Molecular mechanisms in the pyrolysis of unsaturated chlorinated hydrocarbons: Formation of benzene rings. 2. Experimental and kinetic modeling studies

摘要

The mechanism of formation of benzene rings during the pyrolysis of dichloro- and trichloroethylenes has been investigated by the method of laser powered homogeneous pyrolysis coupled with product analysis by gas chromatography. Additionally, selected (co)pyrolyses between the chlorinated ethylenes, CH_2Cl_2, C_4Cl_4, C _4Cl_6, and C_2H_2 have been performed to explicitly probe the roles of 2C_3 and C_4/C_2 reaction pairs in aromatic growth. The presence of odd-carbon products in neat C_4Cl_6 pyrolyses indicates that 2C_3 processes are operative in these systems; however, comparison with product yields from C_2HCl_3 suggests that C_4/C_2 processes dominate most other systems. This is further evidenced by an absence of C _3 and other odd-carbon species in (co)pyrolyses with dichloromethane which should seed C_3-based growth. The reactions of perchlorinated C_4 species C_4Cl_5, C_4Cl_3, and C_4Cl_4 with C_2Cl_2 were subsequently explored through extensive kinetic simulations of the possible reaction pathways based on previous kinetic models and the exhaustive quantum chemical investigations of our preceding work. The experimental and theoretical results strongly suggest that, at moderate temperatures, aromatic ring formation from chlorinated ethylenes normally follows a Diels-Alder coupling of C _4 and C_2 molecular units followed by internal shifts; the one exception is the C_4Cl_4 + C_2Cl_2 system, where steric factors lead to the formation of nonaromatic products. There is little evidence for radical-based routes in these systems.