description- – Reversible benzene ring contractions are suggested to be the common mechanistic feature of automerization reactions, skeleton transformations and interconversions of polycyclic aromatic hydrocarbons. Experiments conducted with $/sp[13]$C-labeled PAH bring new insights into the acid catalyzed automerizations of (1-$/sp[13]$C) biphenyl and (1-$/sp[13]$C) anthracene and deepens the understanding of the 1,2-switch of carbon atoms in the thermal automerizations of (1-$/sp[13]$C) anthracene, (6-$/sp[13]$C) benzo (c) phenanthrene and (1-$/sp[13]$C) and (4-$/sp[13]$C) phenanthrenes at high temperatures (900-1100$/sp/circ$C). Eight bay region PAH (chrysene, benz (a) anthracene, phenanthrene, picene, perylene, benzo (ghi) perylene, benzo (e) pyrene, benz (a,c) anthracene) were discovered to undergo an intriguing transformation when heated at 1100$/sp/circ$C in flash pyrolyses. The novel information extracted from the experimental data can be summarized as follow: loss of two hydrogens to increase the C/H ratio, the creation of five-membered rings from a purely benzenoid compound, an increase in the total number of rings, permanent destruction of a benzene ring and generation of a fullerene subunit. Tests of the proposed mechanistic pathways were achieved by independent generation of the proposed intermediates and by $/sp[13]$C-labeling experiments. Thermal formation of five-membered rings was used to synthesize a series of interesting new nonalternant hydrocarbons (cyclopenta (cd) fluoranthene, dicyclopenta (cd,fg), (cd,jk) and (cd,mn) pyrene, cyclopenta (cd) coronene and benzo (ghi) cyclopenta (cd) perylene, all of which are products that are formed in pyrolyses of other PAH and/or in ethylene flames. In the search for an authentic example of a Stone-Wales rearrangement i.e., benzo (d) pyracylene to cyclopenta (cd) fluoranthene, the first example of migration of an ethynyl group from one benzene ring to another along the edge of an anthracene ring was discovered. All the transformations presented could play a significant role in the chemistry of hydrocarbons in flames, in the mechanism of formation of carcinogens in smoke and in the production and/or interconversions of fullerenes.
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