Molecular mechanisms in the pyrolysis of unsaturated chlorinated hydrocarbons: Formation of benzene rings. 1. quantum chemical studies

摘要

Analogues of important aromatic growth mechanisms in hydrocarbon pyrolysis and combustion systems are extended to chlorinated systems. We consider the addition of C_2Cl_2 to both C_4Cl_3 and C_4Cl_5 radicals at the M06-2X/6-311+G(3df,3p)//B3LYP/6- 31G(d) level of theory, and we demonstrate that these reaction systems have much in common with those of nonchlorinated species. In particular, we find that these radicals appear to lead preferentially to fulvenes, and not to the observed aromatic products, as is found in nonchlorinated systems. We have therefore also considered nonradical C_4/C_2 channels by way of Diels-Alder cyclization of C_4Cl_4/C_2Cl _2 and C_4H_2Cl_2/C2HCl pairs to describe aromatic formation. While the latter pair readily leads to the formation of partially chlorinated benzenes, the fully chlorinated congeners are sterically prohibited from ring closing directly; this leads to a series of novel rearrangement processes which predict the formation of hexachloro-1,5-diene-3-yne, in addition to hexachlorobenzene, in good agreement with experiment. This suggests, for the first time, that facile nonradical routes to aromatic formation are operative in partially and fully chlorinated pyrolysis and combustion systems.