Mass‐Spectrometric Study of the Reaction of Atomic Hydrogen with Acetaldehyde

The reaction of atomic hydrogen with acetaldehyde has been studied with a discharge‐flow system coupled to a time‐of‐flight mass spectrometer. The results are consistent with the mechanism Primary step: $${\mathrm{CH}}_3\mathrm{CHO}+\mathrm{H}\to {\mathrm{CH}}_3\mathrm{CO}+{\mathrm{H}}_2;$$ Secondary steps: $${\mathrm{CH}}_3\mathrm{CO}+\mathrm{H}\to {\mathrm{CH}}_2\mathrm{CO}+{\mathrm{H}}_2,$$ $${\mathrm{CH}}_2\mathrm{CO}+\mathrm{H}\to {\mathrm{CH}}_3{\mathrm{CO}}^{*}\to {\mathrm{CH}}_3+\mathrm{CO},$$ and $${\mathrm{CH}}_3+\mathrm{H}\to {\displaystyle {lim}_{\mathrm{wall}}}{\mathrm{CH}}_4.$$ The rate constant for the abstraction of the aldehydic hydrogen in the primary step (1) for the isotopic modification CD₃CDO+D has been determined directly to be 3.2±0.5×10⁻¹⁴ cc molecule⁻¹·sec⁻¹ at 300°K. Under the conditions studied, the major reaction of the acetyl radical is its further reaction with hydrogen atoms to form ketene (6). The final carbon containing products, CO and CH₄, are formed from subsequent reactions of ketene with hydrogen atoms (10). The kinetic behavior of the products, CH₂CO and CH₄, confirms the role of ketene as a major intermediate species in this system. Isotopic studies further confirm that methane is formed from a reaction sequence of ketene with H atoms. The above mechanism is compatible with previous studies of this system, with the exception of a recent communication by Lambert, Christie, and Linnett (Chem. Commun. 1967, 338), who suggest a primary step that yields methane and the formyl radical, CHO, rather than Reaction (1).