Reactions of polycyclic alkylaromatics. 4. Hydrogenolysis mechanisms in 1-alkylpyrene pyrolysis

The pyrolyses of 1-methylpyrene and 1-ethylpyrene were conducted at 400, 425, and 450°C in constant-volume batch reactors for holding times up to 300 min. The major products from methylpyrene pyrolysis were pyrene and dimethylpyrene whereas the major products from ethylpyrene pyrolysis were pyrene and methylpyrene. These results show that the aryl-alkyl C-C bond in methyl-and ethylpyrene is susceptible to facile hydrogenolysis. The relative importance of different hydrogenolysis mechanisms was examined through the development of mechanistic models. The model for methylpyrene pyrolysis showed that reverse radical disproportionation played an important role in engendering hydrogenolysis directly and also indirectly through the generation of alkylhydropyrenyl radicals, which subsequently transferred hydrogen. Further hydrogenolysis occurred through a chain mechanism whereby alkylhydropyrenyl radicals, which were formed through methyl radical addition, transferred hydrogen. Mechanistic modeling of ethylpyrene pyrolysis revealed that reverse radical disproportionation was also important, not only in engendering hydrogenolysis and initiating a free-radical chain reaction, but also in generating alkylhydropyrenyl, α-ethylpyrene, and ethyl radicals that participated in radical hydrogen transfer steps. Hydrogen atoms played a minor role as hydrogenolysis agents in methylpyrene pyrolysis, but hydrogen atom ipso-substitution was a more competitive means for engendering hydrogenolysis in ethylpyrene pyrolysis.