Die Kinetik der thermischen Cyclisierung von 2,6‐Dimethyl‐1, cis‐3,5‐heptatrien in der Gasphase

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Abstract
The kinetics of the thermal cyclization of 2,6‐dimethyl‐1, cis‐3, 5‐heptatriene to 1, 5, 5‐trimethyl‐1, 3‐cyclohexadiene have been measured in the gas phase for temperatures ranging between 150.4 and 231.2°C. The observed reaction rates satisfy first order kinetics for pressures ranging between 2.5 and 25 torr. The reaction is homogeneous, as demonstrated by the insensitivity of the calculated rate constants towards a 13‐fold variation in the surface to volume ratio of the reaction vessels used. The least squares regression analysis of the observed rate constants yields (with standard errors) log kcycl. (s−1) = (10.50 ± 0.11) − (31.81 ± 0,24)/θ, where θ equals 4.58 × 10−3 T (°K).The trans isomer is stable under the reaction conditions used. The intramolecular reaction proceeds via a 6‐center cyclic transition state, involving a loss in entropy of ∼ 13.8 cal/°‐mole (based on the transition state formulation for unimolecular reactions, assuming a transmission coefficient of unity). When compared with estimated and with reported activation parameters for the cyclization of 1, cis‐3,5‐hexatriene, this amounts to an additional loss of ∼ 4.5 e.u., and an increase in Ea by ∼ 2 kcal/mole. This is attributed to the additional steric repulsions in the transition state, primarily owing to the methyl group in 6‐position.

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