Homogeneous pyrolysis kinetics of ethyl 3-hydroxy-3-methylbutanoate in the gas phase

The pyrolysis kinetics of ethyl 3-hydroxy-3-methylbutanoate have been examined over the temperature range of 286–330°C and pressure range of 29–108 Torr. In a seasoned vessel and in the presence of the free radical inhibitor cyclohexene or toluene the reaction is homogeneous, unimolecular and obeys a first-order rate law. The elimination products are mainly acetone and ethyl acetate, and very small amounts of ethyl 3-butenoate, acetic acid, ethylene and H₂O. The rate coefficient is expressed by the following equation: log k₁(s^–1)=(12.39±0.46)–(174.5±5.2) kJ mol^–1 (2.303RT)^–1. The mechanism appears to proceed via a six-membered cyclic transition state, where polarization of the (CH₃)C(OH)^+...-CH₂COOCH₂CH₃ bond is rate determining.     

Reexamination of the intimate ion pair mechanism in the gas phase pyrolysis kinetics of ethyl 4-bromobutyrate

The gas phase pyrolysis of ethyl 4-bromobutyrate in the temperature range of 354–375 °C and pressure range of 47–152 Torr obeys a first-order rate law. The rate coefficient for the unimolecular elimination is expressed by the following Arrhenius equation: log k₁(s^–1)=(13.71±±0.60)–(209.9±7.3) kJ mol^–1/2.303 RT. A more complete analysis for the parallel elimination and consecutive reactions of the bromoester confirms the argument of an intimate ion pair type of mechanism for the debromination processes. The carboethoxy substituent through neighbouring group participation exerts a small but significant accelerating effect in some of the parallel eliminations.

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4- 354–375°C 47–152 . : log k₁(^–1)=(13,71±0,60)–(209,9±7,3) ·^–1/2,303 RT. . - , .

     

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.     

Steric factors in the gas phase elimination kinetics of ethyl N‐benzyl‐N‐cyclopropylcarbamate and ethyl diphenylcarbamate

The elimination kinetics of ethyl N‐benzyl‐N‐cyclopropylcarbamate and ethyl diphenylcarbamate were investigated over the temperature range of 349.9–440.0°C and the pressure range of 31–106 Torr. These reactions have been found to be homogeneous, unimolecular, and obey a first‐order rate law. The products are ethylene, carbon monoxide, and the corresponding secondary amine. The rate coefficient is expressed by the following Arrhenius equations: For ethyl N‐benzyl‐N‐cyclopropylcarbamate log k₁ (s^?1) = (12.94 ± 0.09) ? (198.5 ± 0.9) kJ mol^?1 (2.303RT)^?1 For ethyl diphenylcarbamate log k₁ (s^?1) = (12.91 ± 0.18) ? (208.2 ± 2.4) kJ mol^?1 (2.303RT)^?1 The presence of phenyl and bulky groups at the nitrogen atom of the ethylcarbamate showed a decrease in the rate of elimination. Steric factor may be operating during the process of decomposition of these substrates. These reactions appear to undergo a semipolar six‐membered cyclic transition type of mechanism.© 2001 John Wiley & Sons, Inc. Int J Chem Kinet 34: 67–71, 2002     

The pyrolysis kinetics of ethyl esters in the gas phase. The alkyl substituent effect at the acyl carbon

The gas-phase elimination of ethyl 3-methylbutanoate and ethyl 3,3-dimethylbutanoate has been studied, in a static system, over the temperature range of 360–420°C and in the pressure range of 71–286 torr. The reactions are homogeneous, unimolecular, and follow a first-order rate law. The temperature dependence of the rate coefficients is given by the following Arrhenius equations: for ethyl 3-methylbutanoate, log k₁ (s^?1) = (12.70 ± 0.36) – (202.5 ± 4.4) kJ/mol/2.303RT, and for ethyl 3,3-dimethylbutanoate, log k₁ (s^?1) = (13.04 ± 0.08) – (207.1 ± 1.0) kJ/mol/2.303RT. Alkyl substituents at the acyl carbon of ethyl esters yield very close values in rates. Consequently it is rather difficult to offer some conclusion concerning the effect of these substituents.     

