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

Abstract

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.

Gas-Phase Infrared Spectra of the C7H5 Radical and Its Bimolecular Reaction Products

“…Only recently, it was discussed that the bimolecular reaction of benzyl with phenyl can lead to a diphenylmethyl radical in a well skipping reaction that immediately forms fluorene by ring closure, associated with another H-loss. 46 However, given the intense fluorene mass signal, which is larger than in previous experiments on benzyl, 35 a direct route from 1 to fluorene could be expected. We propose that the reaction of both the allenic and the cyclopentadienyl site of 1 with either phenyl or benzene in ortho position forms fluorene efficiently.…”

“…The formation of diphenylmethane can easily be explained by the reaction of benzyl with a phenyl radical, according to () Formation of fluorene from diphenylmethane proceeds then by ring closure and loss of H 2 . Only recently, it was discussed that the bimolecular reaction of benzyl with phenyl can lead to a diphenylmethyl radical in a well skipping reaction that immediately forms fluorene by ring closure, associated with another H-loss . However, given the intense fluorene mass signal, which is larger than in previous experiments on benzyl, a direct route from 1 to fluorene could be expected.…”

Unconventional Pathway in the Gas‐Phase Synthesis of 9H‐Fluorene (C13H10) via the Radical–Radical Reaction of Benzyl (C7H7) with Phenyl (C6H5)

“…Hydrogen abstraction—if present at all—leads preferentially to the diphenylmethyl radical ( p3 , C 13 H 11 ). Note that a previous study on the reaction of the benzyl (C 7 H 7 ⋅) and phenyl (C 6 H 5 ⋅) radicals proposed the formation of diphenylmethane, but could not uniquely identify the structural isomer associated with signal at m / z =166.08 due to the lack of isomer specific detection schemes [16] …”

A unified reaction network on the formation of five-membered ringed polycyclic aromatic hydrocarbons (PAHs) and their role in ring expansion processes through radical–radical reactions

“…33 Compared to the [C3 + C10] systems [R5/R6], which involved entrance barriers from 7.0 to 14.0 kJ mol À1 , the [C4 + C9] systems [R7/ R8] passed submerged barriers to addition of the radical center to the vinyl moiety of the vinylacetylene reactant following the Hydrogen Abstraction -Vinylacetylene Addition (HAVA) reaction mechanism. 25,30,[34][35][36][37][38] [42][43][44] The present investigation reveals the molecular mass growth processes to 3H-cyclopenta[a]naphthalene (2), 1H-cyclopenta[b]naphthalene (6), and fluorene (8) (C 13 H 10 ) via reactions [R9] to [R11] in the gas phase at elevated temperatures of 973 and 1023 K simulating temperatures under combustion conditions along with circumstellar envelopes of carbon-rich stars and planetary nebulae. These studies exploit a chemical microreactor with products probed isomer-selectively through fragment-free photoionization utilizing tunable vacuum ultraviolet (VUV) light in tandem with the detection of the ionized molecules by a high resolution reflection time-of-flight mass spectrometer (Re-TOF-MS).…”

“…33 Compared to the [C3 + C10] systems [R5/R6], which involved entrance barriers from 7.0 to 14.0 kJ mol −1 , the [C4 + C9] systems [R7/R8] passed submerged barriers to addition of the radical center to the vinyl moiety of the vinylacetylene reactant following the Hydrogen Abstraction – Vinylacetylene Addition (HAVA) reaction mechanism. 25,30,34–38 These pathways eventually produced 3 H -cyclopenta[ a ]naphthalene ( 2 ), 1 H -cyclopenta[ b ]naphthalene ( 6 ), and 1 H -cyclopenta[ a ]naphthalene ( 7 ) in barrierless and exoergic reactions. 39 However, elementary steps involving [C2 + C11] systems are still elusive (Scheme 2).…”