Unimolecular thermal decarbonylation of vanillin stifled by the bimolecular reactivity of methyl-loss intermediate

We studied the thermal decomposition of vanillin in a low-pressure pyrolysis microreactor by photoelectron photoion coincidence spectroscopy. The products were identified isomer-selectively mass-selected threshold spectra (ms-TPES) based on Franck–Condon simulations and reference spectra. Methyl loss homolytic C–O bond fission methoxy group is initial unimolecular step. primary intermediate, 2-hydroxy-5-formyl-phenoxy radical (m/z 137), may undergo multiple decarbonylation steps lose hydrogen atom sequentially to finally form but-1-ene-3-yne at m/z 52. ms-TPES calculated reaction path energetics product intermediates masses 109, 108, 81, 80 amu. 3,4-Dihydroxybenzaldehyde 138) 3,4-dioxocyclohexa-1,5-diene-1-carbaldehyde 136) peaks do not correspond products. Instead, they are formed hydrogen-transfer self-reaction methyl-loss fragment 137. thermally more stable than vanillin, as also shown its high activation energy 137, 110 82 fragments. lighter 136 bimolecular prone CO merges with H-loss-containing channel vanillin. Further reactions discussed. These mechanistic insights, especially (self-)reactivity intermediate resulting preponderance chemistry leading inert dihydroxybenzaldehyde will guide process design involving vanillin-like feedstocks.