| Abstrakt: |
New data on enthalpy and entropy contributions to the energy barrier of β-pinene thermal isomerization were obtained. The rate of β-pinene conversion is higher in supercritical EtOH ( P = 120 atm) than in the gas phase ( P ≤ 1 atm, without solvent, or for inert carrier gas N) at equal temperatures. The highest activation energy E of total β-pinene conversion is also observed in reactions in the supercritical (sc) condition. Activation parameters Δ H, Δ S, and Δ G depend strongly on the reaction pressure. Thus, at P ≤ 1 atm (gas-phase reaction) the values of Δ S are negative, while at sc conditions at P = 120 atm is positive. The linear dependences ln k − E and Δ S − Δ S indicate an isokinetic relation (IKR) and enthalpy-entropy compensation effect (EEC). The isokinetic temperature was calculated ( T = 605.5 ± 22.7 K). It was shown that elevation of temperature reduces the value of Δ G( T) upon sc thermolysis only, whereas in all gas-phase reactions Δ G( T) increases. At equal reaction temperatures, the greatest values of K( T) proved to be typical for thermolysis in sc-EtOH. We hypothesize that the rate of total β-pinene conversion increases dramatically due to a considerable shift in equilibrium toward higher concentrations of activated complex y. A detailed analysis of activation parameters shows that the IKR and EEC coincide, evidence of a common mechanism of β-pinene conversion observed under different reaction conditions, including thermolysis in sc-EtOH. [ABSTRACT FROM AUTHOR] |
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