Pyrolysis of 1,7-Octadiene and the Kinetic and Thermodynamic Stability of Allyl and 4-Pentenyl Radicals

Article abstract of DOI:10.1021/j100200a032

Dilute mixtures of 1,7-octadiene in argon have been decomposed in single-pulse shock tube experiments.The main reaction is the breaking of the carbon-carbon allylic bond leading to the formation of allyl and 4-pentenyl radicals.A minor pathway involving a retroene reaction yielded propene and 1,5-pentadiene.Most of the 4-pentenyl radical decomposes to form another allyl and ethylene.Contributions from a cyclization process resulting in the formation of cyclopentene have also been observed.Allyl radical decomposition rate constants have been measured.The following rate expressions have been determined: k(1,7-octadiene -> allyl + 4-pentenyl) = 1.2 +/- 0.8 * 10¹⁶ exp(-35700 +/- 400/T)s^-1, k(1,7-octadiene -> propene + 1,4-pentadiene) = (3 +/- 1.5) * 10¹² exp(-27900 +/- 250/T)s^-1, k(4-pentyl -> cyclopentene + H)/k(4-pentenyl -> allyl + ethylene) = 6.9 +/- 4) * 10^-3 exp(2118 +/- 500/T), and k(allyl -> allene + H) = 40 +/- 10 s^-1 at 1080 K.When compared with lower temperature literature data on hydrogen addition to allene to form allyl, the present results are consistent with a resonance energy of 51 +/- 4 kJ/mol for the allyl radical, a step-size down for collisional deactivation in argon of 500 cm^-1, and lead to the high-pressure rate expression k(allyl -> allene + H) = 1.5 * 10¹¹T^0.84 exp(-30053/T) s^-1 and k(H + allene -> allyl) = 1.2 * 10⁸T^0.69 exp(-1513/T) L mol^-1 s^-1 over the temperature range 350-1200 K.The uncertainty for the decomposition reaction is estimated to be a factor of 1.3 at 1100 K, but may increase to 1 order of magnitude at the lower temperatures.For the addition reaction, the estimated uncertainties are a factor of 1.5 at 350 K increasing to a factor of 2.5 at 1100 K.The disproportionation to combination ratio for allyl radical under high-temperature conditions is considerably less than 0.005.