Hexanal and 5-Hexenal

n-Hexanal and 5-Hexenal. Photodecomposition of 5-hexenal and n-hexanal in the gas phase has been studied to evaluate the effect of an unconjugated olefinic chromophore on the nonradia-tive processes in aldehydes (99,100,102). Hansen (99) obtained the product quantum yield of 1-butene at low pressures ( 0.10 torr) and room temperature in the photolysis of n-hexanal to be -0.23 at 254 and 313 nm. The product quantum yields in the limit of zero pressure in the photolysis of 5-hexenal were 0.37 for 1,3-butadiene, 0.28 for propylene, and 0.04 for ethylene at 312 nm. An order-of-magnitude-greater yield of ethylene was obtained in the 254-nm photolysis, with other yields remaining the same. 

In the triplet benzene photosensitization, the yield of 1,3-butadiene was about eight times the yield of propylene (with acrolein).  

The formation of an olefin by the Norrish type II photoelimination was not much affected by the presence of an unconjugated olefinic group, but the formation of propylene (and acrolein) in 5-hexenal requires a new photochemical transformation mechanism. It has been suggested that an olefinic, triplet precursor which can undergo intramolecular H-atom abstraction reaction is responsible for this process. 

A similar biradical mechanism was postulated in an earlier study of the triplet-benzene-sensitized decomposition of 4-pentenal (136). This mechanism requires an efficient intramolecular 

The triplet mercury photosensitization of 1-pentene gives methyl-cyclobutane (156) and 1,2-dimethylcyclopropane (192). Likewise, the triplet mercury photosensitization of 1-butene gives methyl-cyclopropane (63). Similar olefin rearrangements have been studied more recently with 3-methyl-1-butene (166). These olefin rearrangements are similar in mechanism to the 5-hexenal rearrangement discussed above. 

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