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(6)H(4)OCH(3)) have been studied using a heated SiC microtubular (mu-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 mu-tubular reactor is subject to a free expansion after a residence time of approximately 50-100 its. The PIMS reveals that, for all three guaiacols, the initial decomposition step is loss of methyl radical: HOC(6)H(4)OCH(3) -> HOC(6)H(4)O + CH(3). Decarbonylation of the HOC(6)H(4)O radical produces the hydroxycyclopentadienyl radical, C(5)H(4)OH. As the temperature of the mu-tubular reactor is raised to 1275 K, the C(5)H(4)OH radical loses a H atom to produce cyclopentathenone, C(5)H(4)=O. Loss of CO from cyclopentadienone leads to the final products, acetylene and vinylacetylene: C(5)H(4)=O -> [CO + 2 HC equivalent to CEI] or [CO + HC--C CH equivalent to CH(2)].The formation of C(5)H(4)=O, HCCH, and CH(2)CHCCH is confirmed with IR spectroscopy. In separate studies of the (1 + 1) resonance-enhanced multiphoton ionization (REMPI) spectra, we observe the presence of C(6)H(5)OH in the molecular beam: C(6)H(5)OH + gimel(275.1) (nm) -> [C(6)H(5)OH (A) over bar]) + gimel(275.1nm) -> C(6)H(5)OH(+). From the REMPI and PIMS signals and previous work on methoxybenzene, we suggest that phenol results from a radical/radical reaction: CH(3) + C(5)H(4)OH -> [CH(3)-C(5)H(4)OH]* -> C(6)H(5)OH + 2H.