The gas phase thermal decarbonylation of a,b-unsaturated aldehydes E-2-butenal and E-3-phenyl-2-methylpropenal was studied in a static system over the temperature range 380.5-490.0 8C and pressure range 55.5-150 Torr. The reactions are homogeneous and unimolecular and obey a first-order rate law. The rate coefficient is represented by the following Arrhenius equations:EÀ2ÀButenal : log k 1 ðs À1 Þ ¼ ð13:18 AE 0:16Þ À ð212:0 AE 2:0Þ kJ mol À1 ð2:303 RTÞ À1 EÀ3ÀphenylÀ2Àmethylpropenal : log k 1 ðs À1 Þ ¼ ð13:23 AE 0:22Þ À ð234:6 AE 3:2Þ kJ mol À1 ð2:303 RTÞ À1The elimination products of 2-butenal are propene and CO gas, while 3-phenyl-2-methylpropenal produces a-methylstyrene, cis-trans-b-methylstyrene, indan, and CO gas. Kinetic and thermodynamic parameters suggest these elimination reactions to proceed through a three-membered cyclic transition state type of mechanisms. However, a two steps mechanisms for the formation of a carbene type of intermediate through a four-membered cyclic transition structure can not be overlooked.
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