Selective Formation of Indene through the Reaction of Benzyl Radicals with Acetylene

The combustion of fossil fuels forms polycyclic aromatic hydrocarbons (PAHs) composed of five-and six-membered aromatic rings, such as indene (C9H8), which are carcinogenic, mutagenic, and deleterious to the environment. Indene, the simplest PAH with single five-and six-membered rings, has been predicted theoretically to be formed through the reaction of benzyl radicals with acetylene. Benzyl radicals are found in significant concentrations in combustion flames, owing to their highly stable aromatic and resonantly stabilized free-radical character. We provide compelling experimental evidence that indene is synthesized through the reaction of the benzyl radical (C7H7) with acetylene (C2H2) under combustion-like conditions at 600 K. The mechanism involves an initial addition step followed by cyclization and aromatization through atomic hydrogen loss. This reaction was found to form the indene isomer exclusively, which, in conjunction with the high concentrations of benzyl and acetylene in combustion environments, indicates that this pathway is the predominant route to synthesize the prototypical five-and six-membered PAH.