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

The pyrolyses of the guaiacols or methoxyphenols (o-, m-, and p-HOC ₆H ₄OCH ₃) have been studied using a heated SiC microtubular (μ-tubular) reactor. The decomposition products are detected by both photoionization time-of-flight mass spectroscopy (PIMS) and matrix isolation infrared spectroscopy (IR). Gas exiting the heated SiC μ-tubular reactor is subject to a free expansion after a residence time of approximately 50 - 100 μs. The PIMS reveals that, for all three guaiacols, the initial decomposition step is loss of methyl radical: HOC ₆H ₄OCH ₃ → HOC ₆H ₄O + CH ₃. Decarbonylation of the HOC ₆H ₄O radical produces the hydroxycyclopentadienyl radical, C ₅H ₄OH. As the temperature of the μ-tubular reactor is raised to 1275 K, the C ₅H ₄OH radical loses a H atom to produce cyclopentadienone, C ₅H ₄≥O. Loss of CO from cyclopentadienone leads to the final products, acetylene and vinylacetylene: C ₅H ₄≥O → [CO + 2 HC≡CH] or [CO + HC≡C - CH≥CH ₂]. The formation of C ₅H ₄≥O, HCCH, and CH ₂CHCCH is confirmed with IR spectroscopy. In separate studies of the (1 + 1) resonance-enhanced multiphoton ionization (REMPI) spectra, we observe the presence of C ₆H ₅OH in the molecular beam: C ₆H ₅OH + λ _275.1nm → [C ₆H ₅OH A] + λ _275.1nm → C ₆H ₅OH ⁺. From the REMPI and PIMS signals and previous work on methoxybenzene, we suggest that phenol results from a radical/radical reaction: CH ₃ + C ₅H ₄OH → [CH ₃ - C ₅H ₄OH]* → C ₆H ₅OH + 2H.