4- methyl-2-pentanone, photolysis

A lower molecular weight methyl ketone and an olefin are isolated as products of this reaction. That the enol is formed as a primary product which rearranges to the ketone follows from its detection in the IR spectrum of gaseous 2-pentanone upon photolysis. 3 In addition to the ketone and olefinic products, one usually obtains varying amounts of cyclobutanols. 

Among the products of the radiolysis of 2-pentanone and 2-hexanone, ethylene and propene were found in considerable amounts . The distribution of the butenes in the radiolysis of 4-methyl 2-hexanone (both in the vapour and in the liquid phase) was very much similar to that observed in fhe photolysis at short wavelengths . 

The formation of 1-methyl cyclobutanol was observed in the photolysis of 2-pentanone at 3130 A, and later in the wavelength region 2300-3200 A, also. ... 

Values for 0i and 0,i were determined in the photolysis of two substituted 2-pentanones (Table 24). The quantum yields of acetone and olefin formation were found to be equal and independent of temperature. No wavelength dependence of the primary quantum yield 0n was observed between 3130 and 2654 A in the photolysis of methyl neopentyl ketone. 

The results of Barltrop and Coyle , on the photolysis of aliphatic ketones in solution at 3130 A, support the explanation suggested by Wagner and Hammond for the low quantum yields. The overall quantum yield for disappearance of 2-pentanone, 2-octanone and 5-methyl 2-octanone was found to increase and approach unity in hydroxylic solvents. This increase can be attributed to the solvation of the hydroxy biradical intermediate. Since, however, the solvent effect was not observed for products originating from excited singlet molecules, it is probably only the triplet state which decomposes via biradicals. 

The 24 hour photolysis of chlorendic anhydride (III) in acetone in the presence of diethoxyacetophene (IV) yielded at least twelve measurable products as observed by the GC/MS procedure (Table I). Some of the products observed, i.e. 4-methyl-4-hydroxy-2-pentanone, biacetal, 4-methyl-3-pentene-2-one, ethyl acetate, 3-methyl-3-hy-droxy-2-butanone, 4-methyl-2-pentanone, and 2,4-pentane-dlone, are clearly derived from the photochemically induced decomposition of acetone and/or dlethoxyacetophe-none. 

More elaborate functionalizations could be accomplished when the donor or the radical cation underwent more complex reactions. As an example, the above synthesis of alkyltricyanobenzenes could be varied into a ROCAS reaction (Scheme 14.9, right part) [48, 49]. Thus, photolysis of a mixture of TCB as acceptor, 2-methyl-2-phenyl-l,3-dioxolane (13) as donor and an electron-poor olefin formed a 3-aryl substituted 2-pentanone, which cyclized during purification on silica gel to afford isocoumarine 14 in 75% yield [48], This three-component reaction is based... 

The reaction of OH with 3,3-dimethyl-2-butanone has been studied by Wallington and Kurylo (1987a), who used flash photolysis with resonanee fluorescence detection. They determined a rate coefficient of (1.21 0.05) x 10" cm molecule" s" at 298 3 K. In the absence of other measurements, we recommend this rate coefficient, but with an increased uncertainty of 40%. The reaction probably has a negative temperature dependence similar to that of 4-methyl-2-pentanone. Most of the reaction... 

The data available are insufficient to allow a definitive estimate of the quantum efficiencies as a function of wavelength for photolysis of 3-methyl-2-butanone in air that are required to define y-values. The photochemistry of 3-methyl-2-butanone may be similar to that observed for 2-butanone and 3-pentanone (section IX-D-2 and IX-D-3). However, the CH3C(0)—CH(CH3)2 bond is somewhat weaker than the CH3C(0)—C2H5 bond, and it seems reasonable that the y-values for use with 3-methyl-2-butanone should be somewhat greater than those of acetone and 2-butanone. 

The absorption spectrum of 4-methyl-2-pentanone is compared with that of several other ketones in figure IX-D-18, and tabular data are given in table IX-D-8. Data from a single gas-phase, 313 nm photolysis experiment with the pure ketone (120 C, pressure 49Torr) by Martin and Pitts (1955), suggest that there is little discrimination in bond breakage in (I) and (II). From co, c3H6-0rv 0.35. 

The only extensive study of the photolysis of 4-methyl-2-pentanone was carried out in degassed solutions of hexane confined in Pyrex test tubes that were irradiated with a medium-pressure Hg lamp (Encina et al., 1975). Extrapolation of this information to the photolysis in 1 atm. of air is impossible. However, the information may shed some light on the gas-phase photolysis. In the hexane solutions, 4>isobutane (pY 0.034 0.003(76°C). 4>ch3C(0)CH3 0iv 0.21 0.02 (-5 to 69°C). Experiments with added 1,3-pentadiene were used to quench excited triplet ketone molecules. The authors estimated from experiments at 20°C that the quantum yield of quenchable triplets excited under their conditions was 0.75 0.03 about 88% of the acetone product of (IV) was formed from the Ti state with about 12% from the Si state. These results point to the involvement of both the singlet and triplet states as well in the gas-phase photolysis, but the extent of participation of each is unclear. Until the necessary data are available to evaluate properly the wavelength dependence of the quantum... 

D Anna, B., S.R. Sellevig, K. Wirtz, and C.J. Nielsen (2005a), Photolysis study of perfluoro-2-methyl-3-pentanone under natural sunlight conditions. Environ. Sci. TechnoL, 39, 8708-8711. 

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