Pyrolysis of hexachloroethane in the gas phase: Computer aided kinetic study

The pyrolysis of C₂Cl₆ has been studied between 652 and 735 K at pressures ranging from 19 to 50 torr. The observed total pressure- and Cl₂ pressure-time curves show S-shapes with an induction period depending on temperature and pressure. Further, the total pressure goes through a maximum to finally reach a lower constant value. These curves are explained in terms of a recently proposed reaction model using a parameter optimization computer program. © 1996 John Wiley & Sons, Inc.     

The mechanisms of the homogeneus,unimolecular elimination kinetics of ethyl 4-chlorobutyrate and 4-chlorobutyric acid in the gas phase

Ethyl 4-chlorobutyrate, which is reexamined, pyrolyzes at 350–410°C to ethylene, butyrolactone, and HCl. Under the reaction conditions, the primary product 4-chlorobutyric acid is responsible for the formation of γ-butyrolactone and HCl. In seasoned vessels, and in the presence of a free-radical inhibitor, the ester elimination is homogeneous, unimolecular, and follows a first-order rate law. For initial pressures from 69–147 Torr, the rate is given by the following Arrhenius expression: log k₁(s^?1) = (12.21 ± 0.26) ? (197.6 ± 3.3) kJ mol^?1 (2.303RT)^?1. The rates and product formation differ from the previous work on the chloroester pyrolysis. 4-Chlorobutyric acid, an intermediate product of the above substrate, was also pyrolyzed at 279–330°C with initial pressure within the range of 78–187 Torr. This reaction, which yields γ-butyrolactone and HCl, is also homogeneous, unimolecular, and obeys a first-order rate law. The rate coefficient, is given by the following Arrhenius equation: log k₁(s^?1) = (12.28 ± 0.41) ? (172.0 ± 4.6) kJ mol^?1 (2.303RT)^?1. The pyrolysis of ethyl chlorobutyrate proceeds by the normal mechanism of ester elimination. However, the intermediate 4-chlorobutyric acid was found to yield butyrolactone through anchimeric assistance of the COOH group and by an intimate ion pair-type of mechanism. Additional evidence of cyclic product and neighboring group participation is described and presented.     

The unimolecular elimination kinetics of benzaldoxime in the gas phase

The kinetics of the gas‐phase elimination of benzaldoxime was determined in a static reaction system over the temperature and pressure range 350°C–400°C and 56–140 Torr, respectively. The products obtained were benzonitrile and water. The reaction was found to be homogeneous, unimolecular, and tend to obey a first‐order rate law. The observed rate coefficient is represented by the following Arrhenius equation: According to kinetic and thermodynamic parameters, the reaction proceeds through a concerted, semi‐polar, four‐membered cyclic transition state type of mechanism. © 2007 Wiley Periodicals, Inc. 39: 145–147, 2007     

Reaction of recoil tritium atoms with ethyl alcohol in the gas phase

Reaction of recoil tritium atoms with ethyl alcohol in the gas phase has been studied in the presence of moderator and scavenger. The total amount of tritium produced from³He (n, p) T reaction under given irradiation conditions is determined by adding methane as a monitor for each set of sample. The HT, CH₃T, C₂H₅T and C₂H₄TOH yields were due to the decrease of hot reaction with increasing moderator pressure. On the other hand, the C₂H₃T yield, due to the unimolecular reaction of excited CH₄TOH^* or C₂H₅T^* moleculas, decreased with increasing pressure. All tritiated compounds were analyzed by radio gas chromatography.     

Master equation methods in gas phase chemical kinetics

In this article, we discuss the application of master equation methods to problems in gas phase chemical kinetics. The focus is on reactions that take place over multiple, interconnected potential wells and on the dissociation of weakly bound free radicals. These problems are of paramount importance in combustion chemistry. To illustrate specific points, we draw on our experience with reactions we have studied previously.     

