Biacetyl quenching

Borkowski and Ausloos pointed out that biacetyl quenches the formation of ethylene, but has no influence on the formation of propane in the photolysis of n-butyraldehyde at 3340 A. The rate of formation of C2H4 levels off at a biacetyl pressure (about 8 torr) which is approximately the same as that where induced phosphorescence attains its limiting efficiency. The triplet aldehyde molecules seem to decompose according to step III, while primary process I originates from the singlet state (or from the higher vibrational levels of the triplet state) at 3340 A. 

Wagner and Liu60 have studied the reaction of the ketone (60) in an endeavour to obtain more accurate values for the lifetime of 1,4-biradicals produced by the Norrish Type II process. In this example the biradical produced, (61), can follow several reaction paths, i.e. oxetan formation, fission, reversion to ground state by disproportionation, and formation of norbornanol (62). The products obtained are illustrated in Scheme 3. From the analysis of the system the authors 59 concluded that cyclization and cleavage of the biradical (61) each occur with a rate constant of 6 x 10 s 1. Coyle and Kingston51 have reported a study of Norrish Type II cleavage in a series of 2-(dimethylamino)ethylbenzoates (63). Since neither penta-1,3-diene nor biacetyl quenches the reaction, although both... 

Backstrom and Sandros<54-55) found that the phosphorescence of biacetyl in benzene solution at room temperature was quenched at a diffusion-controlled rate by aromatic hydrocarbons when the triplet energy of the hydrocarbon was sufficiently below that of biacetyl. 

The photoreduction of nitrobenzene using p5o ex filtered light from a medium pressure mercury arc was studied in petroleum, toluene, ether, 2-propanol, tert-butyl alcohol, diethylamine, triethylamine, aqueous solutions of 2-propanol and diethylamine and also in aqueous t-butylalcohol containing sodium boro-hydiide 3 >. Varying amounts of aniline, azo- and azoxybenzene were obtained. In the presence of a fourty-fold excess of benzophenone, a six-fold increase in the rate of aniline formation in ethereal solution was observed, and aniline formation was completely suppressed by addition of biacetyl or octafluomaphthalene Since unreacted nitrobenzene could be recovered in these experiments, it is demonstrated that the triplet state of nitrobenzene was quenched. 

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. 

In studies of the vapor phase photolysis of biacetyl it was observed that a new product, which quenched both the phosphorescence and primary dissociation of biacetyl, was formed. A strong absorption at 275 nm, which was associated with the quenching activity, was also observed. Since no new product which displayed significant quenching activity could be isolated, it was concluded that the quenching was due to the enol of biacetyl (8).57 This assignment was supported by the disappearance of the absorption at 275 nm when IC1 was added to an irradiated aqueous solution of biacetyl.58... 

The sensitized photo-Fries rearrangement of 55 in benzene, toluene, and in concentrated polystyrene solution in dioxane is effectively quenched with biacetyl.4 This phenomenon must again be attributed to quenching of the aromatic energy donor, because in pure dioxane the photorearrangement of 55 is not influenced by biacetyl (vide supra). 

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 a study of the quenching of benzophenone photoreduction with biacetyl, it was observed that the biacetyl was consumed.116 This phenomenon also is readily explained by chemical quenching, transfer of a hydrogen atom from the benzophenone ketyl radical to biacetyl to form the biacetyl ketyl radical and ultimately the pinacol derived from biacetyl. 

Biacetyl as sensitizer phosphorescence quenched fluorescence not affected, release of NH S No apparent degradation of biacetyl ... 

Biacetyl sensitized photoisomerization of l,2-di-9-anthrylethane 7a does not lead to the 4n + 4n cyclomer 8a but yields exclusively the An + 2n cycloadduct 26 with a quantum yield of 0.1 [72]. Since the phosphorescence of biacetyl is quenched by dianthrylethane 7a at nearly diffusion controlled rate, the photochemical Diels-Alder reaction is explicable by triplet energy transfer from biacetyl to 7a. The photochemical isomerization of 10-benzoyl-l,2-di-9-anthrylethane 27 also proceeds exclusively by An + 2n cycloaddition and gives cycloadduct 28 with a quantum yield of 0.005 [73], The low fluorescence quantum yield of 27 (excited triplet state. Biacetyl sensitization of 27 leads to 28... 

B ckstrom, H. L. J., and K. Sandros The quenching of the long-lived fluorescence of biacetyl in solution. Acta chem. scand. 12, 823 (1958). 

Before the concerted vs. two-step question was further elucidated, another basic mechanistic puzzle was raised. One research group found that type II cleavage of 2-pentanone was quenched by biacetyl [6], which was known to quench excited triplets rapidly. Another group found that the reaction of 2-hexanone was not quenched under the same conditions [7]. The two groups obviously differed as to which excited state undergoes the reaction. The apparent conflict was neatly solved by the revelation that each of the two ketones reacts from both states, with 2-hexanone undergoing more unquenchable singlet reaction than 2-pentanone [8,9]. 

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