reaction of Cl atoms with propane yields of 43 % 1-propyl and 57 % 2-propyl radicals, while the reaction of Cl atoms with n-butane produces 29% 1-butyl and 71% 2-butyl radicals. Butyl radicals were found to react with Cl2 with rates which are 3 times greater than the corresponding reactions with O2.
- Tyndall,Orlando,Wallington,Dill,Kaiser
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- Reduction of 1-Substituted 2,4,6-Triphenylpyridinium Ions in Aprotic Media
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Some 1-substituted 2,4,6-triphenylpyridinium salts have been studied by cyclic voltammetry.The compounds exhibit two or three reduction peaks depending on the sweep rate and the rate of loss of the 1-substituent.The rate of cleavage of the one-electron reduction product (the radical) was measured by cyclic voltammetry and double potential step chronoamperometry, and the rate of cleavage of the two-electron reduction product (the anion) found by simulation of the voltammetric curves; the simulation suggested that the rate of cleavage of the radical was faster than the rate of cleavage of the anion.This unexpected finding is tentatively explained by different conformations of the phenyl groups in the 2 and 6 positions in the radical and in the anion.
- Joergensen, Lars Viborg,Lund, Henning
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p. 759 - 764
(2007/10/02)
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- Thermal and photochemical fragmentation of α,α-dialkoxybenzyl radicals: A comparison of the thermal reactions with laser induced fragmentations by using laser flash and laser-jet photolyses
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The thermal and photochemical cleavage of α,α-dialkoxybenzyl radicals has been examined using a combination of techniques, including two-laser two-color laser flash photolysis and the laser-jet technique. For the parent α,α-dimethoxybenzyl radical photofragmentation occurs with a quantum yield of 0.80. The study of several unsymmetrically substituted radicals (e.g., methoxyisopropoxy) leads to the conclusion that the photoinduced fragmentation shows no selectivity. In contrast, the thermal decomposition of the radicals shows that fragmentation follows the expected radical stabilities, i.e., isopropyl > ethyl > methyl, the differences being almost exclusively due to changes in the activation energy. By comparing with literature data for methyl elimination it is possible to estimate the rate constants for fragmentation at 56°C as 213, 1380, and 16 600 s-1 for methyl, ethyl, and isopropyl elimination.
- Banks,Scaiano,Adam, Waldemar,Oestrich, Rolf Schulte
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p. 2473 - 2477
(2007/10/02)
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- CIDEP Studies on Radicals Produced from Photochemical Reactions of Some Aromatic Carbonyl Compounds
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CIDEP spectra of radicals generated from photoinduced hydrogen abstraction and α-cleavage reactions of several alkyl phenyl ketones were studied.The participation of radical pair mechanism on the spectra was discussed from the magnitude of hyperfine interaction.
- Ikoma, Tadaaki,Akiyama, Kimio,Tero-Kubota, Shozo,Ikegami, Yusaku
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p. 1491 - 1494
(2007/10/02)
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- New General Processes of Homolytic Alkylation of Heteroaromatic Bases by t-BuOOH or (t-BuO)2 and Alkyl Iodides
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New general, selective processes of homolytic alkylation of protonated heteroaromatic bases have been developed using alkyl iodides and t-BuOOH or (t-BuO)2 as sources of alkyl radicals.Both processes are based on the generation of methyl radical from the peroxides, and on iodine abstraction from the alkyl iodide by the methyl radical.The selective processes are the result of combined enthalpic and polar effects.The enthalpic factor governs the equilibrium of iodine abstraction, whereas the polar effect governs the reactivity of the alkyl radicals with the protonated heteroaromatic ring.A redox chain is operative with t-BuOOH and an unusual free-radical chain process is involved with (t-BuO)2.Both chains are particularly effective because of the electron-transfer oxidation of the pyridyl radical intermediate, the ionization potential of which (5.4-6.0 eV) is close to that of lithium (5.39 eV) or sodium (5.14 eV).
