Biacetyl, photolysis photoreduction

The phosphorescence of a 5 X lO" M solution of biacetyl in de-aerated 2-propanol at room temperature could be quenched completely by 1 a,d,e (10 8 M) 84). In all three cases, the corresponding photoreduction products 2a,d,e emerge from analogous preparative scale biacetyl sensitized runs. Since 2e is also formed, steric hindrance to hydrogen abstraction from solvent cannot be too effective when a (probably longer-lived) triplet is populated, whereas it might be effective in the direct photolysis ot 1 e 88) where isomerisation competes with reduction probably in the (short-lived) singlet state. 

Photoreduction was quenched by high concentrations of biacetyl, slightly retarded by iodonaphthalene, but not affected by azulene or anthracene.113 These observations led to the unsatisfying conclusion that reduction proceeded via a triplet state which could be only selectively quenched. However, later work114 using flash photolysis showed that the benzophenone ketyl radical was generated upon irradiation of solutions of benzophenone and acridine, and that its predominant mode of disappearance was by reaction with... 

In the presence of an alcohol or ether the excited acetone abstracts an H atom from the carbon alpha to the 0 atom of the alcohol or ether. Similar photoreduction of perinaph-thenone (365) has been observed. However, the nature of the excited state of acetone for liquid photoreduction in these studies has not been established. Free radicals produced from photoreduction of acetaldehyde, biacetyl and acetoin in the presence of good H-atom donors have been observed by Zeldes and Livingston (216). These authors also studied the photolysis of oxalic acid and esters (366). 

Hexafluorobiacetyl. In the gas phase, hexafluorobiacetyl behaved 169> in the same manner as biacetyl, affording carbon monoxide and hexa-fluoroethane in stoichiometric ratio. However, in the presence of hydrocarbon vapor or in hydrocarbon solution, the disappearance of dione was not accompanied by formation of cleavage products. For example, photolysis (A >3000 A, 65°) of hexafluorobiacetyl in 150 molar excess of 2,3-dimethylbutane until 84% of starting material had disappeared resulted in only 0.5% of carbon monoxide, an equivalent amount of trifluoromethane, and a complex mixture of other products. On the basis of carbonyl and hydroxyl absorption in the infrared, it was assumed that the excited dione abstracted hydrogen from hydrocarbons to give products of photoreduction, photoaddition, etc. Occurrence of H-abstraction in the gas phase is unique. 

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