Mechanism of the I2-Catalyzed Decompositions of Acetaldehyde and Propionaldehyde in the Vapor Phase

The kinetic data of Rollefson and Faull for the iodine-catalyzed decomposition of acetaldehyde are summarized, and the RF mechanism of the reaction is critically reviewed. A new mechanism for the iodine-catalysis reaction is presented. Properties predicted from the new mechanism are compared with the observed reaction rates, reaction orders, and the unusual experimental behavior of the reactive system. Excellent agreement is found. The reaction is initiated by an iodine atom attack on acetaldehyde to abstract a hydrogen atom and produce hydrogen iodide and an acetyl radical. The rate-determining step for the reaction is the decomposition of the acetyl radical, which is a unimolecular pressure-dependent decomposition. The characteristic falloff of the rate of decomposition in the later stages of reaction is shown to be the result of this pressure dependence. The mechanism is also applied to the iodine-catalyzed decomposition of propionaldehyde. Satisfactory agreement with observed reaction orders and reaction rates is obtained. From these data the decomposition kinetics of the propionyl radical is predicted to be pressure-dependent at normal experimental pressures.