No abstract

Rate constants for the thermal isomerizations of vinylcyclopropane to cyclopentene have been measured over a wide temperature range, 577-1054 K, in a static reactor and a single-pulse shock tube; they are well represented by log(A, s -1 ) ) 14.3 and E a ) 51.7 kcal/mol. This activation energy is higher than two previously reported values by some 2 kcal/mol; rate constants at high temperatures are about a factor of 2 larger than calculated from the Arrhenius parameters derived from the earlier low-temperature data. The thermal decomposition and isomerization reactions of vinylcyclobutane to give ethene plus 1,3-butadiene and cyclohexene have also been followed in shock-tube kinetic studies at 839-965 K. Combining the new rate constants with those from two lower-temperature studies gives the following: for the total consumption of vinylcyclobutane, log(A, s -1 ) ) 14.5 and E a ) 49.3 kcal/mol; for production of ethene and butadiene, log(A, s -1 ) ) 14.5 and E a ) 49.8 kcal/mol; and for isomerization to cyclohexene, log(A, s -1 ) ) 13.4 and E a ) 47.5 kcal/mol. These values are close to previously reported Arrhenius parameters based on lower temperature static-reactor kinetic investigations. The diradical transition structure for the vinylcyclobutane to cyclohexene isomerization appears to be strain free, while the transition structure for the vinylcyclopropane to cyclopentene conversion retains some 4.6 ( 0.9 kcal/mol of ring strain and torsional strain energy.

. Can. J. Chem. 57, 1324Chem. 57, (1979. Gas samples containing either 0.20% or 1.0% cyclopentane plus 0.25% tert-butyl alcohol in argon have been heated to 1185-1257 K in a single pulse shock tube. The previously studied dehydration of the alcohol was used as an internal standard reaction for temperature measurement. Pyrolysis of the cyclopentane yielded a broad spectrum of Cl-C, hydrocarbon products; however, the products and their concentration ratios were in good agreement with those recently reported by Tsang, in a comparative rate study of cyclopentane pyrolysis vs. cyclohexene decyclization. Thus strong support is offered for Tsang's proposed complex mechanism involving (a) molecular isomerization to 1-pentene and subsequent degradation of that product, (b) molecular decomposition to cyclopropane and ethene, followed by isomerization of cyclopropane to propene, and (c) induced decomposition to a variety of C1-C5 hydrocarbons plus H2 via hydrogen atom abstraction from cyclopentane by various radicals generated from 1-pentene. The only significant disagreement with Tsang's study is a -40" temperature discrepancy: possible sources of this are discussed. [Traduit par le journal]

No abstract