Unimolecular Thermal Decomposition of Dimethoxybenzenes

The unimolecular thermal decomposition mechanisms of o-, m-, and p-dimethoxybenzene (CH₃O-C₆H₄-OCH₃) have been studied using a high temperature, microtubular (μtubular) SiC reactor with a residence time of 100 μs. Product detection was carried out using single photon ionization (SPI, 10.487 eV) and resonance enhanced multiphoton ionization (REMPI) time-of-flight mass spectrometry and matrix infrared absorption spectroscopy from 400 K to 1600 K. The initial pyrolytic step for each isomer is methoxy bond homolysis to eliminate methyl radical. Subsequent thermolysis is unique for each isomer. In the case of o-CH ₃O-C₆H₄-OCH₃, intramolecular H-transfer dominates leading to the formation of o-hydroxybenzaldehyde (o-HO-C₆H₄-CHO) and phenol (C₆H₅OH). Para-CH₃O-C₆H₄-OCH₃ immediately breaks the second methoxy bond to form p-benzoquinone, which decomposes further to cyclopentadienone (C₅H₄=O). Finally, the m-CH ₃O-C₆H₄-OCH₃ isomer will predominantly follow a ring-reduction/CO-elimination mechanism to form C ₅H₄=O. Electronic structure calculations and transition state theory are used to confirm mechanisms and comment on kinetics. Implications for lignin pyrolysis are discussed.