Thermochemical kinetics of the retro-ene reactions of molecules with the general structure (allyl)XYH in the gas phase. 6. Concerted unimolecular decomposition of hepta-1,6-diene

Abstract

The kinetics of the thermal decomposition of hepta-1,6-diene (HD) have been studied in the gas phase over the temperature range 628-744 °K. Propylene and butadiene in essentially equimolar concentrations are the major reaction products, with 4-methylcyclohexene, cyclopentene, and ethylene as minor detectable side products at large conversions. First-order rate constants for the disappearance of HD, obtained by an internal standard technique, fit the Arrhenius relationship, log k, sec-1 = 11.31 ± 0.44 -(47.00 ± 0.73 kcal mol-1)/2.303R7. The reaction is homogeneous, and up to a 20-fold excess over HD of added but-l-ene, propylene, or toluene did not affect the rate constants. The entropy of activation derived from these data is AS* = -11.3, in excellent agreement with prediction. The extent of nonsynchronous bond breaking and the nature of the transition state involved in these reactions is discussed. The reaction is best described as a concerted "retro-ene" elimination of propylene.The observed activation parameters are in line with a cyclic six-center transition state and with the results obtained for similar reactions of compounds of the general structure (allyl)XYH, where X equals NR, O, CO, or CR2 and Y equals CR2 or O. An independent study of the kinetics of the thermal l decomposition of hepta-1,6-diene (HD) has not been reported to date. Ellis and Frey,3 when pyrolyzing bicyclo[3.2.0]heptane (BCH) observed a fast secondary reaction of the primary product HD, yielding propylene and butadiene in practically equimolar quantities.We have become interested in reaction 1 in connection with a series of investigations in this laboratory4•5 of the thermochemical kinetics involved in reactions of the general type (allyl)XYH ->-+ X==YBased on these studies it was to be expected, that n,-(n + 5)-diolefins should in principle be capable of undergoing a concerted 6-center "retro-ene" decomposition in analogy with allyl alkyl ethers,6 but-3-enoic acids,7 but-3-enols,8 and jV-allyl-jV-alkylamines.4 5 Furthermore, the absence of a heteroatom should yield interesting information regarding the nonsynchroneity involved in these reactions, i.e., the amount of charge separation in the six-center transition state. For

Kinetic study of the thermal rearrangement of cis- and trans-2-pinanol

“…For the thermal intramolecular retroene reaction (H) Smith and Yates [60] reported an activation energy of 170 kJ mol −1 for the retro-ene reaction of 4-hydroxy-4-methyl-1-pentene producing propylene and acetone. Egger et al [61] found similar values for other types of compounds that are able to undergo thermal retro-ene rearrangements. It was found that this activation energy resulted in retro-ene kinetics that underestimated yields of compound 9 and produced significant yields of compound 8.…”

“…Smith and Yates [60] reported a pre-exponential factor of 2.0 × 10 11 s −1 for the retro-ene reaction of 4-hydroxy-4-methyl-1-pentene producing propylene and acetone. Egger et al [61] found similar values for other types of compounds that are able to undergo retro-ene rearrangements. This value is adopted in the current kinetic models.…”

Very low‐pressure pyrolysis (VLPP) of hex‐1‐ene. Kinetics of the retro‐ene decomposition of a mono‐olefin

“…Unimolecular rate constants based on the disappearance of hex-1-ene and the formation of ethylene were obtained over the temperature range of 915-1153 K using all three reactor exit apertures. They were calculated from the mass-spectral peak intensities and calibration data in the usual way [12,15]. The rate constants for the retro-ene pathway were then obtained by difference.…”

An effort to generalize the thermal isomerization of 6,6-dimethylbicyclo[3.1.1]heptanes and 6,6-dimethylbicyclo[3.1.1]heptenes: Comparative pyrolysis of pinane, α-pinene, and β-pinene

“…4, 5 and 8). Thermolysis of similar 1,6-heptadienes yields fragmentation products exclusively [48]. The only condition for those cyclizations that has to be fulfilled, is the absence of an internal carbon-carbon double bond in conjugation to one of the others in 1-or 6-position (e.g.…”