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J. Phys. Chem. 1981, 85,2370-2303
Thermal Decomposition of Propyl-, Isobutyl-, and Neopentylbenzene David A. Robaugh, Barrie D. Barton,+ and Stephen E. Stein” Department of Chemistty, West Virginia University, Morgantown, West Virginia 26506 (Received: November 2 1, 1980; In Final Form: April 6, 198 1)
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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(CH& PhCH2. + i-C3H7 (2); PhCH2C(CH3), PhCH2- + t-C4Hg(3). The following high-pressure Arrhenius rate expressions for reactions 1and 2 were obtained from radical thermochemistry and rate meakz/s-’ = 1015.”.8/8. A rate expression for reaction surements (Olkcal mol-’ = 0.004577‘): kl/s-’ = 1015.3-69.6/0; 3 was obtained from rate measurements and an assumed reverse recombination rate constant ( k d ) of M-’ s-l: k3/s-’ = 1015.5-64.3/0. The latter rate expression is consistent with an enthalpy of formation for tert-butyl radicals of 9.8 f 2 kcal mol-’ at 298 K. These results are compared to results from carrier systems and shock tube experiments.
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Introduction At the present time there is considerable debate concerning both absolute and relative values of enthalpies of formation of simple alkyl radicals-l These uncertainties, coupled with general difficulties involved in accurate estimation of preexponential factors, prevent accurate estimation of homolysis rate constants for many reactions yielding simple alkyl radicals. To improve predictive ability for rates of these reactions, and to establish a set of thermochemical properties for alkyl radicals consistent with empirical bond dissociation rates, we have examined /3 C-C bond scission in a series of alkylbenzenes PhCH2R, where R = C2H6,i-C3H7,and t-C4H,. Studies of ethylbenzene dissociation (R = CH,) have been reported earAlthough “carrier” methods have yielded rate constants for bond homolysis in a wide variety of molecules, propylbenzene is the only molecule in the series studied here that has been previously studied by these method^.^,^ Kerr and co-workers4attempted to study isobutylbenzene dissociation by carrier methods, but they could not obtain reliable kinetic results. Tsang studied the kinetics of isobutylbenzene dissociation relative to cyclohexene dissociation by the comparative single-pulse shock tube met h ~ d .No ~ studies of neopentylbenzene dissociation have previously been reported. Experimental Section The experimental apparatus has been described in detail in a previous publication.2 Two separate single-aperture quartz reactors were used, one with an exit aperture area of 9.1 mm2 and a collision number of 1160 and the other with an exit aperture area of 1.1 mm2 and a collision number of 10060. Empirical unimolecular rate constants, kUni,were calculated by means of eq 1 where f is the
fraction of reactant decomposed and k, = 4.0(T/M)l12 for the 3-mm aperture reactor and k, = 0.55(T/M)1/2for the 1-mm aperture reactor. Reagents were obtained from the Chemical Samples Co. and thoroughly degassed until a constant vapor pressure was obtained. t Monsanto Research Corporation, Mound Facility, Miamisburg,
OH 45342
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Results and Discussion Product Analysis. The most thermodynamically favorable bond homolysis path for each molecule studied leads to the formation of a resonance-stabilized benzyl radical and an alkyl radical PhCH2R PhCH2. R* where R- = ethyl, isopropyl, or tert-butyl radicals. Reaction products were determined a t low ionization energies (12-16 eV) to minimize fragmentation of the parent molecular ions. For each reaction, product spectra were found to be independent of flow rate and reaction temperature. Presented below are the observed mass spectral peaks and corresponding product assignments. Reaction products from the pyrolysis of n-propylbenzene (reaction 1)produced the following mass spectral peaks: PhCH2CH2CH3 ---* PhCHy + C2H5 (1) mle 91,29, and 28. Signals a t mle 91 and 29 are assumed to correspond to benzyl and ethyl radicals, respectively, the expected products of the primary bond scission. The peak at mle 28, which was as intense as the signal at mle 29 a t 16 eV, may be attributed to the decomposition of ethyl radicals in the reactor to form ethylene. In VLPP studies, radical products with labile H atoms /3 to the radical center often lose an H atom prior to escape from the reactore6 In addition to the above peaks, three minor signals a t mle 92, 104, and 15 were observed that were
