PHOTOCHEMISTRY OF BIACETYL

The photochemistry of biacetyl has been extensively studied, both in the vapor phase and in solution. In the vapor phase the products include carbon monoxide, ethane, methane, acetone, ketene, and 2,3-pentanedione. It has been shown that the primary process is cleavage of the carbon-carbon bond between the two carbonyl groups to yield acyl radicals, which on further reaction give the observed products.14,43... 

Biacetyl is a major product of the atmospheric oxidation of many aromatic compounds, e.g., toluene, o-xylene, 1,2,3- and 1,2,4-trimethylbenzene. Its photochemistry is of interest to atmospheric scientists since it is a likely source of free radicals within the troposphere. The absorption of biacetyl extends well into the visible region of the sunlight see figure IX-F-14. The analysis of the absorption bands of biacetyl have been studied and rationalized theoretically (e.g., see Brand and Mau, 1974) Huang et al., 2005). The photochemistry of biacetyl in the absence of oxygen has been the subject of numerous studies since the early 1940s (e.g., see Henriques and Noyes, 1940 Roof and Blacet, 1941 Anderson and RoUefson, 1941 Blacet and Bell, 1953 Bell and Blacet, 1954 Sheats and Noyes, 1955a, b Ausloos and Steacie, 1955 Okabe and Noyes, 1957 Heicklen, 1959 Noyes et al., 1962 Parmenter and Poland, 1969 Caro et al., 1969 Abuin et al., 1971 Horowitz and Calvert, 1972 Sidebottom et al.. 

Sheats, G.F., and W.A. Noyes, Jr. (1955b), The long wave photochemistry of biacetyl and its correlation with fluorescence at temperatures over 100°, J. Am. Chem. Soc., 77, 4532-4533. 

Photodecomposition. As reviewed by Cundall and Davies (61), both of these absorption systems have been studied, and product yields resulting from excitation from 435.8 to 238 nm have been reported (22,30,176). Since the last review by Cundall and Davies (61), most studies have dealt with the A X system, since both singlet and triplet emission have provided valuable tools in the exploration of biacetyl photochemistry (see ref. 185). Three photodecomposition processes have been proposed for biacetyl ... 

The photochemistry of o-quinones and non-enolic a-diketones has a venerable history dating at least to 1886 when Klinger 82> reported sunlight irradiations of benzil and 9,10-phenanthrenequinone. The latter compound and biacetyl have been extensively investigated for many years and interest has broadened to embrace a wide variety of substances. The basic processes involved depend in large measure on the presence of the vicinal dicarbonyl system rather than on classification as diketone or o-quinone it appears both justified and desirable to consider their photochemistry jointly. a-Diketones which exist in the enolic form behave for the most part as substituted a,/9-unsaturated ketones and will not be considered. For convenience, the term dione will be used when reference is intended to both a-diketones and o-quinones. 

Photochemistry of diones began with Klinger s 82> report of the sunlight reactions of benzil and phenanthrenequinone with diethyl ether but reactions of diones with ethers have been investigated to a very limited extent since then. The reported reactions include biacetyl (dioxane 20>), benzil (diethyl ether 82>, dioxane 128>), tetramethyltetralindione (73, dioxane 60>), tetrachloro-o-benzoquinone (dioxane 128>), acenaphthene-quinone (dioxane 128>) and phenanthrenequinone (diethyl ether 82>, di-isopropyl ether 127>, di-n-butyl ether 127>, tetrahydrofuran 128>, dioxane 128>, anisole 128>, methoxycholestane (86) 128>, methoxy-5-cholestene 128>). 

Triplet states in organic photochemistry. Triplet states are often more important than singlets, and so are important in preparative organic photochemistry. Only a few triplet states, however, can be made in good yield by illumination of the ground state of these, that of biacetyl is the best known. Many other triplet states, however, can be prepared from the ground-state molecules by transfer of electronic energy from biacetyl excited by a flash (see below. Section 4.S.2.2), e.g. ... 

Photochemistry. The cw-isomer of [PtCl2(py)2] undergoes two primary photoprocesses, isomerization and dissociation, with quantum yields of ca. 0.04 and 0.025 respectively (313 nm irradiation). /ra 5-(PtCl2(py)2] also undergoes parallel photoisomerization and dissociation, but with very much lower quantum yields of ca. 10" . The effect of acidity on the photochemistry of cw-[Pt(gly)2] has been examined-the balance between photoisomerization and photodissociation is pH-dependent. Photochemical cis trans isomerization of [Pt(gly)2] is sensitized by pyrazine or by xanthone, but not by thioxanthone, quinoline, naphthalene, or biacetyl. Ni + quenches the photoisomerization, but Mn + does not. The sensitization process was shown to involve triplet-triplet intermolecular energy transfer. ... 

Due to a fast and efficient intersystem crossing process (cf. Table 3.2) most ketones perform mainly triplet photochemistry [25,73], In Table 3.6, the photoreactivity of triplet-excited acetone, biacetyl, and benzophenone is compared with the triplet-excited azoaUcane DBH-T. The data for both chromophores follow similar trends. Namely, dienes and amines quench with quite high rate constants, while ethers and aromatic compounds react rather inefficiently. 

---