Mechanism of the Photolysis of Propionaldehyde

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Abstract
The photolysis of propionaldehyde at λλ2537, ∼2900, and ∼3200A has been studied by the Paneth mirror method. The alkyl radicals produced are exclusively ethyl; no atomic hydrogen can be detected by the guard mirror method in amounts exceeding 2 percent of the total number of mirror‐active particles present. The yield of C2H5 radicals has been determined and compared with the yields of CO, H2, and C2H6. The results are most readily interpreted in terms of competing primary decomposition processes producing free radicals in one reaction and ultimate molecules in another. Whereas the former reaction increases in importance at shorter wave‐lengths, the latter becomes less significant. On the other hand, an explanation for the results can be given on the basis of a mechanism involving only the production of free radicals in the primary process at all wave‐lengths. The role played by excited propionaldehyde molecules (of life which may be >10−3 sec.) and by freshly formed (energy‐rich) C2H5 and HCO radicals on their first collision is also considered. The mechanism of energy transfer within the propionaldehyde molecule is discussed and it is shown that the products of the primary decomposition are probably determined by geometric considerations and by the relative heights of the dissociation levels involved in the normal state.

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