Transient radicals, persistent products: flash pyrolysis of 4-ethylguaiacol and eugenol at 1000 °C

The thermal decomposition of 4-ethylguaiacol and eugenol was probed in a low-pressure pyrolysis SiC microreactor at 1000 °C using imaging photoelectron photoion coincidence (iPEPICO) spectroscopy. Pyrolysis products were identified using photoion mass-selected threshold photoelectron spectroscopy (ms-TPES), supported by ionization energy calculations and Franck-Condon simulations. DFT calculations were conducted to elucidate detailed reaction mechanisms for the thermal degradation pathways, augmented with single-point CBS-QB3 composite method energy calculations and RRKM analysis to assess the competition between radical-initiated and closed-shell reaction channels. Both samples undergo homolytic O-CH₃ bond cleavage to substituted hydroxyphenoxyl radicals, which is calculated to be the kinetically preferred initial step compared to the closed-shell alternative pathways. Although methyl radicals are detected in the mass spectrum, the corresponding CH₃-loss products, m/z 137 for 4-ethylguaiacol and m/z 149 for eugenol, are elusive under the present conditions. This is consistent with the potential energy surface, which predicts rapid secondary chemistry. In 4-ethylguaiacol, the radical pathway accounts for a 122 Da product assigned to 2-hydroxy-p-quinone methide rather than 4-ethylphenol. In eugenol, an analogous pathway leads to a 120 Da product assigned to 2-hydroxy-6-vinylfulvene rather than 3-allylcyclopentadienone. Both pathways converge toward 94 Da products involving phenol, cyclohexadienone-related species, and ring-contracted intermediates. These results refine the radical pyrolysis mechanisms of alkylated guaiacols under low-pressure flash pyrolysis conditions and distinguish the observed downstream intermediates from plausible closed-shell alternatives.