2.3- Butanedione, from photolysis

This method was first applied 87) in 1972 for the synthesis of disubstituted barre-lene 83 from dione 81, which was prepared as illustrated. Photolysis of 81 in dilute solution afforded 63 % of 83 in addition to the other products shown. Neither a cyclo-butanedione nor the purported diene intermediate 82 were observed although this might be possible under appropriate conditions. 

Butanedione is removed from the troposphere primarily by photolysis estimates of the lifetime for photodissociation within the lower troposphere with an overhead Sun are uncertain but estimated to be in the range from 0.8 h to 4 h (see figure IX-F-26 and table IX-M-1). The lifetime with respect to reaction with OH, for [OH] = 10 ... 

Figure iX-D-i i. Quantum yield of C2H4, CHsQOlCHs, and CO in 2-pentanone as a function of added 2,3-butanedione for photolyses at 313 and 253.7 run data are from Michael and Noyes (1963). The rise in the co values at the higher pressures of 2,3-butanedione probably reflects the increasing importance of CO formation from direct photolysis of the added 2,3-butanedione at 253.7 mn co increases reflect the stronger absorption by 2,3-butanedione at this wavelength. 

Figure IX-D-13. Plot of the wavelength dependence of the quantum yields of C2H4 and CH3C(0)CH3 in photolysis of 2-pentanone as a function of wavelength as estimated from the data referenced in table IX-D-5. Data are insensitive to pressures of 2-pentanone and temperature. Experiments with pure ketone, circles and triangles 4>w, unquenched by 2,3-butanedione or dienes, short dashed line 4>iv< adjusted for additional air quenching at 1 atm. using k /ki for acetone as interpolated for the selected wavelength, solid line. Both the short-dashed and the solid lines were used in the estimation of y-values for 2-pentanone given in figure IX-D-16.

Zahra and Noyes found that at 313 nm the addition of small amounts of oxygen (0.03-0.14 Torr) to 26 Torr of ketone (27°C) lowered the emission yield from 3-methyl-2-butanone by 21%, comparable to the effect seen in other small ketones. The emission yields observed by Zahra and Noyes in photolyses of pure ketone and in 2,3-butanedione-ketone mixtures, indicate clearly that the triplet state plays an important role in the photolysis of 3-methyl-2-butanone at 313 nm. There was no evidence of the triplet excited state observed in experiments at 253.7 nm. Lissi et al. (1973/1974) carried out experiments at 313 nm both with added 2,3-butanedione and with added cfi-l,3-pentadiene. These experiments confirm the importance of the triplet precursor to products for photolysis at 313 nm. 

---