- Fontana, Francesca,Minisci, Francesco,Barbosa, Maria Claudia Nogueira,Vismara, Elena
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p. 995 - 999
(2007/10/02)
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- Photoreactivity of σ-bonded metalloporphyrins. 1. Formation of zwitterionic indium and gallium porphyrin complexes in tetrahydrofuran
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Continuous irradiation and laser-flash photolysis experiments were carried out in THF on σ-bonded indium and gallium porphyrins of the type (P)M(R), where P is the dianion of tetraphenylporphyrin (TPP) or octaethylporphyrin (OEP), M is In or Ga, and R is CH3, C2H5, C4H9, CH(CH3)2, C(CH3)3, CH=CHC6H5, or C≡CC6H5. Steady-state photolysis of (P)M(R), where R = CH3, C2H5, C4H9, CH(CH3)2, or C(CH3)3, leads to photodissociation of the metal-carbon bond. In contrast, indium and gallium porphyrins with σ-bonded CH=CHC6H5 or C≡CC6H5 groups do not undergo this photodissociation reaction. Laser photolysis studies were carried out both in the presence and in the absence of ferrocene and indicate that the photoreaction occurs via a triplet state originating from the porphyrin macrocycle. The resulting ESR spectra obtained after photolysis of (TPP)In(C2H5) in THF are compared with ESR spectra obtained after electrochemical reduction of (TPP)InCl. On the basis of these results, a photochemical generation of zwitterionic metalloporphyrin radicals of the type (P)-In+ is demonstrated to occur in THF. Similarly, the (P)Ga(R) complexes also form a zwitterionic photoproduct upon irradiation as evidenced by ESR and UV-visible spectra. The photoreactivity of (P)In(R) and (P)Ga(R) is discussed with respect to the porphyrin macrocycle, the σ-bonded R group, and the solvent polarity.
- Kadish,Maiya,Xu
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p. 2518 - 2523
(2008/10/08)
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- Photolysis of, and the reactions of H-atoms with, isopropanethiol
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Detailed investigation of the photolysis of i-C3H7SH has been carried out in the absence and presence of the inert gas, n-C4H10.A mechanism consisting of three primary photochemical steps : i-C3H7SH /-> i-C3H7S + H, φ1 = 0.93 +/- 0.02; i-C3H7SH /-> i-C3H7 + SH, φ2 = 0.057 +/- 0.024; i-C3H7SH /-> H2S + C3H6, φ3 = 0.009 +/- 0.001; six hot reaction steps and seven thermal reaction steps adequately explains all the experimental observations.As in the case of hot H* atoms both the H-atom abstraction, H + i-C3H7SH /-> H2 + i-C3H7S, and the SH-displacement, H + i-C3H7SH /-> H2S + i-C3H7, reactions occur with thermalized H-atoms, with the rate constant ratio k7/k8 = 19.2 +/- 4.3 at 25 deg C.The Arrhenius expressions have been determined over the temperature range 25-145 deg C, to be : k7 = (4.0 +/- 0.7) x 1012 exp and k8 = (2.8 +/- 0.6) x 1012 exp cm3mol-1s-1.It was found that the overall mechanism for the photolytic decomposition of i-C3H7S is analogous to that established for C2H5SH in previous investigations.
- Lam, W.W.,Yokota, T.,Safarik, I.,Strausz, O. P.
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p. 2025 - 2033
(2007/10/02)
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- ON THE REMOVAL OF METALLIC MIRRORS BY FREE RADICALS.
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Large radicals can be formed by passing chlorinated organic compounds at pressures of a few mm. , through a furnace containing a pellet of sodium and heated to 350-400 degree C. It is found that the only radicals that will remove metallic mirrors (of tellurium or antimony, etc. , previously deposited beyond the furnace) are those that can decompose into methyl or ethyl radicals plus an unsaturated molecule, without undergoing any transmigration of atoms. The authors also found, especially in the case of larger monochlorinated molecules, that there was some decomposition, approximately half, even in the absence of metallic sodium.
- Rice,Tweedell
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p. 995 - 997
(2007/10/02)
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- Mechanistic Investigation of Carbon-Carbon Bond Formation in the Reduction of Alkyl Halides by Organonickel Complexes in Aqueous Solution
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The macrocyclic alkylnickel(II) complexes, RNi(tmc)+ (R = C2H5, C3H7, and CH2-c-C5H9; tmc = (1R,4R,8S,11S)-1,4,8,11-tetramethyl-1,4,8.11-tetraazacyclotetradecane), react with alkyl halides R'X (R'= CH3, C2H5, 1-C3H7, 2-C3H7, and PhCH2) in alkaline aqueous solutions, yielding combination and disproportionation products of R and R'.The reactivity of R'X increases in the order methyl . + 2-C3H7I -> 2-C3H7. + C2H5I in aqueous solution was determined by competition with V(H2O)62+.The value k2-PrI ca. 6E5 M-1s-1 is significantly higher than in aprotic solvents.
- Bakac, Andreja,Espenson, James H.
