Thermal Decomposition Mechanisms of the Methoxyphenols: Formation of Phenol, Cyclopentadieneone, Vinylacetylene and Acetylene

The pyrolyses of the methoxyphenols (o-, m- and p-HO-C ₆H ₄-OCH ₃) have been studied using a hyperthermal μ-tubular reactor with both photoionization time-of-flight techniques and matrix isolation infrared spectroscopy. Gas exiting the reactor is subject to a free expansion after a residence time of approximately 65 μs. This allows the detection of highly reactive radicals and other intermediates. Our results show the initial steps in the pyrolyses of the methoxyphenols are analogous to those observed in anisole (C ₆H ₅OCH ₃). First, methyl radical is lost to form hydroxy-phenoxy radical followed by ejection of a CO to form hydroxy-cyclopentadienyl radical; HO-C ₆H ₄-OCH ₃ ® OH-C ₆H ₄-O + CH ₃; OH-C ₆H ₄-O ® c-C ₅H ₄-OH + CO. At high temperatures (Twall ≥ 1000 °C), the c-C ₅H ₄-OH loses the terminal hydrogen atom to form cyclopentadieneone (C ₅H ₄=O); c-C ₅H ₄-OH ® C ₅H ₄=O + H. The C ₅H ₄=O then loses a CO and either fragments to two acetylene molecules; C ₅H ₄=O ® 2 HCCH + CO or forms vinylacetylene CH ₂CHCCH; C ₅H ₄=O ® CH ₂CHCCH + CO. The formation of C ₅H ₄=O, HCCH and CH ₂CHCCH is confirmed with matrix FTIR. A recombination reaction involving methyl radical and c-C ₅H ₄-OH is observed resulting in CH ₃-C ₅H ₄-OH. This is followed by stepwise ejection of two hydrogen atoms to form phenol; CH ₃ + c-C ₅H ₄-OH ® CH ₃-C ₅H ₄-OH ® C ₆H ₅OH + 2H. Phenol formation is verified with 1+1 resonance-enhanced multiphoton ionization. This bimolecular reaction confirms the analogous reaction observed in anisole pyrolysis in which benzene is generated.