PHOTOCHEMISTRY OF ALIPHATIC KETONES WITH A HYDROGEN ATOM IN THE y POSITION

Photochemistry of aliphatic ketones with a hydrogen atom in the y position 

The aliphatic ketones with a hydrogen atom on the y-carbon atom constitute a well-defined group with respect to their photochemistry, showing definite differences from the photochemical behaviour of other ketones. Recent investigations have considerably extended our knowledge in this field. 

There are four primary processes to be taken into consideration, 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. 

The occurrence of primary step III was observed in the photolysis of ketones having several carbon atoms. In spite of the limited information available, it seems likely that only ketones with y-hydrogen atoms show this type of reaction. 

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