2-hexanone, photolysis

Quantum yield data for several processes that occur on photolysis of 5-4-methyl-1-phenyl-l-hexanone have been determined. The results are tabulated below for benzene as solvent. 

The major fate mechanism of atmospheric 2-hexanone is photooxidation. This ketone is also degraded by direct photolysis (Calvert and Pitts 1966), but the reaction is estimated to be slow relative to reaction with hydroxyl radicals (Laity et al. 1973). The rate constant for the photochemically- induced transformation of 2-hexanone by hydroxyl radicals in the troposphere has been measured at 8.97x10 cm / molecule-sec (Atkinson et al. 1985). Using an average concentration of tropospheric hydroxyl radicals of 6x10 molecules/cm (Atkinson et al. 1985), the calculated atmospheric half-life of 2-hexanone is about 36 hours. However, the half-life may be shorter in polluted atmospheres with higher OH radical concentrations (MacLeod et al. 1984). Consequently, it appears that vapor-phase 2-hexanone is labile in the atmosphere. 

Hexanone is a ketone, and ketones are generally not degraded by hydrolysis (Lande et al. 1976 Morrison and Boyd 1974). Based on its reactions in air, it seems likely that 2-hexanone will undergo photolysis in water, however no information was located. Based on studies with microorganisms (see Section 5.3.2.3), it is probable that 2-hexanone will be biodegraded in water. 

One of the most interesting molecular elimination reactions was first discovered by Norrish and Appleyard65 in 1934 and studied further by Bamford and Nor-rish66-69 in papers appearing in 1935 and 1938. These authors found that, on photolysis, aliphatic ketones with hydrogen atoms on carbons in the gamma position to the carbonyl yielded olefins and a methyl ketone. An early example was found in 2-hexanone, viz. 

Steps I and II are often referred to as Norrish type I and Norrish type II processes, respectively. Free radicals are formed in the former, while the latter leads to stable molecules without the participation of free radicals. The terms, type I and type II, were suggested by Bamford and Norrish following the discovery of primary decomposition mode II by Norrish and Appleyard in the photolysis of 2-hexanone. 

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 minor products (CO, CH4, C2H6, -C4Hio, I-C4H8, n-CsHn, CH3CHO) of the photolysis of 2-hexanone vapour were formed by the secondary reactions (recombination, disproportionation, hydrogen abstraction and decomposition) of the radicals originating from primary process conversions, the... 

The occurrence of primary process III in the photolysis of 2-hexanone vapour has not been verified so far. 

The results of Srinivasan convincingly proved the occurrence of isotope exchange in the photolysis of 2-hexanone-5,5-rf2, as well as in that of 2-hexanone carried out in a vessel pretreated with D2O. 

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 . 

Very little is known regarding the nature of the electronic state responsible for the decomposition of other ketones. The low quenching efficiencies observed in the photolysis of 2-hexanone may be interpreted by the short-life-time of the triplet state of 2-hexanone. The similar distribution of the butene isomers found in the photolysis of 4-methyl 2-hexanone at 2537 A and in its mercury-sensitized decomposition, indicates the important role played by the triplet state in reaction II even at short wavelengths. Finally, it is to be mentioned that the presence of secondary H atoms in the y position seems to enhance the contribution of the excited singlet state to the decomposition step II. 

Quantum yield data for three competing processes that occur on photolysis of 5 -methyl-l-phenyl-1-hexanone at 313 nm in benzene have been determined and are tabulated below. When the reaction is run in r-butanol, the racemization is entirely suppressed and the Type-II fragmentation is the major reaction. What information do these data provide about the mechanism operating under these conditions ... 

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