Experimental Observation of Hydrocarbon Growth by Resonance‐Stabilized Radical–Radical Chain Reaction

Rapid molecular‐weight growth of hydrocarbons occurs in flames, in industrial synthesis, and potentially in cold astrochemical environments. A variety of high‐ and low‐temperature chemical mechanisms have been proposed and confirmed, but more facile pathways may be needed to explain observations. We provide laboratory confirmation in a controlled pyrolysis environment of a recently proposed mechanism, radical–radical chain reactions of resonance‐stabilized species. The recombination reaction of phenyl (c‐C6H5) and benzyl (c‐C6H5CH2) radicals produces both diphenylmethane and diphenylmethyl radicals, the concentration of the latter increasing with rising temperature. A second phenyl addition to the product radical forms both triphenylmethane and triphenylmethyl radicals, confirming the propagation of radical–radical chain reactions under the experimental conditions of high temperature (1100–1600 K) and low pressure (ca. 3 kPa). Similar chain reactions may contribute to particle growth in flames, the interstellar medium, and industrial reactors. Radical–radical chain reactions may contribute to rapid growth of polycyclic aromatic hydrocarbons in combustion and astrochemical environments. Phenyl and benzyl radicals were experimentally observed to complete this chain reaction, producing diphenylmethyl and triphenylmethyl radicals by prompt H‐atom loss (see scheme). This observation confirms the plausibility of such chain reactions at high temperature and low pressure.