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

The kinetics of the gas-phase elimination of α-methyl-trans- cinamaldehyde catalyzed by HCl in the temperature range of 399.0-438.7 °C, and the pressure range of 38-165 Torr is a homogeneous, molecular, pseudo first-order process and undergoing a parallel reaction to produce via (A) α-methylstyrene and CO gas and via (B) β-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 44-150 Torr 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 α-methyl-trans-cinamaldehyde α-methylstyrene: logk1′s-1lmol-1=12.67±0.02-183.3±0.31kJmol-12.303RT-1 β-methylstyrene: logk1′s-1lmol-1=13.19±0.03-183.0±0. 45kJmol-12.303RT-1 Products formation from E-2-methyl-2-pentenal E-2-pentene: logk1′s-1lmol-1=12.79±0.06-174.5±0.80kJmol-12.303RT-1 The kinetic and thermodynamic parameters for the thermal decomposition of α-methyl-trans-cinamaldehyde suggest that via (A) proceeds through a bicyclic transition state type of mechanism to yield α-methylstyrene and carbon monoxide, whereas via (B) through a five-membered cyclic transition state to give β-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 α-β-unsaturated aldehydes decarbonylate through a three-membered cyclic transition state type of mechanism.