Origin of Pyrene under High Temperature Conditions in the Gas Phase. The Pivotal Role of Phenanthrene

4-Ethynylphenanthrene (15), and the latent precursors for 2-ethynyl- (18) and 3-ethynylphenanthrene (19), viz., 2-(1-chloroethenyl)- (16) and 3-(1-chloroethenyl)phenanthrene (17), respectively, have been subjected to flash vacuum thermolysis (FVT). Whereas at 800 °C 15 is quantitatively converted into pyrene (1), 16 and 17 only give 18 and 19, respectively. Both 18 and 19 contain redundant ethynyl substituents, i.e., after ethynyl−ethylidene carbene equilibration neither five- nor six-membered ring formation can occur by carbene C−H insertion. At T ≥ 1000 °C 16 and 17 gave pyrolysates containing the same set of 11 (non)-alternant polycyclic aromatic hydrocarbons (PAH), albeit in a different ratio. The different product ratio suggests that redundant ethynyl substituents migrate along the phenanthrene periphery presumably via transient cyclobuta-PAH intermediates toward positions suitable for either five- or six-membered ring formation by carbene C−H insertion. The results provide an explanation for the ubiquitous formation of pyrene (1), acephenanthrylene (9), and fluoranthene (3) during (incomplete) combustion. Phenanthrene (2) appears to be a point of divergence in PAH growth by C2 addition.