Photooxidation of n-Hexanal in Air

Jovan Tadić,Ivan Juranić,Geert K Moortgat

doi:10.3390/60400287

Abstract

Dilute mixtures of hexanal in synthetic air (up to 100 ppm) were photolyzed with fluorescent UV lamps (275-380 nm) in air at 298 K. The main photooxidation products, identified and quantitatively analyzed by FTIR spectroscopy, were butene, CO, vinylalcohol and ethanal. The photolysis rates and the absolute quantum yield Φ were found to be slightly dependent on the total pressure. At 100 Torr, Φ100 = 0.43 ± 0.02, whereas at 700 Torr the total quantum yield was Φ700 = 0.38 ± 0.02. These results may be explained by the collisional deactivations of photoexcited molecules. Two decomposition channels were identified: the radical channel C5H11CHO → C5H11 + HCO, and the molecular channel C5H11CHO → C4H8 + CH2=CHOH, having the relative yields of 27 and 73 % at 700 Torr. The product CH2=CHOH tautomerizes to ethanal.

Highlights

Aldehydes play a significant role in a variety of atmospheric reactions, such as formation of photochemical smog, peroxyacetyl nitrate (PAN), and regional ozone
The photolysis rates and the absolute quantum yield Φ were found to be slightly dependent on the total pressure
We have investigated products and absolute quantum yields in the pressure range 100 to 700 Total pressure (Torr)

Summary

Introduction

Aldehydes play a significant role in a variety of atmospheric reactions, such as formation of photochemical smog, peroxyacetyl nitrate (PAN), and regional (tropospheric) ozone. Photodissociation of aldehydes represents an important source of free radicals in the lower atmosphere, and may significantly influence the atmospheric oxidation capacity [1,2,3]. Smaller alkyl aldehydes are products of the atmospheric photooxidation of hydrocarbons, ethers, alcohols, and other organic compounds. Aldehydes absorb in the near UV range, and dissociate upon absorption of light. Aliphatic aldehydes exhibit a weak absorption band in the wavelength range 240-360 nm as a result of a symmetry forbidden n - π* transition [6,7]. Photodissociation of aldehydes produces free radicals, which are subsequently involved in other atmospheric reactions and reaction cycles. There have been a number of studies devoted to the photodissociation of the simplest alkyl aldehydes, such as HCHO, CH3CHO, C2H5CHO [8,9,10,11,12,13,14,15,16], but a relatively small number devoted to longer chain aldehydes, such as C3H7CHO, C4H9CHO [17,18] and iso-pentanal and tert-pentanal [32]

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