Reactions of Polycyclic Alkylaromatics. 2. Pyrolysis of 1,3-Diarylpropanes

We pyrolyzed 2-(3-phenylpropyl)naphthalene (PPN) and l,3-bis(l-pyrene)propane (BPP) at temperatures between 315 and 450 °C. PPN pyrolysis followed [formula omited]-order kinetics with apparent Arrhenius parameters of [log A (M^-1/2 s^-1), E* (kcal/mol)] = [13.7 ± 0.1, 51.7 ± 2.9]. The primary pyrolysis pathway led to 2-methylnaphthalene plus styrene as one pair of major products and toluene plus 2-vinylnaphthalene as a second major product pair. Toluene and 2-methylnaphthalene were thermally stable, but both styrene and 2-vinylnaphthalene underwent rapid secondary decomposition. The selectivity of PPN to toluene plus 2-vinylnaphthalene was slightly greater than its selectivity to 2-methylnaphthalene plus styrene. This difference in selectivity was consistent with the difference in resonance stabilization energies of the benzyl and 2-methylnaphthyl radicals being about 0.8 kcal/mol. The kinetics for PPN disappearance were reliably predicted by using rate constants previously determined for 1,3-diphenylpropane pyrolysis but adjusted to reflect the relevant difference in resonance stabilization energies. BPP pyrolysis followed [formula omited]-order kinetics, and the apparent Arrhenius parameters were [log A (M^-1/2 s^-1), E* (kcal/mol)] = [10.5 ± 0.8, 40 ± 6]. The reaction network for BPP included parallel primary pathways, with one pathway being much more important. The major pathway, which was completely analogous to that for PPN, led to vinylpyrene plus methylpyrene, whereas a minor primary pathway led to the production of pyrene via hydrogenolysis of the aryl-alkyl C-C bond. Although pyrene was only a minor primary product, high yields (36%) were obtained at complete BPP conversion. These high yields were produced via secondary pathways that involved dealkylation of alkylpyrenes formed in primary reactions. Pyrolyses of BPP in the presence of 1,6-dimethylnaphthalene, a hydrogen-transfer reporter molecule, implicated selective hydrogenolysis mechanisms as likely candidates for the aryl-alkyl bond cleavage.