Gas-Phase Thermolysis of 1,5-Hexadiene: The Continuous Wave CO2 Laser-Induced Reaction and Studies of the Cyclization Pathway

Article abstract of DOI:10.1021/ja00297a002

The continuous wave CO2 laser-induced chemistry of 1,5-hexadiene has been studied at low pressures and at laser powers between 40 W/cm², the threshold for reaction at 40 torr, and 125 W/cm².Irradiation of a 40-torr sample at 100 W/cm² to 85percent conversion gave methane (4.4percent), ethene (14percent), ethyne (7percent), propene (46percent), 1,2-propadiene (2.1percent), 1-butene (8.1percent), 1,3-butadiene (6.6percent), 1,3-cyclopentadiene (2.7), and benzene (5.9) along with several minor and trace products.The hexadienyl radical 1 is a likely cyclization intermediate.However, 4-methylcyclopentene, the major product expected from cyclization of 1 and hydrogen abstraction, is present to the extent of less than 0.1percent, suggesting that this radical cyclization pathway is either unimportant or the cyclopentenylmethyl radical undergoes rapid reversion to 1.Radical 1 has been independently prepared in the gas phase and in solution, and products of its reactions have been identified.In the gas phase, 1 appears to principally undergo hydrogen atom loss giving (E)- and (Z)-1,3,5-hexatriene which, under the conditions employed, give 1,3-cyclohexadiene.In solution 1 gives 1,4- and 1,5-hexadiene (60.6percent) and 4-methylcyclopentene (37.3percent), but no cyclohexene.The mechanism of formation of cyclic products upon pyrolysis of 1,5-hexadiene are discussed in light of the results obtained.Factors accounting for the observed differences between solution and gas-phase products are also discussed.