Pyrolysis kinetics of polypropylene

The pyrolysis of oil shale and plastic wastes is being presently considered as an alternative means of partial substitution of fossil fuels to generate the necessary energy to supply the increasing energy demand and as well as new technology to reduce the negative environment of plastic wastes. However, Knowledge of pyrolysis kinetics is of great imponrtance for the design and simulation of the reactor and in order to establish the optimum process conditions. In this study, the thermal decomposition of polypropylene, oil shale and their mixture was studied by TG under a nitrogen atmosphere. Experiments were carried out for various heating rates (2, 10, 20, 50 K min⁻¹) in the temperature range 300–1273 K. The values of the obtained activation energies are 207 kJ mol⁻¹ for polyethylene, 57 kJ mol⁻¹ for the organic matter contained in the oil shale and 174 kJ mol⁻¹ for the mixture. The results indicate that the decomposition of these materials depends on the heating rate, and that polypropylene acts as catalyst in the degradation of the oil shale in the mixture.     

The elimination kinetics of secondary Alkyl Bromides in the gas phase

Elimination kinetics of 2-bromohexane and 2-bromo-4-methylpentane in the gas phase were examined over the temperature range of 310–360°C and pressure range of 46–213 torr. The reactionsin seasoned, static reaction vessels, and in the presence of the free radical inhibitor cyclohexene, are homogeneous, unimolecular, and follow first order rate laws. The overall rate coefficients are described by the following Arrhenius equations: For 2-bromohexane, log??₁(s^?1) = (13.08 ± 0.70) ? (185.7 ± 8.2) kJ mol^?1 (2.303RT)^?1; for 2-bromo-4-methylpentane, log??₁(s^?1) = (13.08 ± 0.33) ? (183.4 ± 3.8) kJ mol^?1 (2.303RT)^?1. The electron releasing effect of alkyl groups influences the overall elimination rates. The olefin products isomerize in the presence of HBr gas until an equilibrium mixture is reached.     

Ab initio kinetics of gas phase decomposition reactions

The thermal and kinetic aspects of gas phase decomposition reactions can be extremely complex due to a large number of parameters, a variety of possible intermediates, and an overlap in thermal decomposition traces. The experimental determination of the activation energies is particularly difficult when several possible reaction pathways coexist in the thermal decomposition. Ab initio calculations intended to provide an interpretation of the experiment are often of little help if they produce only the activation barriers and ignore the kinetics of the decomposition process. To overcome this ambiguity, a theoretical study of a complete picture of gas phase thermo-decomposition, including reaction energies, activation barriers, and reaction rates, is illustrated with the example of the β-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) molecule by means of quantum-chemical calculations. We study three types of major decomposition reactions characteristic of nitramines: the HONO elimination, the NONO rearrangement, and the N-NO(2) homolysis. The reaction rates were determined using the conventional transition state theory for the HONO and NONO decompositions and the variational transition state theory for the N-NO(2) homolysis. Our calculations show that the HMX decomposition process is more complex than it was previously believed to be and is defined by a combination of reactions at any given temperature. At all temperatures, the direct N-NO(2) homolysis prevails with the activation barrier at 38.1 kcal/mol. The nitro-nitrite isomerization and the HONO elimination, with the activation barriers at 46.3 and 39.4 kcal/mol, respectively, are slow reactions at all temperatures. The obtained conclusions provide a consistent interpretation for the reported experimental data.     

