2-Pentanone photolysis

RATE OF FORMATION OF PRODUCTS FROM LIQUID 2-PENTANONE PHOTOLYSIS (iN 10 MOLE.I" .SEC UNITS) AT 3130A AND / = 8xl0 QUANTA.l" .SEC" ... 

Data directly related to 2-pentanone photolysis in the atmosphere are very limited, but some useful, related information is available for the pure ketone and its mixture with various added compounds. 

Figure IX-D-14. Quantum yields of C2H4 in 2-pentanone photolysis as a fnnction of added 1,3-butadiene or 1,3-pentadiene. Data from Wettack and Noyes (1968) wavelength, 313nm temperature, 25°C.

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. 

Vapor-Phase Photolysis of 2-Pentanone. Effect of Additives (Ausloos and Rebbert )... 

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 . 

As expected, ethylene and acetone are formed in equal amounts in the photolysis of 2-pentanone . The primary quantum yield, n, is practically independent of temperature and of 2-pentanone pressure, but increases with decreasing wavelength (Table 21). 

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 Borkowski and Ausloos on the photolysis of 2-pentanone-4,5,5- 3 also indicate a significant preference for the abstraction of the H atom of the weaker C-H bond in the primary step II, i.e. the splitting of the weaker C-H bond is preferred to that of the C-D bond. However, as the vibrational excitation of the molecule increases, the D atom abstraction becomes more significant. 

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. 

Michael and Noyes observed a long-lived, 2-pentanone-sensitized biacetyl emission, the efficiency of which was lower at 2537 A than at 3130 A. A very weak sensitized emission was reported by them to occur in the photolysis of 2-hexanone, in the presence of biacetyl, at 3130 A, while at 2537 A emission was not observed. In the photolysis of 2-pentanone, biacetyl decreased the value of 0n at 3130 A (a Stern-Volmer type relation was obeyed), but exerted no influence at 2537 A . Biacetyl was foimd to have no influence on reaction II at either wavelength in the photolysis of 2-hexanone . 

In the photolysis ol 2-pentanone, the relative quenching efficiency ol biacetyl and m-butene-2, measured in terms of reaction II, is considerably greater than 1 ( Bi/Bu = 214) . This means that primary process II originates from low vibrational levels of the triplet state. 

A recent investigation by Wettack and Noyes on the photolysis of 2-pentanone with added diolefins, indicates that both the excited singlet and the triplet states are the precursors of reaction II. On the basis of the assumption that diolefins quench the triplet state, it was concluded that at 3130 A about 62 % of reaction II arises from the triplet state, while at 2654 A only about 18 %. Furthermore, their results suggest that both the excited singlet and the triplet states may decompose according to reaction I. 

Results of the photolyses of acetone, 2-butanone, and 2-pentanone adsorbed on Vycor glass are shown in Table 2. It is well known that alkyl ketones with / hydrogen atoms, such as 2-pentanone, undergo the Norrish Type II processes (intramolecular elimination) as well as the Norrish Type I processes (C -cleavage into radical pairs), as shown in the following reaction mechanisms. In the gas phase photolysis of 2-pentanone at room temperature, the amount of products derived from the Type I processes is less than 5-15% of that derived from the Type II process (26). As seen in Table 2, the rate of CgHg formation is more than 75% that of C2H formation. 

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