Pathways, kinetics, and mechanisms for 2-dodecyl-9,10-dihydrophenanthrene pyrolysis

We pyrolyzed 2-dodecyl-9,10-dihydrophenanthrene (DDPh) in batch microreactors. The reaction conditions included neat pyrolyses between 375-450 °C for times of 15-240 min and also pyrolyses in benzene at 400 °C and 90 min but at different initial DDPh concentrations ranging from 0.0095 to 0.238 mol/L. The disappearance of DDPh followed first-order kinetics, and the global first-order rate constant had Arrhenius parameters of (log₁₀ A (s^-1) = 13.6 ± 2.8 and E (kcal/mol) = 54.5 ± 9.1, where the uncertainties are the 95% confidence intervals. The decomposition of DDPh can be described by a reaction network that possesses four parallel primary pathways. The major primary path, which involves dehydrogenation, leads to 2-dodecylphenanthrene. The other three primary paths involve C-C bond cleavage, and they lead to 2-methyl-9,10-dihydrophenanthrene plus undecene, to 2-vinyl-9,10-dihydrophenanthrene plus decane, and to numerous minor products. Important secondary and tertiary reactions include the rapid reduction of 2-vinyl-9,10-dihydrophenanthrene to 2-ethyl-9,10-dihydrophenanthrene and the facile dehydrogenation of 2-methyl- and 2-ethyl-9,10-dihydrophenanthrene to form 2-methyl- and 2-ethylphenanthrene, respectively. The identities and relative abundances of the major products are consistent with a free-radical chain reaction mechanism for DDPh pyrolysis.