Acetaldehyde photodecomposition

Studies that relate to the quantum efficiency of the photodecomposition of acetaldehyde have continued since the early work of Leighton and Blacet (1933), Blacet and Heldman (1942), and Blacet and Loeffier (1942). These and the more recent studies of acetaldehyde photodecomposition (Weaver et al 1976/1977 Gill and Atkinson, 1979 Horowitz et al., 1982 Horowitz and Calvert, 1982 Meyrahn et al., 1982 Simonaitis and Heicklen, 1983 Horowitz, 1986 Yadav and Goddard, 1986) have been reviewed recently by Calvert et al. (2008). These studies confirm that radical formation in process (1) is the dominant mode of photodecomposition at the longer wavelengths present within the troposphere. Processes (11) and (III) increase in importance at the shorter wavelengths. 

Table IX-B-4. Recommended values for (pi,

Photodecomposition. The photochemistry of acetaldehyde has been studied by several investigators in the past. Both the primary processes and the free-radical chain processes have been studied, but it has not been until recently that a reasonable understanding of the primary processes has been achieved. If free-radical chain reactions are assumed to be secondary processes, three decomposition pathways can be envisaged ... 

The spectrophone technique has shown that a transient chemical species formed in the photolysis of acetaldehyde vapour, previously identified as an oxetan, is in fact the acetyl radical formed by the sensitized decomposition of trace amounts of biacetyl,84 and the involvement of biacetyl in the photodecomposition of other simple aliphatic carbonyl compounds may be much more important than has been realized hitherto. 

We were able to observe clear evidence for the chain-type mechanism in experiments, involving acetaldehyde decomposition in the gas-phase [98], similar to those already discussed for 2-propanol. With acetaldehyde, the values exceeded the maximum value obtained for a similar film for 2-propanol oxidation (0.28) (Fig. 6). As already discussed, the latter value may be considered to be an intrinsic maximum value for this particular film. Therefore, if < > exceeds the intrinsic maximum value, it indicates that radical chain reactions are important, that is, a single photon can cause more than one photodecomposition reaction. 

Ti, Nb, Ta, Mo, W, Fe, or Mn. One member of this family BisTisFeOis (with a bandgap of about 2-2.5 eV) has been reported to have photocatalytic activity for the decomposition of rhodamine B and acetaldehyde under visible light irradiation [84] or isopropyl alcohol photodecomposition [85]. 

Egerton TA, Murphy DM, Jenkins CA and Rowlands CC (1997) An EPR study of spin trapped free radical intermediates formed in the heterogeneously assisted photodecomposition of acetaldehyde. Journal of the Chemical Society, Perkin Transactions 2 2479-2485. 

IX-B-2.3. Photolysis Frequencies for the Photodecomposition of Acetaldehyde in the Lower Troposphere... 

Propanal (C2H5CHO) photodecomposition in the troposphere follows the pattern exhibited by acetaldehyde. Although five different photodecomposition paths have been considered in explanation of its photochemistry, only process (I) is important for... 

At wavelengths of sunlight within the troposphere, only primary processes (I) and (III) are important. Primary processes (II), (IV), (V), (VI), and (Vn) may become significant for photolysis at the shorter wavelengths that are available only in the upper atmosphere. While processes (I), (II), and (VI) are analogous to those observed with acetaldehyde and propanal, process (III) is unique to carbonyl compounds that contain a p-H-atom, an H-atom attached to the third C-atom down-chain from the carbonyl group. This photodecomposition path is often called a Norrish type-II process (Bamford and Norrish, 1935 Davis and Noyes, 1947). The initial enol-form of acetaldehyde formed in (III),... 

CH2=CH0H, ultimately forms the more stable acetaldehyde molecule (CH3CHO), a transformation catalyzed by contact with a surface. The commonly accepted mechanism for this type of photodecomposition involves a cyclic, 6-membered ring intermediate ... 

Bacher et al. (2001) and Magneron et al. (2005) have studied the photodecomposition of 2-hydroxyethanal in air. Bacher et al. (2001) employed 240-400nm radiation from a filtered xenon lamp. Their data suggested that process (I) is the major photodecomposition mode, analogous to the bond cleavage seen in acetaldehyde photolysis, although some CH3OH was detected also, presumably via reaction (III). 

Bacher et al. (2001) used acetone and acetaldehyde as reference compounds in pho-tolyses within the reaction chamber to estimate the average quantum yield of process (I) in 1 atm. of air <(>i>0.5 for X = 285 25nm. From measuredy-values in sunlight in the EUPHORE chamber, Magneron et al. (2005) concluded that the effective total quantum yield of photodecomposition was near unity (< = 1.3 0.3). 

As with acetaldehyde, the dominant mode of photodecomposition in the troposphere may occur by reaction (I) with a rupture of the R—CHO bond. 

Raber and Moortgat (1996) and Koch and Moortgat (1998) does not confirm the occurrence of process (VI), since CH3CHO appears to have been formed by heterogeneous photodecomposition of methylglyoxal at the wall of the quartz reactor. No acetaldehyde was observed in the stainless steel reactor of Staffelbach et al. (1995), and it appears likely that (pw is near zero. No methane has been detected among the products, and (pv must be near zero. 

The major product of ethyl nitrite photodecomposition in the troposphere is expected to be acetaldehyde, formed from the ethoxy radical reaction, C2H5O -I- O2 CH3CHO + HO2, as well as some ill-defined amount of CH2O as well as CH3CHO formed in process (II). 

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