Arrhenius parameters for pyrolysis

TABLE 1. Comparative rates and Arrhenius parameters for pyrolysis of alkyl fluorides at 450 °C... 

TABLE 17. Arrhenius parameters for pyrolysis of 2-phenyl and 3-phenylalkyl bromides at 420 °C... 

The homogeneous thermal decomposition of HC1 has only been studied in shock tubes. Fishbume51 investigated the shock pyrolysis of HC1 diluted with Ar in the temperature region 3300-5400 °K and obtained Arrhenius parameters for... 

The kinetics of the pyrolysis of CHC13 was determined behind incident and reflected shock waves at 1050-1600 K92. This decomposition was reported to be unimolecular and the process is an a,a-elimination CHC13 - CC12 + HC1. The Arrhenius parameters for this process are log A - 14.3, E = 228 kJmoT1. 

An IR laser pyrolysis method was developed for the determination of Arrhenius parameters for unimolecular elimination of chloroalkanes97. This technique was reported to be a clean, efficient method for the measurement of high-temperature homogeneous gas-phase reaction rates. A pulse of C02 laser radiation was used to excite multiphoton SiF4... 

TABLE 16. Relative rates and Arrhenius parameters for co-phenylalkyl chloride pyrolysis at 440 °C... 

It is extremely difficult from direct studies of hydrocarbon oxidation to unravel the contributions made to the overall mechanism at these temperatures by the additive, abstractive and pyrolysis routes. Baldwin, Walker and their co-workers have recently developed a new technique, however, which has overcome this problem to some extent and has also yielded quantitative Arrhenius parameters for many elementary reactions [108,157—161]. This technique utilizes the fact that the kinetics of slowly reacting hydrogen and oxygen mixtures in aged boric acid coated... 

Significant contribution towards elucidation of the overall mechanism comes from investigations of the pyrolysis on ethyl nitrite " . Arrhenius parameters for the pyrolysis of some alkyl nitrites are shown in Table In the... 

The main reaction occuring in the gas-phase pyrolysis of diallyldimethylsilane 91 is the retro-ene elimination of propene with formation of 1-silabuta-l,3-diene 81 followed by a ring closure reaction to yield the l-silacyclobut-2-ene 80 (equation 20In the presence of methanol the adduct of 81 was observed. The Arrhenius parameters for the retro-ene fragmentation of 91 (log A (s )... 

H. K. Moffat, K. F. Jensen, and R. W. Cart J. Phys. Chem., 96, 7695 (1992). Estimation of Arrhenius Parameters for the 1,1-Elimination of H2 from Si2H and the Role of Chemically Aaivated Disilane in Silane Pyrolysis. 

This fragmentation mode is not altered for silacyclobutanes bearing a vinyl group at the silicon17, as the same Arrhenius parameters are found for the decomposition of 1 and of 1 -methyl-1-vinylsilacyclobutane 3 (logA = 15.64 s 1, E = 62.6 kcalmol-1), in sharp contrast to the pyrolysis of cyclobutanes where a vinyl group accelerates the pyrolysis by a factor of nearly 60018. 2-Silabuta-l,3-diene 4 was produced in a laser-photosensitized (SFg) decomposition (LPD) of 1-methyl-1-vinylsilacyclobutane 3... 

The kinetics associated with the reactions shown in Figure 7 are summarized in Table n. Detailed mechanistic studies on the pyrolysis of alkylaromatics (12,13,15), alkylnaphthenes (14) and alkyltetralins (14) have allowed for the formulation of the Arrhenius parameters and stoichiometric coefficients shown. The kinetics for paraffin and olefin pyrolyses were extracted from the abundant literature data (16-18). Finally, the issue of kinetic interactions have been both theoretically and experimentally addressed (11,19). These interactions in general cause the reaction of the mixture to be different then the linear combination of the pure component rates. 

The velocity of the decomposition flame was first measured by Gerstein et al. [62] using upward flame propagation in a tube. The experimental value of 12.5 cm. sec , corrected to 1 atm and room temperature, was in reasonable agreement with values calculated from theories of flame propagation using the Arrhenius parameters obtained by Mueller and Walters [63] for the first-order pyrolysis of ethylene oxide at much lower temperatures. 

Support for the c/j-nature of the elimination reaction has come from the work of Barton et on the pyrolysis of menthyl chloride, and the results of this study have recently been confirmed by Bamkole > who also examined the pyrolysis of neomenthyl chloride. The product ratio of menthene-3 to methene-2 is 3 1 in the case of menthyl chloride and 1 6 in the case of neomenthyl chloride, thus demonstrating a preference for m-elimination in each case. These two decompositions do, however, have some unusual characteristics the Arrhenius parameters are considerably lower than those reported for other secondary chlorides, and the rate of elimination of hydrogen chloride from each compound is appreciably faster than from cyclohexyl chloride . (The relative rate of pyrolysis of menthyl chloride and cyclohexyl chloride at 300 °C is about fifty.)... 

Early kinetic experiments on the thermal decomposition of nitro compounds established that for the simplest derivative, nitromethane, the process was first order, but that the reaction was chemically complex owing to further reactions between the products and nitromethane. Cottrell et re-examined the nitromethane pyrolysis and reported values of = 53.2 kcal.mole" and log A = 13 for the Arrhenius parameters of the homogeneous decomposition a radical mechanism was proposed, initiated by C-N cleavage... 

Within experimental error, the activation energy is the same as that measured for the condensed phase pyrolysis of hexanitroethane" , 38.9 kcal.mole, also suggesting a simple rate determining rupture of a C-N bond in the latter reaction. Slightly smaller activation energies and pre-exponential factors have been report-ed for C(N02)4 and C2(N02)e decompositions (Table 30). Trinitroalkanes" also follow first-order kinetics and it appears that C-N cleavage is rate-determining. Table 30 lists the Arrhenius parameters of thermolysis. 

Table 1. Arrhenius activation parameters for gas phase pyrolysis of alkyl...

This report by Conlin raised a lot of questions in my mind, but these questions remained dormant until in a futile attempt to prepare a,a-silylenevinylene polymers we accidentally synthesized 1,3-di-methylene-l,3-disilacyclobutane 5. This serindipitious synthesis allowed us to establish a two-case generality for the isomerization of methylenesilacyclobutanes since gas-phase pyrolysis of 5 cleanly and solely produced methylenedisilacyclopentene 7 (Eq. 9). This isomerization was kinetically followed in a stirred-flow reactor to afford Arrhenius parameters that clearly revealed this to be a concerted reaction. (Well, maybe not that clearly since you wouldn t bet your life on a log A of 12.5, but there certainly aren t any 54 kcal/mol sigma bonds in 5.)... 

The remaining issue in the formulation of Arrhenius parameters concerns the size of combined activation energies and frequency factors that often appear in overall rate constants of mechanistic rate expressions. Since mechanistic constants are much more commonly encountered than elementary constants, we need to know if they too are subject to limits such as the ones described above. For example, in the pyrolysis of ethane (see Chapter 3) the mechanistic rate expression is ... 

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