The photo-oxidation of propionaldehyde

C₂H₅CHO vapor was photolyzed with 313 nm radiation at 22 °C in the presence of O₂ and O₂He, O₂N₂, O₂NO and O₂cis-2-C₄H₈ mixtures. The contents of the reaction mixture were allowed to bleed through a pinhole into a quadrupole mass spectrometer providing for continous monitoring of the C₂H₅OH and C₂H₅C(O)O₂H produced. CO, CO₂, C₂H₄ and CH₃CHO concentrations were determined by gas chromatography. From CO quantum yield determinations we found that the only photolytic process of importance is the following: {A figure is presented} The CO quantum yields also indicate that the reactive state of C₂H₅CHO is pressure quenched, and the half-quenching pressures for the various reactant gases were measured. The primary quantum yield in 1 atm of air is 0.30 ± 0.05. The data suggest that the reacting state is a vibrationally excited triplet state which consists of levels which are quenched with different efficiencies by any quenching gas. The more rapidly quenched leves account for about 40% of the total, while the less rapidly quenched levels account for about 60% of the total. Large quantum yields were measured for CO₂, C₂H₅OH and C₂H₅C(O)O₂H indicating a chain reaction initiated by abstraction of the aldehydic hydrogen of C₂H₅CHO by C₂H₅O radicals. The C₂H₄ quantum yields are dependent on the total pressure, and we believe that C₂H₄ is produced from the decomposition of vibrationally excited C₂H₅CO₃ radicals: {A figure is presented} The deactivated C₂H₅CO₃ radicals will either react with HO₂ or C₂H₅CHO or at the walls to produce C₂H₅CO₃H, or ultimately decompose to CO₂ + C₂H₅. We found that the following reactions are unimportant at 22 °C: {A figure is presented}.