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p. 719 - 723
(2007/10/02)
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- A Direct Study of the Reactions of CH2 (3B1)-Radicals with Selected Hydrocarbons in the Temperature Range 296 K = T = 705 K
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The kinetics of the reactions of CH2 (3B1)-radicals with five selected organic compounds has been studied in an isothermal discharge flow system in the temperature range 296 .Two basic reaction mechanisms, either direct H-atom abstarction by 3CH2 or thermal excitation of 3CH2 to the low lying 1A1 state followed by consecutive reactions of 1CH2, are of importance.For acetaldehyde, isobutane, and propane direct H-atom abstraction by 3CH2 predominates.After separation of the small contribution attributed to the singlet reaction the following rate constants for the reactions of CH2 (3B1) with acetaldehyde, isobutane, and propane are obtained: .Presuming the reactions of 1CH2 with hydrocarbons are fast the thermal excitation mechanism dominates the reaction system in the cases of methane and ethane.The activation energy of EA(CH4) = 40 +/- 8 kJ/mol measured for methane is concluded to be determined by the singlet-triplet energy splitting in CH2. - Keywords: Chemical Kinetics / Elementary Reactions / Laser Magnetic Resonance / Methylene / Radicals
- Dobe, S.,Boehland, T.,Temps, F.,Wagner, H. Gg.
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p. 432 - 441
(2007/10/02)
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- Kinetics of the Reactions between CH2(3B1)-Radicals and Saturated Hydrocarbons in the Temperature Range 296 K = T = 707 K
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The reaction between CH2-radicals in their ground electronic state (3B1) and n-hexane CH2() + n-C6H14 --> CH3 + C6H13 was studied in a discharge flow system with LMR detection of CH2.In the temperature regime 413 K 4 = 1E(13.22 +/- 0.20)*exp(-3380 +/- 240/T) cm3/mol s.The reaction proceeds both via direct H-atom abstraction by CH2() and via thermal excitation of CH2() to the low-lying singlet state (1A1) followed by fast consecutive reactions of CH2().The contributions due to thermal excitation and singlet reaction were evaluated for the present work as well as for a recent study of the reactions of CH2() with a series of other hydrocarbons.Corrected rate constants kT for the direct reactions of CH2() with the reactants HR = CH4 (1), C2H6 (2), C3H8 (3), n-C6H14 (4), i-C4H10 (5), and CH3CHO (6) in the temperature range 296 K 1T = 4.3E12*exp(-42 kJ mol-1/RT) cm3/mol s, k2T = 6.5E12*exp(-33.1 kJ mol-1/RT) cm3/mol s, k3T = 4.9E12*exp(-27.7 kJ mol-1/RT) cm3/mol s, k4T = 7.8E12*exp(-25.6kJ mol-1/RT) cm3/mol s, k5T = 2.5E12*exp(-22.5 kJ mol-1/RT) cm3/mol s, k6T = 1.7E12*exp(-14.7 kJ mol-1/RT) cm3/mol s.The activation energies for the reactions studied are described by an Evans Polanyi type relation.Arrhenius expressions are proposed for the rate constants of H-atom abstraction by CH2(3B1) from primary, secondary, tertiary, and aldehydic C-H bonds.The results are compared to the isoelectronic reactions of O(3P). - Keywords: Chemical Kinetics / Elementary Reactions / Radicals / Spectroscopy, Laser Magnetic Resonance
- Boehland, T.,Dobe, S.,Temps, F.,Wagner, H. Gg.
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p. 1110 - 1116
(2007/10/02)
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- Abschaetzung kinetischer und thermodynamischer Daten fuer Wasserstoffabspaltungsreaktionen von niederen Olefinen
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Mit Hilfe der BEBO-Methode sind fuer 500 ... 1200 K die Aktivierungsparameter der H-Uebertragungsreaktionen .R1 + HR2 --> R1H + R2.(R1: H, CH3, C2H5, n-C3H7, i-C3H7, CH3CO, CH2COCH3; R2H: Ethen, Propen, But-1-en) berchnet worden.Unter Verwendung entsprechender Daten der Rueckreaktion wurden daraus ΔRGθ, ΔRHθ und ΔRSθ bestimmt und mit den aus thermochemischen Inkrementen zugaenglichen Werten verglichen.Die Beweiskraft der guten Uebereinstimmung beider Datensaetze fuer die Anwendbarkeit des BEBO-Verfahrens auf H-Abstraktionen von Olefinen wird diskutiert.Ausserdem wird gezeigt, dass bei der Pyrolyse von Olefinen die H-Abstraktion aus der Vinylgruppe bis ueber 1500 K gegenueber der aus den Alkylgruppen zu vernachlaessigen ist.
