Pressure dependence of the photooxidation of selected carbonyl compounds in air: n-butanal and n-pentanal

Abstract

Dilute mixtures of n-butanal and n-pentanal in synthetic air (up to 100ppm) were photolyzed with fluorescent UV lamps (275–380nm) in air at 298K. The main photooxidation products, identified and quantitatively analyzed by FT-IR spectroscopy, were ethene (n-butanal) or propene (n-pentanal), CO, vinylalcohol and ethanal. The photolysis rates and the absolute quantum yields Φ of both compounds were found to be slightly dependent on the total pressure. At 100Torr, in the photolysis of n-butanal and n-pentanal Φ100=0.48±0.02 and 0.40±0.04, whereas at 700Torr, the total quantum yields were Φ700=0.38±0.02 and 0.32±0.01, respectively. These results could be explained by the collisional deactivation of photoexcited molecules. Two decomposition channels were identified in both cases: the radical channel RCHO→R+HCO, and the molecular channel CxHyCHO→Cx−1Hy−3+CH2CHOH, having the relative yields of 68±3 and 32±3% (n-butanal), and 20±4 and 80±4% (n-pentanal) at 700Torr. The product, CH2CHOH, tautomerizes to ethanal. UV absorption cross-sections were measured prior to photolysis. For the comparison, the products were also analyzed after photolysis of both compounds in teflon bags using direct sunlight.