Thermal geometrical isomerization of 1,trans-3,trans-5-heptatriene and the concept of a biradical intermediate

Abstract

The gas-phase unimolecular isomerization of 1,trans-3,trans-5-heptatriene has been followed in the temperature range 247-332°C and at pressures ranging from 1.6 to 22 torr. Gas-liquid partition chromatography has been used to analyze the product mixture. 1,cis-3,trans-5-heptatriene is apparently the primary reaction product and undergoes fast cyclization to 5-methyl-1,3-cyclo-hexadiene which is isomerized further via intramolecular 1,5-hydrogen transfer, to 1- and 3-methyl-1,3-cyclohexadienes. The reaction is apparently homogeneous and clean under the reaction conditions used. The only observable side reaction contributing measurably to the overall conversion at lower temperatures and with higher pressures is the Diels-Alder addition reaction.

The observed rate constants satisfy the first-order rate law yielding for the high pressure limit the Arrhenius relationship (with standard errors) : log k(s^–1)=(12.28 ± 0.19)–(42.37 ± 0.49)/θ where θ equals 4.58 × 10^–3T(K). These rate parameters are in agreement with those expected for a 3,4-biradical transition state with the planes of the two radical ends at 90° angle to each other. Compared with literature data on 2-olefins and taking the differences in π-bond-energies into account, the observed activation energy implies that the full amount of two “monoallylic” stabilization energies is developed in the biradical transition state. The results indicate that the geometrical isomerization about the 5,6-π-bond is kinetically not a competitive process.