Photolysis in the liquid phase and solution

The results of earlier photolytic investigations ° , carried out in the liquid phase and in solution, revealed certain similarities and, at the same time, definite differences compared to those obtained in the vapour phase. 

The reactions taking place in the vapour phase also occur in the condensed phase, and their mechanisms are probably similar. However, as may be expected on the basis of the results obtained for the gas phase photolysis, the formation of olefins, cycloparaffins, and CO is of less importance, while that of the saturated aldehydes is more important in the liquid phase or solution, where energy dissipation by collision is more efficient. The decarbonylation products were shown to be only of minor importance in the photolysis of liquid cyclopentanone and cyclohexanone . The unsaturated aldehyde was found to be the main product in the liquid-phase photolysis of cyclopentanone ( A4-pent nai = 0.37 0.02), of cyclohexanone (where, at 3130 A, approximately 90 % of the ketone decomposed gives 5-hexenal), as well as of 2-methyl cyclohexanone . Unsaturated aldehydes were also identified in the photolysis products of other cyclic ketones in the liquid phase as well as in solution .  

In the photolysis of 6-, 7- and 8-membered cyclic ketones and their alkyl derivatives, the occurrence of a reaction which can be shown as 

In addition to the products mentioned hitherto, the formation of various condensed products was also reported in the liquid phase and in solu-tion in addition, the formation of cyclohexanone pinacoP and caproic acid in cyclohexanol and water solutions, respectively, was observed. Moreover, aliphatic alcohols with carbon atom numbers equal to those of the original ketones were found in the liquid phase and also in solution It is probable that these products were formed by the interaction between the excited and the normal ketone molecules or in the reaction of the excited ketone with the solvent. These processes may be symbolized as 

Cyclopentanone emits both fluorescence and phosphorescence in the liquid phase ° The results of Dunion and Trumbore show that the quantum yield of 4-pentenal formation, in the photolysis of cyclopentanone, is decreased by O2 or piperylene . Since both substances are known to be triplet state quenchers, it may be suggested that the triplet state participates in the photolytic decomposition of cyclic ketones in the liquid phase and in solution. 

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