The Synthesis of Methylacetylene by the Pyrolysis of Propylene. VI. The Pyrolysis of Allyl Iodide

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Abstract
Allyl iodide has been pyrolyzed in a flow system over a wide range of conditions (temperature, 800–1100°C; contact time, 1.44×10−4–14.0×10−4 sec; concentration, 1.8–6.5 mol%; pressure, atmospheric pressure) in order to find suitable conditions for producing methylacetylene and allene and in order to elucidate the reactions of the allyl radical with iodine or the iodine atom at high temperatures. In addition, a study of the mechanism of the pyrolysis of allyl iodide at high temperatures (800–1100°C) has been made. A total yield of allene and methylacetylene of 10 mol per 100 mol of allyl iodide pyrolyzed was obtained under suitable conditions. In the pyrolysis, little cleavage of the C–C bonds of allyl iodide occurred, and propylene, allene, methylacetylene,diallyl, benzene, and an unidentified product were found to be the main products. By means of the zero-conversion method, diallyl alone has been found to be the major product in the early stage of the pyrolysis, while propylene, allene, methylacetylene, and benzene have been found to be the chief products at higher conversions. On the basis of the observed results, a free-radical mechanism has been proposed for the main reactions. It has further been concluded that the pyrolysis is a radical decomposition initiated by the C3H5I → C3H5·+I reaction and that the overall mechanism in the early stage of the pyrolysis may be represented essentially by the 2C3H5I → C6H10+I2 reaction.