Radical Chemistry in the Thermal Decomposition of Anisole and Deuterated Anisoles: An Investigation of Aromatic Growth

The pyrolyses of anisole (C₆H₅OCH₃), d₃-anisole (C₆H₅OCD₃), and d ₈-anisole (C₆D₅OCD₃) have been studied using a hyperthermal tubular reactor and photoionization reflectron time-of-flight mass spectrometer. Gas exiting the reactor is subject to an immediate supersonic expansion after a residence time of approximately 65 s. This allows the detection of highly reactive radical intermediates. Our results confirm that the first steps in the thermal decomposition of anisole are the loss of a methyl group to form phenoxy radical, followed by ejection of a CO to form cyclopentadienyl radical (c-C₅H₅); C ₆H₅OCH₃ → C₆H₅O + CH₃; C₆H₅O → c-C₅H₅ + CO. At high temperatures (T_wall = 1200 °C 1300 °C) the c-C₅H₅ decomposes to propargyl radical (CH₂CCH) and acetylene; c-C₅H₅ → CH₂CCH + C ₂H₂. The formation of benzene and naphthalene is demonstrated with 1 + 1 resonance-enhanced multiphoton ionization. Propargyl radical recombination is a significant benzene formation channel. However, we show the majority of benzene is formed by a ring expansion reaction of methylcyclopentadiene (C₅H₅CH₃) resulting from methyl radical addition to cyclopentadienyl radical; CH₃ + c-C ₅H₅ → C₅H₅CH₃ → C₆H₆ + 2H. The naphthalene is generated from cyclopentadienyl radical recombination; 2c-C₅H₅ → C₅H₅C₅H₅ → C₁₀H ₈ + 2H. The respective intermediate amu 79 and 129 species associated with these reactions are detected, confirming the stepwise nature of the decompositions. These reactions are verified by pyrolysis studies of cyclopentadiene (C₅H₆) and C₅H ₅CH₃ obtained from rapid thermal dissociation of the respective dimer compounds, as well as pyrolysis studies of propargyl bromide (BrCH₂CCH).