Kinetics and mechanisms of gas-phase decarbonylation of [alpha]-methyl-trans-cinamaldehyde and E-2-methyl-2-pentenal under homogeneous catalysis of hydrogen chloride

The kinetics of the gas-phase elimination of [alpha]-methyl-trans-cinamaldehyde catalyzed by HCl in the temperature range of 399.0-438.7°C, and the pressure range of 38-165Torr is a homogeneous, molecular, pseudo first-order process and undergoing a parallel reaction to produce via (A) [alpha]-methylstyrene and CO gas and via (B) [beta]-methylstyrene and CO gas. The decomposition of substrate E-2-methyl-2-pentenal was performed in the temperature range of 370.0-410.0°C and the pressure range of 44-150Torr also undergoing a molecular, pseudo first-order reaction gives E-2-pentene and CO gas. These reactions were carried out in a static system seasoned reactions vessels and in the presence of toluene free radical inhibitor. The rate coefficients are given by the following Arrhenius expressions: Products formation from [alpha]-methyl-trans-cinamaldehyde [alpha]-methylstyrene: log k 1 ' s - 1 lmo l - 1 = 12.67 ± 0.02 - 183.3 ± 0.31 kJmo l - 1 2.303 RT - 1 [beta]-methylstyrene: log k 1 ' s - 1 lmo l - 1 = 13.19 ± 0.03 - 183.0 ± 0.45 kJmo l - 1 2.303 RT - 1 Products formation from E-2-methyl-2-pentenal E-2-pentene: log k 1 ' s - 1 lmo l - 1 = 12.79 ± 0.06 - 174.5 ± 0.80 kJmo l - 1 2.303 RT - 1 The kinetic and thermodynamic parameters for the thermal decomposition of [alpha]-methyl-trans-cinamaldehyde suggest that via (A) proceeds through a bicyclic transition state type of mechanism to yield [alpha]-methylstyrene and carbon monoxide, whereas via (B) through a five-membered cyclic transition state to give [beta]-methylstyrene and carbon monoxide. However, the elimination of E-2-methyl-2-pentenal occurs by way of a concerted cyclic five-membered transition state mechanism producing E-2-pentene and carbon monoxide. The present results support that uncatalyzed [alpha]-[beta]-unsaturated aldehydes decarbonylate through a three-membered cyclic transition state type of mechanism. Copyright © 2014 John Wiley & Sons, Ltd. [PUBLICATION ABSTRACT]