Methyl 2-hexanones, 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. 

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. 

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 . 

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 ... 

The photolysis of 4-methyl-2-hexanone follows the pattern of photodecompositions observed in other ketones (Calvert et al., 2(X)8) ... 

Ausloos, P. (1961). Intramolecular rearrangements. II. Photolysis and radiolysis of 4-methyl-2-hexanone, J. Phys. Chem., 65, 1616-1618. 

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