Photolysis of acetaldehyde in aqueous solution

The photolysis of acetaldehyde in solution has been the subject of only a few studies. 

Hirshberg and Farkas photolyzed acetaldehyde in aqueous solution by the full light of a mercury arc at room temperature and at aldehyde concentrations of 10 -10 M. They concluded that the main product of the photolysis was aldol, formed in an association reaction of an excited and a normal aldehyde molecule. 

Experimental data suggest that the main reactions occurring in aqueous solution and in the gas phase are the same. However, the photolysis in solution shows some characteristic features. The question of the mechanism of hydrogen formation is still unsolved. Chen and Volman s observation that (f n decreased with increasing aldehyde concentration led them to conclude that reaction 

Allyl alcohol decreases the quantum yields of product formation. The quantum yields level off at higher allyl alcohol concentrations. The limiting value is approximately the same at 27.6 and 73.0 °C. It may well be assumed that the limiting quantum yields are related to an intramolecular non-radical decomposition. However, according to Chen and Volman, the residual reaction is essentially a geminate one occurring between methyl and formyl radicals that have been formed from the same aldehyde molecule in a solvent cage, viz. 

Finally it is to be noted that allyl alcohol does not completely suppress ethane formation, which indicates that ethane may also be formed in a non-radical process of some sort. 

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