Photochemistry of Carbonyl Compounds

Abstract

Carbonyl compounds undergo various photochemical reactions in both gas and liquid phases. The electronic excited states of saturated ketones are n {\textrightarrow} $π$* states, whereas of conjugated ketones are $π$ {\textrightarrow} $π$* states. Both these excited states in singlet or triplet states take part in chemical reactions. The activation energies for singlet and triplet states of saturated ketones are about 80--85 and 75--80 kcal/mol, respectively, whereas of unsaturated ketones, these are in the range of 45--75 kcal/mol. For this reason, the photoreactions of saturated ketones occur in the UV region, 270--290 nm, and of unsaturated ketones in the UV region, 310--330 nm. The excited carbonyl compounds have radical characters at both carbon and oxygen atoms of the carbonyl group, and hence most of their photoreactions proceed through radical intermediates. The important photoreactions of carbonyl compounds are the reduction of carbonyl compounds by hydrogen abstraction, fragmentation including the Norrish types I and II cleavages, cycloaddition to alkenes (the Paterno- Büchi reaction), and rearrangement (the lumiketone and oxa-di- $π$-methane rearrangements) reactions. All these reactions are discussed in details with several examples and mechanisms.