- Scherzer, K.,Lohse, K.,Loeser, U.
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p. 196 - 204
(2007/10/02)
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- Absolute Rate Constants for the Reaction of Triethylsilyl Radicals with Organic Halides
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The absolute rate constants for the reactions of Et3Si. radicals with a number of organic halides have been measured in solution by using laser flash photolysis techniques.The reactivities cover a wide range; e.g., the rate constants at ca. 300 K are 4.3 * 109, 1.1 * 108, 2.5 * 106, and 5 M-1 s-1 for CH3CH2I, C6H5Br, (CH3)3CCl, and C6H5CH2F, respectively.Arrhenius parameters were determined for a few representative substrates.Thus, for CCl4, Ea = (0.78 +/- 0.24) kcal/mol and log (A/M-1 s-1) = 10.2 +/- 0.2, data which can be combined with relative rates from the literature to yield absolute rate constants for some additional substrates.Comparison of the Arrhenius parameters for halogen abstraction from several substrates suggests the involvment of charge-transfer interactions in some of these reactions.Such rate-enhancing interactions can have a greater influence on the preexponential factor than on the activation energy.An explanation for this phenomenon is advanced.Combination of our rate constants with data from literature also allows evaluation of the rates of inversion at the silicon center in triorganosilyl radicals as being in the range (3-12) * 109 s-1 at temperatures from 0 to 80 deg C which implies that a considerable activation barrier is involved in such process.
- Chatgilialoglu, C.,Ingold, K. U.,Scaiano, J. C.
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p. 5123 - 5127
(2007/10/02)
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- Addition of 2,2,3-Trimethylbutane to Slowly Reacting Mixtures of Hydrogen and Oxygen at 480 deg C
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The oxidation of 2,2,3-trimethylbutane(TRIMB) has been studied by adding traces of the alkane to slowly reacting mixtures of H2+O2 in aged boric-acid-coated vessels at 480 deg C.Rate constants for H and OH attack on TRIMB have been obtained and, by use of an additivity principle, Arrhenius parameters of log10(A/dm3mol-1s-1)=9.32+/-0.13 and E=460+/-950 J mol-1 are suggested for OH attack at a tertiary C-H bond in alkanes from a combination of the present results with studies at lower temperatures.Propene, isobutene, and 2,2,3-trimethylbut-1-ene (TRIMB-1) are the major initial products, the yield of the latter increasing markedly at high O2 pressures.The proportions of the three species of trimethylbutyl radicals formed in each mixture have been estimated.Two of the species of trimethylbutyl radicals react almost completely by homolysis of the strained central C-C bond to give propene and isobutene as products.The tird radical (CH3)3CC(CH3)2 is removed by either reaction (1C) or (2C), and it is suggested that (1C) occurs by a concerted mechanism: (CH3)3CC(CH3)2 i-C4H8 + i-C3H7 (1C) (CH3)3CC(CH3)2 + O2 (CH3)3CC(CH3)=CH2 + HO2. (2C) Rate constants have been obtained for a number of the homolysis reactions of the trimethylbutyl radicals, for which there is a good correlation between log k and ΔU, the internal energy change.
- Baldwin, Roy R.,Walker, Raymond W.,Walker, Robert W.
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p. 2157 - 2174
(2007/10/02)
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- Thermal Decomposition of Propyl-, Isobutyl-, and Neopentylbenzene
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Kinetics of the following bond dissociation reactions have been determined over a range of temperature near 1000 K by using the technique of very low-pressure pyrolysis: PhCH2CH2CH3 ---> PhCH2* + *C2H5 (1); PhCH2CH(CH3)2 ---> PhCH2* + i-*C3H7 (2); PhCH2C(CH3)3 ---> PhCH2* + t-*C4H9 (3).The following high-pressure Arrhenius rate expressions for reactions 1 and 2 were obtained from radical thermochemistry and rate measurements (θ/kcal mol-1 = 0.00457T):k1/s-1 = 1015.3-69.6/θ; k2/s-1 = 1015.6-67.8/θ.A rate expression for reaction 3 was obtained from rate measurements and an assumed reverse recombination rate constant (k-3) of 109.3 M-1s-1: k3/s-1 = 1015.5-64.3/θ.The latter rate expression is consistent with an enthalpy of formation for tert-butyl radicals of 9.8+/-2 kcal mol-1 at 298 K.These results are compared to results from carrier systems and shock tube experiments.
- Robaugh, David A.,Barton, Barrie D.,Stein, Stephen E.
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p. 2378 - 2383
(2007/10/02)
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