The products of the thermal decomposition of CH₃CHO

We have used a heated 2 cm × 1 mm SiC microtubular (μtubular) reactor to decompose acetaldehyde: CH₃CHO → products. Thermal decomposition is followed at pressures of 75-150 Torr and at temperatures up to 1675 K, conditions that correspond to residence times of roughly 50-100 μs in the μtubular reactor. The acetaldehyde decomposition products are identified by two independent techniques: vacuum ultraviolet photoionization mass spectroscopy (PIMS) and infrared (IR) absorption spectroscopy after isolation in a cryogenic matrix. Besides CH₃CHO, we have studied three isotopologues, CH₃CDO, CD₃CHO, and CD₃CDO. We have identified the thermal decomposition products CH₃ (PIMS), CO (IR, PIMS), H (PIMS), H₂ (PIMS), CH₂CO (IR, PIMS), CH ₂CHOH (IR, PIMS), H₂O (IR, PIMS), and HC≡CH (IR, PIMS). Plausible evidence has been found to support the idea that there are at least three different thermal decomposition pathways for CH₃CHO; namely, radical decomposition: CH₃CHO + Δ → CH₃ + [HCO] → CH₃ + H CO; elimination: CH₃CHO + Δ → H₂=CH₂=CO; isomerizationelimination: CH ₃CHO + Δ → CH₂=CH-OH → HC≡CH + H₂O. An interesting result is that both PIMS and IR spectroscopy show compelling evidence for the participation of vinylidene, CH₂=C:, as an intermediate in the decomposition of vinyl alcohol: CH₂=CH-OH + Δ → [CH₂=C:] + H₂O → HC≡CH + H ₂O.