DFT Study of substituent effects of 2‐substituted alkyl ethyl methylcarbonates in homogeneous,unimolecular gas phase elimination kinetics

Theoretical studies on the gas phase elimination of 2‐substituted alkyl ethyl methylcarbonates were performed at the B3LYP/6‐31G* and B3LYP/6‐31+G** level of theory. The results of these calculations provide additional evidence that the mechanism of carbonates with a C_β H bond proceeds through a concerted nonsynchronous six‐membered cyclic transition state to produce methylcarbonic acid and the corresponding olefin. The unstable intermediate, methylcarbonic acid, rapidly decomposes through a four‐membered cyclic transition state to methanol and carbon dioxide. The correlation of the logarithm of theoretical rate coefficients against Hancock's steric parameters E gave an approximate straight line (δ = 0.30, r = 0.996 at 400°C). An additional fact is that when experimental log k_re.l is plotted against the theoretical log k_re.l. for 2‐alkyl ethyl methylcarbonates an approximate straight line (r = 0.997 at 400°C) is obtained. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 184–193, 2006     

Elimination kinetics of tertiary alcohols catalyzed by HBr in the gas phase

The homogeneous, molecular, gas phase elimination kinetics of several tertiary alcohols in seasoned, static reaction vessels, catalyzed by hydrogen bromide at temperatures of 320–392 °C and pressures of 40–174 torr are described. The steric factor appears to be responsible for rate enhancement in the dehydration process. A six-membered cyclic transition state appears to be a reasonable explanation for the mechanism of these reactions.

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, 320–392°C 40–174 . . .

     

The pyrolysis kinetics of 4-chloro-2-butanone in the gas phase

The rates of pyrolysis of 4-chloro-2-butanone in the gas phase have been determined in a static system seasoned with the products of decomposition of allyl bromide. The reaction is catalyzed by hydrogen chloride. Under maximum catalysis of HCl, the kinetics were found to be of order 1.5 in the substrate suggesting that a complex elimination is involved. The reaction, when maximally inhibited with propene, appears to undergo a unimolecular elimination and follows a first-order law kinetics. The products are methylvinyl ketone and hydrogen chloride. The kinetics have been measured over the temperature range of 402.0–424.4°C.The rate coefficients are given by the Arrhenius equation \documentclass{article}\pagestyle{empty}\begin{document}$ \log k_1 (\sec ^{ - 1}) = (13.67 \pm 0.69) - (225.2 \pm 8.6)\,{\rm kj}/{\rm mol}/2.303RT\angle $\end{document}. Thepyrolysis of 4-chloro-2-butanone is 31 times greater in rate than that of ethyl chloride at 440°C. This large difference in rate may be attributed to the -M effect of the acetyl substituent in the pyrolysis of the former halo compound.     

The kinetics of the gas phase decomposition reactions of the hexafluoro-t-butoxy radical

Hexafluoro-t-butoxy radicals have been generated by reacting fluorine with hexafluoro-2-methyl isopropanol: Over the temperature range of 406–600 K the hexafluoro-t-butoxy radical decomposes exclusively by loss of a CF₃ radical [reaction (-2)] rather than by loss of a CH₃ radical [reaction (-1)]: (1) The limits of detectability of the product CF₃COCF₃, by gas-chromatographic analysis, place a lower limit on the ratio k_?2/k_-1 of ～80. The implications of this finding in relation to the reverse radical addition reactions to the carbonyl group are briefly discussed. A thermochemical kinetic calculation reveals a discrepancy in the kinetics and thermodynamics of the decomposition and formation reactions of the related t-butoxy radical:      

The mechanism in the elimination kinetics of 2-chloropropionic acid in the gas phase

The elimination kinetics of 2-chloropropionic acid have been studied over the temperature range of 320–370.2°C and pressure range of 79–218.5 torr. The reaction in seasoned vessel and in the presence of the free radical suppressor cyclohexene, is homogeneous, unimolecular, and obeys a first-order rate law. The dehydrochlorination products are acetaldehyde and carbon monoxide. The rate coefficient is expressed by the following Arrhenius equation: log k₁(s^?1) = (12.53 ± 0.43) – (186.9 ± 5.1) kJ mol^?1 (2.303RT)^?1. The hydrogen atom of the carboxylic COOH appears to assist readily the leaving chloride ion in the transition state, suggesting an intimate ion pair mechanism operating in this reaction.